Meteorology

METEOROLOGY

INTRODUCTION

Meteorology, study of the earth’s atmosphere and especially the study of weather. A meteorologist is a person who studies the atmosphere. Meteorology is divided into a number of specialized sciences. Physical meteorology deals with the physical aspects of the atmosphere, such as the formation of clouds, rain, thunderstorms, and lightning. Physical meteorology also includes the study of visual events such as mirages, rainbows, and halos. The study of the winds and the laws that govern atmospheric motion is called dynamic meteorology. Synoptic meteorology is the study and analysis of large weather systems that exist for more than one day. Weather forecasting is part of synoptic meteorology. Agricultural meteorology deals with weather and its relationship to crops and vegetation. The study of atmospheric conditions over an area smaller than 1 sq km (0.4 sq mi) is called micrometeorology. Climate describes the average weather of a region. Climatology, a division of meteorology, is the study of a region’s average daily and seasonal weather events over a long period.

PHYSICAL CHARACTERISTICS OF AIR

A. Air CompositionThe earth’s atmosphere is a mixture of gases, mainly nitrogen (N2) and oxygen (O2), that are held to the earth by gravity. Near the earth’s surface, air is composed of about 78 percent nitrogen and about 21 percent oxygen. Small amounts of other gases, such as carbon dioxide (CO2), argon (Ar), methane (CH4), nitrous oxide (N2O), and water vapor (H2O), are also present. The concentration of the invisible water vapor varies from place to place and from time to time. Near the ground in warm tropical locations, the concentration of water vapor may reach 4 percent, while in polar areas its concentrations may be only a small fraction of a percent. Clouds are comprised of billions of small droplets of condensed water or tiny ice crystals.

B. Air TemperatureAir molecules are in constant motion. The speed of air molecules corresponds to their kinetic energy, which in turn corresponds to the amount of heat energy in the air. Air temperature is a measure of the average speed at which air molecules are moving; high speeds correspond to higher temperatures. The temperature of a substance is measured by a thermometer.

C. Air Pressure

Air is held to the earth by gravity. This strong invisible force pulls the air downward, giving air molecules weight. The weight of the air molecules exerts a force upon the earth and everything on it. The amount of force exerted on a unit surface area (a surface that is one unit in length and one unit in width) is called atmospheric pressure or air pressure. The air pressure at any level in the atmosphere can be expressed as the total weight of air above a unit surface area at that level in the atmosphere. Higher in the atmosphere, there are fewer air molecules pressing down from above. Consequently, air pressure always decreases with increasing height above the ground. Because air can be compressed, the density of the air (the mass of the air molecules in a given volume) normally is greatest at the ground and decreases at higher altitudes.

A column of air 1 sq cm (.16 sq in) in area, extending from the ocean surface (sea level) up to the top of the atmosphere would contain slightly more than 1 kg (about 2.35 lb) of air. If more air molecules are packed into the column, the total weight of air at the bottom of the column would increase, and the air pressure there would increase. If air is removed from the column, the total weight of the air at the bottom of the column would decrease, and the air pressure would decrease. The most common unit of pressure found on surface weather maps is the millibar (1 millibar equals 100 newtons/sq m, where newtons are the metric unit of force). Inches of mercury is a pressure unit commonly used in television and radio weather broadcasts. On average, at sea level, the standard value of the atmospheric pressure is 1013.25 millibars, 29.92 inches of mercury, and 14.7 lbs/sq in. Barometers are instruments that measure air pressure.

D. WindWind is air in motion. It is caused by horizontal variations in air pressure. The greater the difference in air pressure between any two places at the same altitude, the stronger the wind will be. The wind direction is the direction from which the wind is blowing. A north wind blows from the north and a south wind blows from the south. The prevailing wind is the wind direction most often observed during a given time period. Wind speed is the rate at which the air moves past a stationary object.

A variety of instruments measure wind. A wind vane measures wind direction. Most wind vanes consist of a long arrow with a tail that moves freely on a vertical shaft. The arrow points into the wind and gives the wind direction. Anemometers measure wind speed. Most anemometers consist of three or more cups that spin horizontally on a vertical post. The rate at which the cups rotate is related to the speed of the wind.

E. PrecipitationPrecipitation is any form of water (either liquid or solid) that falls from the atmosphere and reaches the ground, such as rain, snow, or hail. Rain gauges are instruments that measure rainfall. The standard rain gauge consists of a funnel-shaped collector that is attached to a long measuring tube.

F. HumidityHumidity refers to the air’s water vapor content. Hygrometers are instruments that measures humidity. The maximum amount of water vapor that the air can hold depends on the air temperature; warm air is capable of holding more water vapor than cold air. Relative humidity is the ratio of the amount of water vapor in the air compared to the maximum amount of water vapor that the air could hold at that particular temperature. When the air is holding all of the moisture possible at a particular temperature, the air is said to be saturated. Relative humidity and dew-point temperature (the temperature to which air would have to be cooled for saturation to occur) are often obtained with a device called a psychrometer. The most common type of psychrometer is a sling psychrometer. This instrument consists of two thermometers mounted side by side and attached to a handle that allows the thermometers to be whirled. A cloth wick covers one thermometer bulb. The wick-covered thermometer bulb (called the wet bulb) is dipped in water, while the other thermometer bulb (the dry bulb) is kept dry. Whirling both thermometers allows water to evaporate from the wick, which cools the wet bulb. By looking up the dry and wet bulb temperatures in a set of tables, known as humidity tables, it is possible to find the corresponding relative humidity and dew-point temperature.

SPECIAL METEOROLOGICAL INSTRUMENTS

A. Radiosonde

A radiosonde measures air temperature, air pressure, and humidity from the earth’s surface up to an altitude of about 30,000 m (about 100,000 ft). The radiosonde consists of a small box attached to a gas-filled balloon. As the balloon rises, a barometer measures air pressure, a thermometer measures temperature, and a hygrometer measures humidity. All of this information is transmitted by radio back to the ground. Special tracking equipment monitors the movement of the radiosonde, and this tracking information is then converted into wind speed and wind direction. When the balloon bursts, the radiosonde descends to the earth by parachute.

B. Doppler Radar

Radar provides meteorologists with information about precipitation and storms. A radar unit sends out a pulse of microwaves. When the microwaves strike objects, such as falling precipitation, some of the microwaves are reflected back to the radar unit, where they are detected by an antenna and displayed on a screen. The elapsed time between transmission and return indicates how far away the precipitation is. Doppler radar can determine wind speed by measuring the speed at which precipitation is moving horizontally toward or away from the radar antenna. It does this by measuring the change in frequency of the returning microwaves—the frequency of the returning waves decreases if the rain is moving away from the radar unit and increases if the rain is moving toward it. This change in frequency is called the Doppler effect. Meteorologists also use Doppler radar to peer into severe thunderstorms and locate tornadoes. Presently, there is a network of 135 Doppler radar units at selected sites within the continental United States.

C. Weather Satellites

A weather satellite is a cloud-observing platform in space. Satellites provide cloud observations day and night over vast regions. There are two main types of weather satellites: geostationary satellites and polar orbiting satellites. Geostationary satellites orbit the earth at the same rate that the earth spins. Hence, they remain about 36,000 km (about 22,000 mi) above a fixed spot on the equator and constantly monitor a specific region below them. Successive cloud photographs from geostationary satellites provide meteorologists with valuable information about the development, movement, and dissipation of weather fronts, storms, and clouds. Polar orbiting satellites, situated about 850 km (about 530 mi) above the earth’s surface, pass over the North and South poles on each orbit photographing the clouds directly beneath them. Because the earth rotates beneath the satellite, each orbit enables the satellite to monitor an area that is west of its previous pass. Thus, the satellite photographs the entire surface of the earth every 12 hours. Since polar orbiting satellites observe clouds at a much lower altitude than geostationary satellites, they provide more photographic detail of cloud systems.

STRUCTURE OF THE ATMOSPHERE

By studying the atmosphere, meteorologists have discovered that it can be divided into a series of layers. Based on a vertical profile of temperature, the layers consist of the troposphere, stratosphere, mesosphere, and thermosphere.

The lowest layer, the troposphere, is warmed by the earth. Sunlight warms the earth’s surface, and the surface warms the air. Therefore, the warmest air is next to the ground and air temperature normally decreases with height. This pattern of decreasing air temperature with altitude occurs usually up to an altitude of between about 8000 m (about 26,000 ft) at the poles and 16,000 m (about 52,000 ft) at the equator. This region of the lower atmosphere where air temperature normally decreases with height is called the troposphere. The troposphere is kept well stirred by rising and descending air currents.

The layer of atmosphere above the troposphere is called the stratosphere. In the stratosphere, air temperature begins to increase with height, mainly because ozone (a type of oxygen) in the stratosphere absorbs energy from the sun, principally ultraviolet radiation. Although the amount of ozone in the stratosphere is quite small, it is important because it protects living things on the earth by absorbing the sun’s harmful ultraviolet radiation. However, chemicals emitted near the earth, such as chlorofluorocarbons (CFCs), can be injected into the stratosphere by the updrafts in thunderstorms. In the stratosphere these chemicals can help to destroy ozone. Occasionally, concentrations of CFCs in the stratosphere are high enough to destroy nearly all of the ozone over large regions, producing a hole in the ozone layer. In recent years, such holes have occurred every spring over Antarctica. Many regions in the temperate latitudes have occasionally experienced a significant thinning of the ozone layer.

Above the ozone-rich stratosphere lies the mesosphere, where air temperature, again, decreases with height. The mesosphere is the coldest layer of the atmosphere and extends from an altitude of about 50 km to about 85 km (about 30 mi to 50 mi). Above the mesosphere lies the hot thermosphere, where air temperatures can exceed 1000° C (1800° F), primarily due to oxygen absorbing the sun’s energetic rays.

ENERGY FLOW AND GLOBAL CIRCULATION

The average amount of energy the earth absorbs from the sun each year is equal to the average amount of energy the earth loses to space. This energy balance, however, is not maintained for each latitude. Annually, tropical regions gain more energy from the sun than they lose, while polar regions lose more energy to space than they gain. The tropics do not continuously grow warmer and the polar regions do not continuously grow colder, however, because the atmosphere transports warm air toward the poles and cold air toward the equator. The oceans do the same with water. The wind pattern generated by the unequal heating of the earth’s surface produces the major wind belts over the globe. This average wind flow is called the general circulation of the atmosphere.

The average wind flow of the general circulation is much less complex than the actual global circulation at any given instant. Winds tend to develop into rotating eddies, called high and low pressure areas, that move across the middle latitudes and complicate the wind flow pattern.

Models make simplifying assumptions about the earth, its atmosphere, and its winds, and meteorologists have developed a simple model, called the three-cell model, to describe the average wind flow of the general circulation. The three-cell model assumes that the earth is covered with water and that the sun is always over the equator. With these assumptions, the model makes the following predictions, each of which matches the average surface wind patterns observed, but not the winds aloft.

In the tropics, the intense sunlight heats the surface, which warms the air, causing it to rise. This reduces the air pressure at the surface, forming a broad region of low pressure. As the warm, humid air rises, it often condenses into huge thunderstorms that provide the tropics with ample rainfall.

Near the top of the troposphere, the rising air branches and moves toward the North and South poles. As the air aloft moves toward the poles, it gradually cools. At the same time, the air slowly squeezes together and becomes more dense.

Near 30° latitude, the air aloft becomes dense enough to produce high-pressure areas at the surface, called the subtropical highs. As the surface air moves outward from the surface highs, the air aloft sinks to replace it and warms by compression, which tends to evaporate any clouds in it. Cloudless skies and little rainfall characterize the region around 30° latitude. Many of the world’s deserts are found near this latitude.

At the surface, some of the sinking air moves back toward the lower pressure at the equator. This flow of air toward the equator is known as the trade winds. Due to the Coriolis force, a force that results from the rotation of the earth, the trade winds are deflected to the west. In the northern hemisphere, the trade winds blow from the northeast, and in the southern hemisphere, they blow from the southeast. The trade winds complete a thermally driven convection cell that begins with the sun warming the tropics, air rising above the equator, flowing toward the poles, then sinking near 30° latitude and returning to the equator. At the equator, the trade

winds from the northern hemisphere meet the trade winds from the southern hemisphere forming a boundary called the intertropical convergence zone (ITCZ).

Another cell occurs in the polar latitudes. At the poles, cold air sinks into an area of surface high pressure. As the cold surface air flows toward the equator, it meets milder middle latitude air near 50° to 60° latitude. Here, the converging air rises for the return trip to the poles. The rising air also cools and often condenses into clouds. Hence, plentiful rainfall characterizes the region between 50° and 60° latitude. In this region, where the rising air moves toward the poles, regions of surface low pressure often form.

The third cell of the three-cell model occupies the mid-latitudes between the other two cells. Some of the rising air between 50° and 60° latitude begins the journey aloft back toward the equator. At about 30° latitude, this air begins to sink in the vicinity of the subtropical highs. The surface winds tend to blow from the high-pressure region at about 30° latitude toward the low-pressure region between 50° and 60° latitude. The Coriolis force deflects these surface winds, producing the prevailing westerlies of the middle latitudes.

Meanwhile, poleward of the prevailing westerlies, cold, polar air moves toward the equator, and the Coriolis force deflects this air, producing a polar wind belt called the polar easterlies. Near 50° to 60° latitude, the polar easterlies meet the prevailing westerlies. Here the winds are blowing in opposite directions along a boundary called the polar front. It is along the polar front that middle latitude storms often develop.

The three-cell model of the general circulation assumes that the sun is always above the equator. In the real world, the zone of maximum surface heating shifts seasonally. Because the sun is overhead in the northern hemisphere in July and overhead in the southern hemisphere in January, the major surface wind belts and pressure systems shift northward in July and southward in January.

CLOUDS

Clouds are made up of tiny water droplets or ice crystals. Clouds form as water vapor either condenses or freezes onto minute floating particles such as dust or tiny salt particles from the sea. These cloud droplets and ice crystals are so small (with an average diameter of 0.002 cm / 0.001 in) that they stay suspended in the air. Raindrops, which typically have a million times more water in them, are heavy enough to fall from clouds.

A system of identifying clouds was proposed by French botanist and zoologist Jean-Baptiste Lamarck in 1802, and a better system was proposed by English naturalist Luke Howard in 1803. With slight modification, Howard’s system is still in use. Howard’s system uses Latin words to describe clouds as they appear to an observer on the ground. High wispy clouds are called cirrus (from the Latin word for curl of hair); sheetlike clouds are called stratus (from the Latin word for layer); billowing, puffy clouds are called cumulus (from the Latin word for heap); and rain-producing clouds are called nimbus (from the Latin word for rain).

Clouds are divided into four main groups based on their height above the ground: high clouds, middle clouds, low clouds, and clouds with vertical development. High clouds have bases generally above 6000 m (20,000 ft). Because at high altitudes the air is thin and cold, high clouds are thin, and their names often include the prefix cirro (from cirrus). They are almost entirely composed of ice crystals. Middle clouds, which have names with a prefix of alto, (from the Latin word for high) typically have bases between 2000 m and 6000 m (between 6500 ft and 20,000 ft) above the ground. They are usually composed of a mixture of water droplets and ice crystals. Low clouds have bases lying below 2000 m (6500 ft). Low clouds are almost always composed of water droplets. Clouds of vertical development are taller than they are wide. Their bases are below 2000 m (6500 ft) while their tops may extend above the top of the troposphere.

Meteorologists divide these four main groups of clouds into ten principal cloud types. The high cloud group consists of cirrus clouds, which are thin and wispy; cirrostratus clouds, which are thin and sheetlike; and cirrocumulus clouds, which are small, white, and puffy. The middle cloud group consists of altostratus clouds, which are gray and sheetlike; and altocumulus clouds, which are gray and puffy. Low clouds consist of stratus clouds, which are low, gray, and sheetlike; nimbostratus clouds, which are sheetlike and dark gray from which rain or snow is falling; and stratocumulus clouds, which are dark, low, and lumpy. Clouds of vertical development consist of cumulus clouds, which are small and puffy; and the giant cumulonimbus clouds, which are thunderstorm clouds with a top that may extend more than 15,000 m (50,000 ft) above the ground.

PRECIPITATION

Precipitation is any form of water, either solid or liquid, that falls from the atmosphere and reaches the ground. There are several types of precipitation, including rain, snow, sleet, and hail. One of the important triumphs of 20th-century meteorology was discovering how precipitation forms in clouds.

A. Formation of Precipitation

Precipitation begins in a cloud when cloud droplets or ice crystals grow large and heavy enough to fall toward the ground. Cloud droplets may grow bigger as large droplets collide and merge with smaller drops. This process is called coalescence.

Ice crystals grow larger through a process called the ice crystal process, or Bergeron process, after the Swedish meteorologist Tor Bergeron, who proposed that raindrops begin as ice crystals. If the temperatures inside a cloud are below freezing, then liquid cloud droplets and ice crystals may coexist. Liquid water droplets existing at below freezing temperatures are called supercooled droplets (see Supercooling). If supercooled droplets and ice crystals are close together, then water vapor may leave the liquid droplets and freeze onto the ice crystals. In this manner, the ice crystals grow larger at the expense of the surrounding supercooled droplets. As ice crystals grow larger by the Bergeron process, they may become heavy enough to fall. Falling ice crystals may collide and stick to other ice crystals, forming a snowflake. Ice crystals may also collide with supercooled cloud droplets, changing the liquid droplets into ice on contact. These ice particles may even stick together producing a chunk of icy matter called graupel.

B. Types of precipitation

Precipitation can take several different forms. Rain is falling drops of liquid water with diameters that are 0.5 mm (0.02 in) or greater. Drizzle is falling drops of water smaller than rain. Some raindrops are cloud droplets that grew by coalescence and fell. However, the majority of raindrops that fall over the middle and higher latitudes begin as snowflakes or graupel. As they fall, they enter warmer layers of air and melt, forming raindrops. If the falling rain evaporates before reaching the ground, it forms streaks in the sky called virga. In the cold air of winter, falling snowflakes and graupel may reach the ground without melting and accumulate as snow. Graupel that reaches the ground is called snow pellets. If rain falls into a deep, subfreezing layer of air near the ground, some of the rain may freeze into tiny ice pellets called sleet. When rain falls into a shallow, subfreezing layer of air near the ground, it may remain as a supercooled liquid and freeze upon striking a cold surface, forming freezing rain. Freezing rain can coat everything with glistening ice, the weight of which can break tree branches and snap power lines.

Hail is the largest form of precipitation, varying in size from peas to golf balls or larger. Hail forms as graupel grows in size by colliding with and sticking to supercooled liquid droplets, all while suspended in violent updrafts in a thunderstorm. When the ice particles become large and heavy enough to overcome the updrafts, they begin to fall as hailstones. Hail damage in the United States alone amounts to hundreds of millions of dollars annually.

Dew and frost are not actually forms of precipitation because they do not fall from the atmosphere. Dew consists of tiny beads of water that form as water vapor condenses onto surfaces near the ground (such as blades of grass) when the surface’s temperature drops to below the air’s dew-point temperature. When the dew-point temperature is below freezing, water vapor changes directly into ice without becoming a liquid first. The white, delicate ice crystals that form in this manner are called frost.

LARGE-SCALE (SYNOPTIC) PHENOMENA

Large-scale, or synoptic, weather phenomena are weather patterns that persist for more than a day and cover thousands of square kilometers. Air masses, large bodies of air of uniform temperature and humidity, may provide several days or weeks of persistent weather. Middle latitude cyclones and tropical cyclones are huge traveling storm systems that cover large areas and may persist for a week or more.

A. Air MassesAn air mass is a body of air that extends over a large area and has nearly uniform temperature and humidity in any horizontal direction. Places where air masses form are called source regions, and they are generally flat with light winds. Ideal source regions are those dominated by large high-pressure areas, such as the arctic plains in winter and the subtropical oceans and desert regions in summer.

Air masses are classified according to their source region. Polar air masses originate over cold regions. Tropical air masses originate over the warm tropics. Continental air masses originate over land, and maritime air masses originate over water. Consequently, a cold, dry air mass that forms over land is called a continental polar air mass and a hot, moist air mass that forms over water is called a maritime tropical air mass.

Generally, the upper-level winds move air masses from one region to another. For example, arctic air masses that form over northern Canada move into the United States when strong upper-level winds, called jet streams, direct these frigid masses of air southward.

B. FrontsA front is a boundary where air masses with sharply contrasting temperature and humidity meet. Many kinds of storms occur along fronts.

A cold front marks the boundary where cold air is replacing warm air. On a weather map, cold fronts are drawn as a solid blue line with triangles. The triangles point in the direction of movement. Typically, warmer, more humid air is found in advance of a cold front, while colder, drier air is behind it. Along the front, the warm, humid air often rises and condenses into towering cumulus clouds that may develop into thunderstorms. A narrow band of heavy precipitation, often in the form of rain showers, usually accompanies the front. As a cold front approaches, atmospheric pressure normally drops. As the cold front moves on by, atmospheric pressure rises and the winds shift direction. The passage of the front is often accompanied by the heaviest

precipitation and the strongest and gustiest winds. Occasionally, however, a line of thunderstorms may develop, out ahead of a cold front. This line is called a squall line and it produces heavy rain and strong, gusty winds.

A warm front marks the region where warm air is replacing cold air. On a weather map, warm fronts are drawn as a solid red line with half circles. The half circles point in the direction of movement. An average warm front has a more gentle slope than that of a typical cold front. As a warm front advances, warmer air glides up and over the colder, denser surface air. This process, called overrunning, produces widespread cloudiness and precipitation well in advance of the front’s surface position.

Warm fronts are best developed in winter. As a typical warm front approaches, the atmospheric pressure drops and high, wispy cirrus clouds form 12 to 24 hours ahead of the front. These clouds give way to thicker and lower clouds (cirrostratus and altostratus). As the warm front moves closer, cloud level descends and steady rain, snow, sleet, or freezing rain may fall from nimbostratus clouds into the cold air ahead of the front. Just before the front passes, there may be low stratus clouds and fog. As the warm front passes, the air temperature and humidity rise, the atmospheric pressure stops falling, the winds shift, the rain ends, and the fog dissipates. However, these weather changes are less noticeable than those of a typical cold front.

Cold fronts usually move faster than warm fronts. Consequently, when a cold front overtakes a warm front, a new front, called an occluded front, forms. Occluded fronts appear on weather maps as a solid purple line with alternating triangles and half circles, both pointing in the direction toward which the front is moving. Generally, the air behind an occluded front is colder than the air ahead of it. The weather and clouds preceding an occluded front are often similar to that of a warm front.

A stationary front is a cold front or warm front that shows little or no movement. On a weather map, stationary fronts are represented as alternating red and blue lines with half circles pointing toward the colder air and triangles pointing toward the warmer air.

C. Middle Latitude Cyclones

Middle latitude cyclones are huge low-pressure storm systems that consist of a cold front and a warm front, and, usually for part of their lifecycle, an occluded front as well. Middle latitude cyclones usually develop along a slow-moving or stationary front. Such fronts are common at the boundary between the midlatitude cell and the polar cell of the three-cell model. The boundary is a trough of low pressure, with (in the northern hemisphere) warm air to the south and cold air to the north. When a jet stream moves over a stationary front, the front may bend, as a cold front pushes southward and, to its east, a warm front moves northward. The junction of the two fronts is the center of the developing storm and has the lowest atmospheric pressure.

Winds at the ground (in the northern hemisphere) blow counterclockwise and inward around the area of low pressure. As the surface winds converge toward the center of the storm, the air gradually rises, often condensing into clouds. The heat released during condensation supplies some of the energy for the storm’s development (see latent heat). Additional energy is derived as the air masses struggle to obtain equilibrium. Warm air rises along the warm front and cold air sinks behind the cold front. The rising and sinking air transforms potential energy into kinetic energy (energy of motion).

The storm’s development and movement depend upon the winds aloft. Strong winds above the storm quickly sweep the rising air downwind. If the winds aloft remove the air above the storm more quickly than the surface air converges, the surface pressure drops and the storm system intensifies. Conversely, if the converging surface air is greater than the removal of air aloft, the surface pressure rises and the storm system weakens. Because the winds above the surface storm typically blow from the southwest (in the northern hemisphere), the center of the surface low normally moves northeastward.

As the storm system moves northeastward, the faster-moving cold front catches up to the slower-moving warm front. Eventually the cold front overtakes the warm front and the storm system becomes occluded. With cold surface air on both sides of the occluded front, warm air is no longer rising and the cold air is no longer sinking. The storm is now without its primary source of energy (the conversion of potential energy into kinetic energy during the forceful lifting of warm air) and the storm system dies out and dissipates.

D. Tropical Cyclone

Tropical cyclones, also known as hurricanes and typhoons, are storms with sustained winds in excess of 120 km/h (74 mph). They form over warm, tropical waters between 5° and 20° latitude, where the winds are light and the humidity is high.

Tropical cyclones form over water when a mass of thunderstorms becomes organized and spirals in toward the storm’s center, or eye. The storm’s highest winds, strongest thunderstorms, and heaviest rain, occur just outside the eye in the region called the eye wall. In the eye itself, winds are usually light and skies are partly cloudy.

Tropical cyclones tend to form along a weak area of low pressure, called a disturbance or wave, in the intertropical convergence zone (ITCZ). As the disturbance becomes more organized, it first becomes a tropical depression, then a tropical storm, and finally a tropical cyclone. For the tropical cyclone to intensify, the outflow of air above the storm must exceed the inflow of air at the bottom. Tropical cyclones derive their energy from the transfer of heat from the warm water and from the latent heat given up to the system during condensation. Tropical cyclones dissipate when they are cut off from their energy sources either by moving over cold water or a large land mass. Although tropical cyclones often take erratic paths, the prevailing easterly winds in the tropics tend to steer tropical cyclones westward or northwestward until they leave the tropics, then the prevailing westerlies tend to sweep them northeastward.

WEATHER PREDICTION

Weather forecasting entails predicting how the present state of the atmosphere will change. Present weather conditions are obtained by ground observations, observations from ships and aircraft, radiosondes, Doppler radar, and satellites. This information is sent to meteorological centers where the data are collected, analyzed, and made into a variety of charts, maps, and graphs. These charts, maps, and graphs are then sent electronically to forecast offices where local and regional weather forecasts are made. In addition, these offices prepare weather advisories and warnings of impending severe weather.

Modern high-speed computers transfer the many thousands of observations onto surface and upper-air maps. Computers draw the lines on the maps with help from meteorologists, who correct for any errors. A final map is called an analysis. Computers not only draw the maps but predict how the maps will look sometime in the future. The forecasting of weather by computer is known as numerical weather prediction.

To predict the weather by numerical means, meteorologists have developed atmospheric models that approximate the atmosphere by using mathematical equations to describe how atmospheric temperature, pressure, and moisture will change over time. The equations are programmed into a computer and data on the present atmospheric conditions are fed into the computer. The computer solves the equations to determine how the different atmospheric variables will change over the next few minutes. The computer repeats this procedure again and again using the output from one cycle as the input for the next cycle. For some desired time in the future (12, 24, 36, 48, 72 or 120 hours), the computer prints its calculated information. It then analyzes the data, drawing the lines for the projected position of the various pressure systems. The final computer-drawn forecast chart is called a prognostic chart, or prog.

A forecaster uses the progs as a guide to predicting the weather. There are many atmospheric models that represent the atmosphere, with each one interpreting the atmosphere in a slightly different way. The forecaster learns the idiosyncrasies of each model and places more emphasis on the ones that do the best job of predicting a particular aspect of the weather.

Weather forecasts made for 12 and 24 hours are typically quite accurate. Forecasts made for two and three days are usually good. Beyond about five days, forecast accuracy falls off rapidly.

WEATHER MODIFICATION

A. Cloud SeedingOne method of weather modification is to seed clouds with tiny particles to try to coax more precipitation from them. There are two primary ways to seed clouds. The first method uses the coalescence process of rain formation. Small water drops or other particles are injected into the base of a cloud. As updrafts carry these particles up through the cloud, the particles grow in size by colliding and merging (coalescing) with drops in their path. Eventually, the drops grow large and heavy enough to fall. On their way down, the drops continue to grow in size and may even fragment into many new drops.

The second method of seeding clouds employs the ice-crystal (Bergeron) process of rain formation. Small particles of silver iodide (AgI) are injected into a cloud that contains both ice crystals and water droplets at below freezing temperatures. Inside

the cloud, the silver iodide particles act like ice crystals. Water vapor from the surrounding liquid droplets evaporates and freezes onto the silver iodide particles, which grow larger at the expense of the surrounding liquid droplets. The growing crystals eventually become heavy enough to fall as precipitation.

The effectiveness of these methods of cloud seeding is disputed because it is difficult to determine how much precipitation would have fallen had the cloud not been seeded. Some studies indicate that seeding under optimum conditions will enhance precipitation by as much as 15 percent. On the other hand, some attempts to seed clouds have reduced the amount of precipitation. It is thought that in some cases the clouds were overseeded, which produced so many small ice particles that there was not enough water droplets and water vapor left to allow the ice crystals to grow large enough to fall. (See Cloud Seeding)

B. Hail SuppressionHail forms in thunderstorms when supercooled liquid droplets accumulate on small clumps of graupel. In an attempt to reduce the destructiveness of hail, large quantities of silver iodide are injected into the thunderstorm. The idea is to overseed the cloud so that many smaller hailstones form, preventing them from growing into large destructive hailstones. Results of hail-suppression experiments have been inconclusive.

C. Fog DispersalFog is a cloud on the ground. Fog-clearing operations have mainly been attempted at airports to improve runway visibility. An early attempt at fog dispersal burned large quantities of fuel oil along runways, so that the air would warm enough to evaporate the fog. This expensive technique proved to be ineffective and very smoky. Another method employs helicopters that hover above the fog layer. The turbulence created by the blades mixes the drier, warmer air above the fog with the cooler, saturated air below. The mixing of the drier air into the fog evaporates the fog. This method works well when the fog is shallow, winds are light, and the air temperature is above freezing.

Fog has also been seeded in an attempt to dissipate it. The seeding usually involves salt particles or dry ice (frozen carbon dioxide). Tiny salt particles cause the fog droplets to grow in size and fall out as drizzle. Dry ice only works in fog at below freezing temperatures. As small pieces of cold dry ice descend, they freeze the liquid fog droplets into ice crystals. The ice crystals grow in size and fall to the ground. The remaining fog droplets evaporate, leaving a clear area in the fog for aircraft operations. No matter how successful the fog-clearing operation, it must be applied continuously or the fog will reform as it moves in from the surrounding area.

D. Hurricane ModificationHurricanes have been seeded with silver iodide in an attempt to reduce their destructive winds. During the 1960s, project STORMFURY, a joint effort of the National Oceanic and Atmospheric Administration (NOAA) and the United States Navy, seeded several hurricanes just outside their centers so that clouds would develop farther away from the main area of severe thunderstorms and high winds. The hope was that this would reduce the air pressure locally and thereby reduce the hurricane’s winds. Although some of the results were encouraging, some uncertainty remains as to the effectiveness of seeding hurricanes.

HUMAN INDUCED GLOBAL WARMING

In 1988, the United Nations Environment Program and the World Meteorological Organization established the Intergovernmental Panel on Climate Change (IPCC) to assess the environmental, social, economic, and scientific information available on climate change. The IPCC consists of more than 200 leading earth scientists. Their Second Assessment Report, published in 1995, concluded that the earth’s average surface air temperature has increased by between 0.3 and 0.6 Celsius degrees (between 0.5 and 1.1 Fahrenheit degrees) in the past 100 years. Their report states that this warming should continue and that global average surface temperature will increase by between 1.0 and 3.5 Celsius degrees (between 1.8 and 6.3 Fahrenheit degrees) by the year 2100 (see Global Warming). If such a warming should occur, sea level should rise by between 15 cm and 95 cm (6 in and 37 in) by the year 2100, with the most likely rise being 50 cm (20 in). Such a rise in sea level might have a damaging effect on coastal ecosystems. Other changes brought on by this warming might include a shift in the world’s wind and rainfall patterns, which might put added stress on important agricultural areas, especially those in the western United States that depend on irrigation water from reservoirs and streams.

Many climate scientists believe that human activity is responsible for global warming. They attribute the main cause of global warming to the burning of fossil fuels, which increases the concentration of carbon dioxide (CO2) gas in the atmosphere. Carbon dioxide levels, presently about 360 parts per million (ppm), have increased 28 percent in the past century. The IPCC estimates that the concentration of CO2 in the atmosphere will surpass 500 ppm, an increase of another 40 percent, before the end of the 21st century.

Carbon dioxide warms the atmosphere through a process known as the atmospheric greenhouse effect. The atmospheric greenhouse effect is caused by certain gases in our atmosphere, called greenhouse gases, selectively absorbing and emitting infrared radiation, or heat energy. The two most plentiful greenhouse gases are water vapor (H2O) and carbon dioxide (CO2). Other less plentiful (and hence less important) greenhouse gases include nitrous oxide (N2O), methane (CH4), and chlorofluorocarbons (CFCs).

A greenhouse gas is like a filter; it allows the shorter wavelengths of radiant energy (such as visible light) to pass through it, but it absorbs some of the longer wavelengths of radiant energy (such as infrared radiation). Visible sunlight readily passes through the greenhouse gases to reach the earth’s surface, where it warms the surface. The earth’s surface, which is much cooler than the sun, emits radiant energy in the form of longer infrared waves. The greenhouse gases absorb some of these infrared waves emitted by the earth’s surface. When greenhouse gases absorb infrared energy, they share this energy with other gases and the atmosphere

warms. The greenhouse gases also emit infrared radiation. Some of the emitted radiation travels back to the earth’s surface, where it warms the earth again. By preventing the rapid escape of infrared energy to space, greenhouse gases act as an insulating layer around the earth, keeping its surface much warmer than it would be if these gases were not present.

The atmospheric greenhouse effect is a natural effect that has been occurring for billions of years. Indeed, without it, the earth would be a frozen planet with an average temperature of about -18° C (about 0° F). Due to the greenhouse effect, the earth’s average surface temperature is a comfortable 15° C (about 59° F).

It is not the greenhouse effect that concerns scientists, but the enhancement of the greenhouse effect by human induced increases in the levels of greenhouse gases. Climate models predict that the world’s average surface temperature should rise by between 1 and 3.5 Celsius degrees (1.8 and 6.3 Fahrenheit degrees) by the year 2100. However, these models show that increasing the concentration of carbon dioxide to 500 ppm and keeping everything else constant only accounts for a global warming of less than 1 Celsius degree (1.8 Fahrenheit degrees). This slight warming, however, would increase the air’s capacity for holding water vapor. The added water vapor, the most plentiful greenhouse gas, would enhance the atmospheric greenhouse effect by producing a positive feedback on the climate system. A positive feedback occurs when an initial change is reinforced by another process. In this situation, the increase in temperature causes an increase in water vapor, which absorbs more of the earth’s infrared energy, thus accounting for the rest of the warming.

The interactions between the earth and its atmosphere are complex. There are many uncertainties in the climate system, especially with regard to clouds (which tend to cool the earth by reflecting sunlight) and the oceans (which act as a huge storehouse of heat energy). It is difficult to prove that increasing concentrations of greenhouse gases are responsible for the recent global warming. Most climate scientists contend, however, that at least part of the warming is due to human induced greenhouse gases.

ATMOSPHERIC OPTICS

Atmospheric optics is the study of how light interacts with the atmosphere and objects in it. It explains, for example, why a mirage occurs, how a rainbow forms, why sunsets are red, and why the sky is blue.

A. MiragesA mirage occurs when an object appears displaced from its true position. Atmospheric mirages are created when light is bent, or refracted, as it travels through layers of air with differing densities (see Optics). Changes in air density are usually caused by changes in air temperature. If the air near the ground is much warmer than the air above, light from the sky will bend up into an observer’s eyes so that an observer looking down at the distant ground sees light from the sky. The image of sky where the distant ground should be produces the mirage of a watery pavement, or water resting on hot desert sand. When the light from an object is bent, making the object appear higher than it actually is, a superior mirage occurs. When an object appears lower than it actually is, the mirage is called an inferior mirage.

B. RainbowsA rainbow is an arc of concentric colored bands that spans a section of the sky. For a rainbow to form, rain must be falling in one part of the sky and the sun must be shining from behind the observer. Rainbows form when sunlight enters a raindrop and the various wavelengths of visible light, representing the different colors, begin to slow and bend. Violet light bends the most and red light bends the least. Most of the light passes through the raindrop. But the refracted light that hits the back of the drop at a certain angle (called the critical angle) is reflected off the back of the drop. The light is then refracted, or bent, a second time as it emerges from the drop. Because each color bends differently, each color emerges from the drop at a slightly different angle, producing a spectrum of colors. Because only a single color from each drop reaches an observer, it takes many raindrops, each one reflecting light back to an observer at slightly different angles, to produce the colors of a primary rainbow.

Fainter, secondary rainbows often form above the primary rainbow. Secondary rainbows form when sunlight enters a raindrop at such an angle that two reflections occur inside the raindrop. The second reflection weakens the light intensity and causes a reversal of colors. The weakened light that emerges produces a dimmer rainbow.

HISTORY OF METEOROLOGY

The scholars of ancient Greece were interested in the atmosphere and its related phenomena. About 340 BC Greek philosopher Aristotle wrote Meteorologica, a treatise on natural philosophy. His works, although speculative, represented the sum of knowledge about the natural science, including weather and climate. At that time, anything that fell from the sky (including rain and snow) and anything that was in the sky (including clouds) were called meteors, from the Greek word meteoros, meaning “high in the sky.” From meteoros comes the term meteorology. Several years later, Theophrastus, a pupil of Aristotle, compiled a book on weather forecasting, called the Book of Signs. His work consisted of ways to foretell the weather by noticing various weather-related indicators, such as a ring around the moon, which is often followed by rain. The work of Aristotle and Theophrastus remained a dominant influence in the study of weather and in weather forecasting for nearly 2000 years.

Although weather records were kept for different locations as early as the 14th century, meteorology did not become a genuine natural science until the invention of weather instruments. These instruments gave scientists data, so that the physical laws could be tested. Italian physicist and astronomer Galileo invented a crude thermometer in the late 1500s. Italian mathematician and physicist Evangelista Torricelli, a student of Galileo, invented the barometer in 1643. A few years later, French mathematician-philosophers Blaise Pascal and René Descartes, using a barometer, demonstrated that atmospheric pressure decreases with increasing altitude. In 1667 Robert Hooke, an English scientist, invented an anemometer for measuring wind speed. In 1714 German physicist Gabriel Daniel Fahrenheit worked on the boiling and freezing of water, and from that work he developed a temperature scale. In 1780 Horace de Saussure, a Swiss geologist and meteorologist, invented the hair hygrometer for measuring humidity.

The science of meteorology benefited from advances in other sciences, technology, and mathematics. In 1660 Irish-born English scientist Robert Boyle discovered the relationship between pressure and volume of a gas. English meteorologist George Hadley, in 1735, used physics and mathematics to explain how the earth’s rotation influences the trade winds in the tropics. By flying a kite in a thunderstorm in 1752, American statesman and scientist Benjamin Franklin demonstrated the electrical nature of lightning. French chemist Jacques Charles, in 1787, discovered the relationship between temperature and volume in a gas. In 1835 French physicist Gaspard de Coriolis mathematically demonstrated the effect that the earth’s rotation has on atmospheric motions.

The first system of classifying clouds was formulated by French botanist and zoologist Jean-Baptiste Lamarck in 1802. In 1803 Luke Howard, an English naturalist, devised a better system of classifying clouds. In 1806 British Admiral and hydrographer Francis Beaufort invented a wind scale for mariners. Enough weather information was available in 1821 that William Redfield, an American saddle maker and amateur meteorologist, was able to draw a crude weather map. By the 1840s ideas about winds and storms were partially understood. Meteorology got a giant boost in 1843 with the invention of the telegraph. Weather observations and information could now be rapidly disseminated.

A significant milestone in meteorology took place about 1920 when a group of Norwegian scientists, led by Vilhelm Bjerknes, and including Tor Bergeron, developed a model explaining the life cycle of a middle latitude storm system. These ideas were expanded as upper air observations became available from aircraft and radiosondes. By 1940 upper-level measurements of temperature, pressure, humidity, and wind gave atmospheric scientists a three-dimensional view of the atmosphere.

Weather radar became available to scientists during the early 1940s. At the same time, high-flying military aircraft discovered the existence of jet streams—swiftly flowing air currents that girdle the earth. In 1946 American chemist and Nobel laureate Irving Langmuir and American atmospheric physicist Vincent Schaefer found that tiny pellets of dry ice could induce supercooled liquid

water droplets to crystallize. During the same year, Bernard Vonnegut, an American chemist, discovered that silver iodide crystals could cause these same water droplets to freeze. These events ushered in an active period of cloud seeding.

The atmospheric sciences advanced again in the 1950s when high-speed computers were able to solve the mathematical equations that describe the behavior of the atmosphere. Today, computers not only plot the observations and draw the lines on surface and upper-level maps, but they also predict the state of the atmosphere up to five days into the future.

In 1960 the National Aeronautics and Space Administration (NASA) launched Tiros 1, the first weather satellite. Subsequent satellites have been more sophisticated and have been capable of monitoring more aspects of the atmosphere. In the mid-1990s, the National Weather Service upgraded its conventional radar with a network of 135 Doppler radar units that are capable of peering into severe thunderstorms, unveiling hail, tornadoes, and strong winds.

Major Air PollutantsSources of major air pollutants include individual actions, such as driving a car, and industrial activities, such as manufacturing products or generating electricity. Note: 1 cubic meter (1m3) is equal to 35.3 cu ft; 1 milligram (1 mg) is equal to 0.00004 oz; 1 microgram (1µg) is equal to 0.00000004 oz.

Troposphere is the lowest layer of the earth's atmosphere and site of all weather on the earth. The troposphere is bounded on the top by a layer of air called the tropopause, which separates the troposphere from the stratosphere, and on the bottom by the surface of the earth. The troposphere is wider at the equator (16 km/10 mi) than at the poles (8 km/5 mi).

The temperature of the troposphere is warmest in the tropical (latitude 0º to about 30º north and south) and subtropical (latitude about 30º to about 40º north and south) climatic zones (see climate) and coldest at the polar climatic zones (latitude about 70º to 90º north and south). Observations from weather balloons have shown that temperature decreases with height at an average of 6.5º C per 1000 m (3.6º F per 1000 ft), reaching about -80º C (about -110º F) above the tropical regions and about -50º C (about -60º F) above the polar regions.

The troposphere contains 75 percent of the atmosphere's mass—on an average day the weight of the molecules in air (see Pressure) is 1.03 kg/sq cm (14.7 lb/sq in)—and most of the atmosphere's water vapor. Water vapor concentration varies from trace amounts in polar regions to nearly 4 percent in the tropics. The most prevalent gases are nitrogen (78 percent) and oxygen (21 percent), with the remaining 1 percent consisting of argon (0.9 percent) and traces of hydrogen, ozone (a form of oxygen), methane, and other constituents. Carbon dioxide is present in small amounts, but its concentration has nearly doubled since 1900. Like water vapor, carbon dioxide is a greenhouse gas (see Greenhouse Effect), which traps some of the earth's heat close to the surface and prevents its release into space. Scientists fear that the increasing amounts of carbon dioxide could raise the earth's surface temperature during the next century, bringing significant changes to worldwide weather patterns. Such changes may include a shift in climatic zones and the melting of the polar ice caps, which could raise the level of the world's oceans.

The uneven heating of the regions of the troposphere by the sun (the sun warms the air at the equator more than the air at the poles) causes convection currents (see Heat Transfer), large-scale patterns of winds that move heat and moisture around the globe. In the Northern and Southern hemispheres, air rises along the equator and subpolar (latitude about 50º to about 70º north and

south) climatic regions and sinks in the polar and subtropical regions. Air is deflected by the earth's rotation as it moves between the poles and equator, creating belts of surface winds moving from east to west (easterly winds) in tropical and polar regions, and winds moving from west to east (westerly winds) in the middle latitudes. This global circulation is disrupted by the circular wind patterns of migrating high and low air pressure areas, plus locally abrupt changes in wind speed and direction known as turbulence.

A common feature of the troposphere of densely populated areas is smog, which restricts visibility and is irritating to the eyes and throat. Smog is produced when pollutants accumulate close to the surface beneath an inversion layer (a layer of air in which the usual rule that temperature of air decreases with altitude does not apply), and undergo a series of chemical reactions in the presence of sunlight. Inversions suppress convection, or the normal expansion and rise of warm air, and prevent pollutants from escaping into the upper atmosphere. Convection is the mechanism responsible for the vertical transport of heat in the troposphere while horizontal heat transfer is accomplished through advection.

The exchange and movement of water between the earth and atmosphere is called the water cycle. The cycle, which occurs in the troposphere, begins as the sun evaporates large amounts of water from the earth's surface and the moisture is transported to other regions by the wind. As air rises, expands, and cools, water vapor condenses and clouds develop. Clouds cover large portions of the earth at any given time and vary from fair-weather cirrus to towering cumulus clouds (see Cloud). When liquid or solid water particles grow large enough in size, they fall toward the earth as precipitation. The type of precipitation that reaches the ground, be it rain, snow, sleet, or freezing rain, depends upon the temperature of the air through which it falls.

As sunlight enters the atmosphere, a portion is immediately reflected back to space, but the rest penetrates the atmosphere and is absorbed by the earth's surface. This energy is then reemitted by the earth back into the atmosphere as long-wave radiation. Carbon dioxide and water molecules absorb this energy and emit much of it back toward the earth again. This delicate exchange of energy between the earth's surface and atmosphere keeps the average global temperature from changing drastically from year to year.

Stratosphere, upper layer of the atmosphere commencing at an altitude of 8 to 16 km (5 to 10 mi) and extending upward to about 50 km (30 mi). In the lower portion of the stratosphere, the temperature remains nearly constant with height, but in the upper portion the temperature increases rapidly with height because of absorption of sunlight by ozone. The stratosphere is almost completely free of clouds or other forms of weather.

Ozone Layer, a region of the atmosphere from 19 to 48 km (12 to 30 mi) above the earth's surface. Ozone concentrations of up to 10 parts per million occur in the ozone layer. The ozone forms there by the action of sunlight on oxygen. This action has been taking place for many millions of years, but naturally occurring nitrogen compounds in the atmosphere apparently have kept the ozone concentration at a fairly stable level. Concentrations this great at ground level are dangerous to breathe and can damage the lungs. However, because the ozone layer of the atmosphere protects life on earth from the full force of the sun's cancer-causing ultraviolet radiation, it is critically important. Thus, scientists were concerned when they discovered in the 1970s that chemicals called chlorofluorocarbons, or CFCs (see Fluorine)—long used as refrigerants and as aerosol spray propellants—posed a possible threat to the ozone layer. Released into the atmosphere, these chlorine-containing chemicals rise and are broken down by sunlight, whereupon the chlorine reacts with and destroys ozone molecules—up to 100,000 per CFC molecule. For this reason, the use of CFCs in aerosols has been banned in the United States and elsewhere. Other chemicals, such as bromine halocarbons, as well as nitrous oxides from fertilizers, may also attack the ozone layer. Destruction of the ozone layer is predicted to cause increases in skin cancer and cataracts, damage to certain crops and to plankton and the marine food web, and an increase in carbon dioxide (see Global Warming) due to the decrease in plants and plankton.

Beginning in the early 1980s, research scientists working in Antarctica have detected a periodic loss of ozone in the atmosphere high above that continent. The so-called ozone “hole,” a thinned region of the ozone layer, develops in the Antarctic spring and continues for several months before thickening again. Studies conducted with high-altitude balloons and weather satellites indicated that the overall percentage of ozone in the Antarctic ozone layer is actually declining. Flights over the Arctic regions found a similar problem developing there.

In 1987 the Montréal Protocol, a treaty for the protection of the ozone layer, was signed and later ratified by 36 nations, including the United States. A total ban on the use of CFCs during the 1990s was proposed by the European Community (now called the European Union) in 1989, a move endorsed by U.S. President George Bush. In December 1995 over 100 nations agreed to phase out developed countries' production of the pesticide methyl bromide, predicted to cause about 15 percent of ozone depletion by the year 2000. Production of CFCs in developed countries ceased at the end of 1995 and will be phased out in developing countries by 2010. Hydrochlorofluorocarbons, or HCFCs, which cause less damage to the ozone layer than CFCs do, are being used as substitutes for CFCs on an interim basis, until 2020 in developed countries and until 2016 in developing countries. To monitor ozone depletion on a global level, in 1991 the National Aeronautics and Space Administration (NASA) launched the 7-ton Upper Atmosphere Research Satellite. Orbiting earth at an altitude of 600 km (372 mi), the spacecraft measures ozone variations at different altitudes and is providing the first complete picture of upper atmosphere chemistry.

The World Meteorological Organization observed a 45 percent depletion of the ozone layer over one-third of the northern hemisphere, from Greenland to western Siberia, for several days during the winter of 1995-1996. The deficiency was believed to have been caused by chlorine and bromine compounds combined with polar stratospheric clouds formed under unusually low temperatures.

Mesosphere, the layer of the Earth’s atmosphere in which temperature decreases rapidly, located between the stratosphere and thermosphere.

Results of the first year of operation of the Meteorological Rocket Network provided the first significant amount of data from the atmosphere between 30 and 80 km (19-50 mi.). This layer, sometimes included within the range of elevations defining the stratosphere, is more frequently referred to as the mesosphere. It is characterized by an increase in temperature from -44° C. at 32 km to -2° C. at 47 km; by a region of constant temperature between 47 and 52 km; and on upward by a temperature decrease to - 92° C. at the top of the layer. These are average values with will be proposed as an extension to the standard atmosphere values of the Manual of the International Civil Aviation Organization. This typical thermal structure is believed to be the result of radiative equilibrium established in the ozone, a gas which absorbs strongly in the ultraviolet portion of the solar spectrum. However, at high latitudes during the winter the mesosphere at 80 km shows a pronounced warming, as much as 50 or 60° C. more than the summer months, probably caused by widespread sinking of the air in this upper region. Consequently, the seasonal change of density in the mesosphere at high latitudes is quite large. The winds of this layer undergo a marked seasonal variation at all latitudes, blowing from the east in the summer and from the west in the winter months. The maximum wind speed occurs at about 50 km, near the level of maximum temperature variously referred to as the stratopause or mesopause.

Thermosphere, the region of the atmosphere above the mesosphere in which temperature steadily increases with height, beginning at about 85 km/53 mi above the Earth’s surface.

Ionosphere, name given to a layer or layers of ionized air in the atmosphere extending from almost 60 km (about 50 mi) above the surface of the earth to altitudes of 1000 km (600 mi) and more. At these altitudes the air is extremely thin, having about the density of the gas in a vacuum tube, and when the atmospheric particles are ionized by ultraviolet radiation from the sun or by other radiation, they tend to remain ionized, because few collisions occur between ions. See Ionization.

The ionosphere exerts a great influence on the propagation of radio signals. Energy that is radiated from a transmitter upward toward the ionosphere is in part absorbed by the ionized air and in part refracted, or bent downward again, toward the surface of the earth. The bending effect makes possible the reception of radio signals at distances much greater than would be possible for waves that traveled along the surface of the earth. Such refracted waves, however, reach the earth only at certain definite distances from the transmitter; the distance depends on the angle of refraction and the altitude. Hence, a radio signal may be inaudible at 100 km (60 mi) from the transmitter but audible at 500 km (300 mi). This phenomenon is known as skip. In certain other areas the ground-wave signals and the refracted signals from the ionosphere may reach the receiver and interfere with each other, producing the phenomenon known as fading. The amount of refraction in the ionosphere decreases with an increase in frequency and for very high frequencies is almost nonexistent. Therefore long-distance transmission of high-frequency radio waves is limited to the line of sight. Both television and frequency-modulation (FM) radio use high-frequency waves. Long-distance transmission can be achieved only in a direct line, such as between the earth and a communications satellite; the signal then may be relayed from the satellite to a distant point on the earth.

The ionosphere is usually divided into two main layers: a lower layer, designated the E layer (sometimes called the Heaviside layer or Kennelly-Heaviside layer), which is between about 80 and 113 km (about 50 and 70 mi) above the earth's surface and which reflects radio waves of low frequency; and a higher layer, the F layer (sometimes called the Appleton layer), which reflects higher-frequency radio waves. The latter is further divided into an F1 layer, which begins at about 180 km (about 112 mi) above the earth; and an F2

layer, which begins at about 300 km (about 186 mi) from the surface. The F layer rises during the night and therefore changes its reflecting characteristics.

At higher elevations, in the ionosphere, the air density has been determined by computing the effect of atmospheric drag as revealed in changes in the period of revolution of satellites. Above 200 km (105 mi.) the density shows large daily variations, caused by solar heating and an upward expansion of the atmosphere during the day, as well as oscillations of about 27 days which are associated with changes in the 20-cm. solar flux, reflecting the sun's period of rotation. A long-period oscillation of density at these levels, associated with the 11-year sunspot cycle, is also indicated.

The region beyond the thermosphere is called the exosphere, which extends to about 9,600 km (about 6,000 mi), the outer limit of the atmosphere.

Atmosphere, mixture of gases surrounding any celestial object that has a gravitational field strong enough to prevent the gases from escaping; especially the gaseous envelope of Earth. The principal constituents of the atmosphere of Earth are nitrogen (78 percent) and oxygen (21 percent). The atmospheric gases in the remaining 1 percent are argon (0.9 percent), carbon dioxide (0.03 percent), varying amounts of water vapor, and trace amounts of hydrogen, ozone, methane, carbon monoxide, helium, neon, krypton, and xenon.

The mixture of gases in the air today has had 4.5 billion years in which to evolve. The earliest atmosphere must have consisted of volcanic emanations alone. Gases that erupt from volcanoes today, however, are mostly a mixture of water vapor, carbon dioxide, sulfur dioxide, and nitrogen, with almost no oxygen. If this is the same mixture that existed in the early atmosphere, then various processes would have had to operate to produce the mixture we have today. One of these processes was condensation. As it cooled,

much of the volcanic water vapor condensed to fill the earliest oceans. Chemical reactions would also have occurred. Some carbon dioxide would have reacted with the rocks of Earth’s crust to form carbonate minerals, and some would have become dissolved in the new oceans. Later, as primitive life capable of photosynthesis evolved in the oceans, new marine organisms began producing oxygen. Almost all the free oxygen in the air today is believed to have formed by photosynthetic combination of carbon dioxide with water. About 570 million years ago, the oxygen content of the atmosphere and oceans became high enough to permit marine life capable of respiration. Later, some 400 million years ago, the atmosphere contained enough oxygen for the evolution of air-breathing land animals.

The water-vapor content of the air varies considerably, depending on the temperature and relative humidity. With 100 percent relative humidity, the water-vapor content of air varies from 190 parts per million (ppm) at -40°C (-40°F) to 42,000 ppm at 30°C (86°F). Minute quantities of other gases, such as ammonia, hydrogen sulfide, and oxides of sulfur and nitrogen, are temporary constituents of the atmosphere in the vicinity of volcanoes and are washed out of the air by rain or snow. Oxides and other pollutants added to the atmosphere by industrial plants and motor vehicles have become a major concern, however, because of their damaging effects in the form of acid rain. In addition, the strong possibility exists that the steady increase in atmospheric carbon dioxide, mainly as the result of the burning of fossil fuels since the mid-1800s, may affect Earth’s climate (see Greenhouse Effect).

Similar concerns are posed by the sharp increase in atmospheric methane. Methane levels have risen 11 percent since 1978. About 80 percent of the gas is produced by decomposition in rice paddies, swamps, and the intestines of grazing animals, and by tropical termites. Human activities that tend to accelerate these processes include raising more livestock and growing more rice. Besides adding to the greenhouse effect, methane reduces the volume of atmospheric hydroxyl ions, thereby curtailing the atmosphere’s ability to cleanse itself of pollutants. See also Air Pollution; Climate; Smog.

The study of air samples shows that up to at least 88 km (55 mi) above sea level the composition of the atmosphere is substantially the same as at ground level; the continuous stirring produced by atmospheric currents counteracts the tendency of the heavier gases to settle below the lighter ones. In the lower atmosphere, ozone, a form of oxygen with three atoms in each molecule, is normally present in extremely low concentrations. The layer of atmosphere from 19 to 48 km (12 to 30 mi) up contains more ozone, produced by the action of ultraviolet radiation from the sun. Even in this layer, however, the percentage of ozone is only 0.001 by volume. Atmospheric disturbances and downdrafts carry varying amounts of this ozone to the surface of Earth. Human activity adds to ozone in the lower atmosphere, where it becomes a pollutant that can cause extensive crop damage.

The ozone layer became a subject of concern in the early 1970s, when it was found that chemicals known as chlorofluorocarbons (CFCs), or chlorofluoromethanes, were rising into the atmosphere in large quantities because of their use as refrigerants and as propellants in aerosol dispensers. The concern centered on the possibility that these compounds, through the action of sunlight, could chemically attack and destroy stratospheric ozone, which protects Earth’s surface from excessive ultraviolet radiation. As a result, industries in the United States, Europe, and Japan replaced chlorofluorocarbons in all but essential uses. See Aerosol Dispenser; Ozone Layer; Photochemistry.

The atmosphere may be divided into several layers. In the lowest one, the troposphere, the temperature as a rule decreases upward at the rate of 5.5°C per 1,000 m (3°F per 3,000 ft). This is the layer in which most clouds occur (see Cloud). The troposphere extends up to about 16 km (about 10 mi) in tropical regions (to a temperature of about -79°C, or about -110°F) and to about 9.7 km (about 6 mi) in temperate latitudes (to a temperature of about -51°C, or about -60°F). Above the troposphere is the stratosphere. In the lower stratosphere the temperature is practically constant or increases slightly with altitude, especially over tropical regions. Within the ozone layer the temperature rises more rapidly, and the temperature at the upper boundary of the stratosphere, almost 50 km (about 30 mi) above sea level, is about the same as the temperature at the surface of Earth. The layer from 50 to 90 km (30 to 55 mi), called the mesosphere, is characterized by a marked decrease in temperature as the altitude increases.

From investigations of the propagation and reflection of radio waves, it is known that beginning at an altitude of 60 km (40 mi), ultraviolet radiation, X rays (see X Ray), and showers of electrons from the sun ionize several layers of the atmosphere, causing them to conduct electricity; these layers reflect radio waves of certain frequencies back to Earth. Because of the relatively high concentration of ions in the air above 60 km (40 mi), this layer, extending to an altitude of about 1000 km (600 mi), is called the ionosphere. At an altitude of about 90 km (55 mi), temperatures begin to rise. The layer that begins at this altitude is called the thermosphere, because of the high temperatures reached in this layer (about 1200°C, or about 2200°F). The region beyond the thermosphere is called the exosphere, which extends to about 9,600 km (about 6,000 mi), the outer limit of the atmosphere.

The density of dry air at sea level is about 1/800 the density of water; at higher altitudes it decreases rapidly, being proportional to the pressure and inversely proportional to the temperature. Pressure is measured by a barometer and is expressed in millibars, which are related to the height of a column of mercury that the air pressure will support; 1 millibar equals 0.75 mm (0.03 in) of mercury. Normal atmospheric pressure at sea level is 1,013 millibars, that is, 760 mm (29.92 in) of mercury. At an altitude of 5.6 km (about 3.5 mi) pressure falls to about 507 millibars (about 380 mm/14.96 in of mercury); half of all the air in the atmosphere lies below this level. The pressure is approximately halved for each additional increase of 5.6 km in altitude. At 80 km (50 mi) the pressure is 0.009 millibars (0.0069 mm/0.00027 in of mercury).

The troposphere and most of the stratosphere can be explored directly by means of sounding balloons (see Ballooning) equipped with instruments to measure the pressure and temperature of the air and with a radio transmitter to send the data to a receiving station at the ground. Rockets carrying radios that transmit meteorological-instrument readings have explored the atmosphere to altitudes above 400 km (250 mi). Study of the form and spectrum of the polar lights (see Aurora) gives information to a height possibly as great as 800 km (500 mi).

Solar EnergySolar Energy, radiation produced by nuclear fusion reactions deep in the Sun’s core (see Nuclear Energy). The Sun provides almost all the heat and light Earth receives and therefore sustains every living being.

Solar energy travels to Earth through space in discrete packets of energy called photons (see Electromagnetic Radiation). On the side of Earth facing the Sun, a square kilometer at the outer edge of our atmosphere receives 1,400 megawatts of solar power every minute, which is about the capacity of the largest electric-generating plant in Nevada. Only half of that amount, however, reaches Earth’s surface. The atmosphere and clouds absorb or scatter the other half of the incoming sunlight. The amount of light that reaches any particular point on the ground depends on the time of day, the day of the year, the amount of cloud cover, and the latitude at that point. The solar intensity varies with the time of day, peaking at solar noon and declining to a minimum at sunset. The total radiation power (1.4 kilowatts per square meter, called the solar constant) varies only slightly, about 0.2 percent every 30 years. Any substantial change would alter or end life on Earth.

INDIRECT COLLECTION OF SOLAR ENERGY

People can make indirect use of solar energy that has been naturally collected. Earth's atmosphere, oceans, and plant life, for example, collect solar energy that people later extract to power technology.

The Sun's energy, acting on the oceans and atmosphere, produces winds that for centuries have turned windmills and driven sailing ships (see Wind Energy). Modern windmills are strong, light, weather-resistant, aerodynamically designed machines that produce electricity when attached to generators.

Approximately 30 percent of the solar power reaching Earth is consumed by the continuous circulation of water, a system called the water cycle or hydrologic cycle. The Sun’s heat evaporates water from the oceans. Winds transport some of the water vapor from the oceans over the land where it falls as rain. Rainwater seeps into the ground or collects into streams or lakes and eventually returns to the ocean. Thus, radiant energy from the Sun is transformed to potential energy of water in streams and rivers. People can tap the power stored in the water cycle by directing these flowing waters through modern turbines. Power produced in this way is called hydroelectric power. See Waterpower; Dam.

The oceans also collect and store solar energy. A significant fraction of the Sun’s radiation reflects or scatters from the water’s surface. The remaining fraction enters the water and rapidly diminishes with depth as the energy is absorbed and converted to heat or chemical energy. This absorption creates differences in temperature between layers of water in the ocean called temperature gradients. In some locations, these differences approach 20°C (36°F) over a depth of a few hundred meters. These large masses of water existing at different temperatures create a potential for generating power. Energy flows from the high-temperature water to the low-temperature water (see Thermodynamics). The flow can be harnessed, to turn a turbine to produce electricity for example. Such systems, called ocean thermal energy conversion (OTEC) systems, require enormous heat exchangers and other hardware in the ocean to produce electricity in the megawatt range. Almost all of the major United States OTEC experiments in recent years have taken place in Hawaii.

Plants, through photosynthesis, convert solar energy to chemical energy, which fuels plant growth. People, in turn, use this stored solar energy through fuels such as wood, alcohol, and methane that are extracted from the plant life (biomass). Fossil fuels such as oil and coal are derived from geologically ancient plant life. People also eat and digest plants, or animals fed on plants, to obtain energy for their bodies.

DIRECT COLLECTION OF SOLAR ENERGY

People have devised two main types of artificial collectors to directly capture and utilize solar energy: flat plate collectors and concentrating collectors. Both require large surface areas exposed to the Sun since so little of the Sun’s energy reaches Earth’s surface. Even in areas of the United States that receive a lot of sunshine, a collector surface as big as a two-car garage floor is needed to gather the energy that one person typically uses during a single day.

A. Flat Plate CollectorsFlat plate collectors are typically flat, thin boxes with a transparent cover that are mounted on rooftops facing the Sun. The Sun heats a blackened metal plate inside the box, called an absorber plate, that in turn heats fluid (air or water) running through tubes within the collector. The energy transferred to the carrier fluid, divided by the total solar energy that falls on the collector, is called the collector efficiency. Flat plate collectors are typically capable of heating carrier fluids up to 82°C (180°F). Their efficiency in making use of the available energy varies between 40 and 80 percent, depending on the type of collector.

These collectors are used for water and space heating. Homes employ collectors fixed in place on roofs. In the Northern Hemisphere, they are oriented to face true south (± 20°); in the Southern Hemisphere, they are oriented to face north. For year-round applications such as providing hot water, they are tilted relative to the horizontal at an angle equal to the latitude ± 15°. In addition to the flat plate collectors, typical hot-water and space heating systems include circulating pumps, temperature sensors, automatic controllers to activate the circulating pump, and a storage device. Either air or a liquid (water or a water-antifreeze mixture) can be used as the fluid in the solar heating system. A rock bed or a well-insulated water storage tank typically serves as an energy storage medium.

B. Concentrating CollectorsFor applications such as air conditioning, central power generation, and many industrial heat requirements, flat plate collectors cannot provide carrier fluids at high enough temperatures to be effective. They may be used as first-stage heat input devices; the temperature of the carrier fluid is then boosted by other conventional heating means. Alternatively, more complex and expensive concentrating collectors can be used. These devices reflect the Sun’s rays from a large area and focus it onto a small, blackened receiving area. The light intensity is concentrated to produce temperatures of several hundred or even several thousand degrees Celsius. The concentrators move to track the Sun using devices called heliostats.

Concentrators use curved mirrors with aluminum or silver reflecting surfaces that coat the front or back surfaces of glass or plastic. Researchers are developing cheap polymer films to replace the more expensive glass. One new technique uses a pliable membrane stretched across the front of a cylinder and another across the back with a partial vacuum between. The vacuum causes the membranes to form a spherical shape ideal for concentrating sunlight.

Concentrating solar energy is the least expensive way to generate large-scale electrical power from the Sun’s energy and therefore has the potential to make solar power available at a competitive rate. Consequently, government, industry, and utilities have formed partnerships to reduce the manufacturing costs of concentrators.

One important high-temperature application of concentrators is solar furnaces. The largest of these, located at Odeillo in the Pyrenees Mountains of France, uses 63 mirrors with a total area of approximately 2,835 sq m (about 30,515 sq ft) to produce temperatures as high as 3200°C (5800°F). Such furnaces are ideal for research requiring high temperatures and contaminant-free environments—for example, materials research to determine how substances will react when exposed to extremely high temperatures. Other methods of reaching such temperatures usually require chemical reactants that would also react with the substances to be studied, skewing the results.

Another type of concentrator called a central receiver, or 'power tower,' uses an array of sun-tracking reflectors mounted on computer-controlled heliostats to reflect and focus the Sun’s rays onto a water boiler mounted on a tower. The steam thus generated can be used in a conventional power-plant cycle to produce electricity. A U.S. demonstration in the Mohave Desert, Solar One, operated through most of the 1980s. During the early 1990s a second demonstration, called Solar Two, used molten salt heated in the boiler to 574°C (1065°F) to produce electricity. The hot salt was stored and later used to boil water into steam that drove a turbine to produce electricity.

PASSIVE SOLAR HEATING

The solar energy that falls naturally on a building can be used to heat the building without special devices to capture or collect sunlight. Passive solar heating makes use of large sun-facing windows (south-facing in the Northern Hemisphere) and building materials such as brick and tile that absorb and slowly release solar heat. A designer plans the building so that the longest walls run from east to west, providing lengthy southern exposures that allow solar heat to enter the home in the winter. A well-insulated building with such construction features can trap the Sun’s energy and reduce heating bills as much as 50 percent. Passive solar designs also include natural ventilation for cooling. Shading and window overhangs also reduce summer heat while permitting winter Sun.

In direct gain, the simplest passive heating system, the Sun shines into the house and heats it up. The house’s materials store the heat and slowly release it. An indirect gain system, by contrast, captures heat between the Sun and the living space, usually in a wall that both absorbs sunlight and holds heat well. An isolated gain system isolates the heated space (a sunroom or solar greenhouse, for example) from the living space and allows the solar heat to flow into the living area via convective loops of moving air.

SOLAR COOLING

Solar energy can also be used for cooling. An absorption air conditioner or refrigerator uses a large solar collector to provide the heat that drives the cooling process (see Refrigeration). Solar heat is applied to the refrigerant and absorbent mixture, which is combined under pressure in a container called a generator or boiler. The Sun’s heat brings the mixture to a boil. The refrigerant (often ammonia) vaporizes, rises as a gas, and reaches the condenser. There it gives off heat and returns to liquid form. As the drops of pure refrigerant fall, they trickle into the evaporator (freezing unit) where they evaporate vigorously. Evaporation requires heat energy, which comes from the surroundings, and results in cooling: The refrigerant absorbs heat from the unit and cools the space. The refrigerant, now a gas again, rejoins the mixture in the boiler to restart the process.

Absorption coolers must be adapted to operate at the normal working temperatures for flatbed solar collectors—between 82° and 121°C (180° and 250°F) alternatively, concentrating collectors may be used.

PHOTOVOLTAICS

Solar cells called photovoltaic made from thin slices of crystalline silicon, gallium arsenide, or other semiconductor materials convert solar radiation directly into electricity. Cells with conversion efficiencies greater than 30 percent are now available. By connecting large numbers of these cells into modules, the cost of photovoltaic electricity has been reduced to 20 to 30 cents per kilowatt-hour. Americans currently pay 6 to 7 cents per kilowatt-hour for conventionally generated electricity.

The simplest solar cells provide small amounts of power for watches and calculators. More complex systems can provide electricity to houses and electric grids. Usually though, solar cells provide low power to remote, unattended devices such as buoys, weather and communication satellites, and equipment aboard spacecraft.

SOLAR ENERGY FROM SPACE

A futuristic proposal to produce power on a large scale envisions placing giant solar modules in geostationary Earth orbit. Energy generated from sunlight would then be converted to microwaves and beamed to antennas on Earth for conversion to electric power. The Sun would shine on a solar collector in geostationary orbit almost 24 hours a day; moreover, such a collector would be high above the atmosphere and so would receive the full power of the Sun’s rays. Consequently, such a collector would gather eight times more light than a similar collector on the ground. To produce as much power as five large nuclear power plants (1 billion watts each), several square miles of solar collectors, weighing 10 million pounds, would need to be assembled in orbit. An Earth-based antenna five miles in diameter would be required to receive the microwaves. Smaller systems could be built for remote islands, but the economies of scale suggest advantages to a single large system (see Space Exploration).

SOLAR ENERGY STORAGE DEVICES

Because of the intermittent nature of solar radiation as an energy source, excess solar energy produced during sunny periods must be stored. Insulated tanks commonly store this energy in hot water. Batteries often store excess electric energy produced from wind or photovoltaic devices. One possibility for the future is the use of excess solar-generated electric energy as a supplemental source for existing power networks. Uncertain economics and reliability, however, make this plan difficult to implement.

Terrestrial Radiation is a radiation originating from Earth at night. It is a electromagnetic radiation in the form of the heat emitted by the Earth as it cools down at night, especially when the air is dry and there are no clouds.

Albedo, ratio between the amount of light a body reflects or scatters and the amount of light that is absorbed. A body that has an albedo of 0.3, for example, reflects or scatters three-tenths, or 30 percent, of the light that falls on it while absorbing the rest.

Various physical characteristics of a body determine its albedo. The earth’s moon has a rough surface that absorbs most of the sunlight that strikes it. The moon, therefore, has a low albedo of 0.12. The planet Venus has a highly reflective cloud layer, which gives the planet an albedo of about 0.65, the highest of any planet in the solar system. The earth’s albedo is about 0.37.

Scientists measure two specific kinds of albedo: bond albedo and normal albedo. Bond albedo is the ratio between the amount of energy that a body reflects and the amount of energy that falls on the body. It is used to keep track of the energy balance of a body, or how much energy a body is gaining or losing.

Normal albedo is the ratio between the amount of light that a surface reflects straight up and the amount of light that falls straight down on the surface. Depending on the composition of the surface, the normal albedo of a surface for different wavelengths of light can be different. By looking at the normal albedo of different wavelengths of light, astronomers can infer the chemical composition of the surface

WATER CYCLE Water is taken in by living things. Some groundwater reappears at the surface as springs. Water is released by living things. The surface water evaporates and enters air as water vapor. The water vapor condenses to form clouds. The water from clouds fall to Earth’s surface as precipitation. The precipitation runs off into bodies of water. The precipitation soaks into ground becoming ground water.

OXYGEN CYCLE During the day, plants undergo photosynthesis and release oxygen into the atmosphere as its waste product. Animals, on

the other hand, breathe oxygen and use it to release energy from their food through respiration. Plants also respire ans take in oxygen.

At night, plants do not photosynthesize and so do not produce waste oxygen. Instead, they take in the oxygen that they need for respiration from the atmosphere.

NITROGEN CYCLE Microorganisms such as those in root nodules of a bean plant, fix nitrogen N2. The ammonium produced, NH+

4 , is assimilated by the bean and other nearby plants. Animals eat the plants and convert the nitrogen into protein and other molecules. Ammonifying bacteria help convert organic nitrogen back into ammonium. Nitrifying bacteria convert ammonium into nitrate, NO-

3. Plants can assimilate ammonium and nitrate. Denitrifying bacteria convert nitrate to nitrogen gas into the atmosphere.

CFC PROBLEM (Chlorofluorocarbons) CFCs are released and rise to the atmosphere. Sunlight breaks down the CFCs, releasing atomic chlorine. Atomic chlorine destroys the ozone. Increased ultraviolet rays reach the earth’s surface, raising the risk of skin cancer and other dangerous consequences.

PHOTOSYNTHESISLight energy

6CO2 + 12H2O ------------------------------------- C6H12O6 + 6O2 + 6H2OChlorophyll

Light energyCarbon dioxide + water ------------------------------------- glucose + oxygen

chlorophyll

FACTORS AFFECTING PHOTOSYNTHESIS: carbon dioxide, sunlight, suitable temperature, water and chlorophyll

Photosynthesis makes use of carbon dioxide. It requires light and water and releases oxygen Light for photosynthesis is absorbed by a green pigment called chlorophyll. The oxygen released during photosynthesis comes from water.

Seawater is a dilute solution of several salts derived from weathering and erosion of continental rocks. The salinity of seawater is expressed in terms of total dissolved salts in parts per thousand parts of water. Salinity varies from nearly zero in continental waters to about 41 parts per 1,000 in the Red Sea, a region of high evaporation, and more than 150 parts per 1,000 in the Great Salt Lake. In the main ocean, salinity averages about 35 parts per 1,000, varying between 34 and 36. The major cations, or positive ions, present, and their approximate abundance per 1,000 parts of water are as follows: sodium, 10.5; magnesium, 1.3; calcium, 0.4; and potassium, 0.4 parts. The major anions, or negative ions, are chloride, 19 parts per 1,000, and sulfate, 2.6 parts. These ions constitute a significant portion of the dissolved salts in seawater, with bromide ions, bicarbonate, silica, a variety of trace elements, and inorganic and organic nutrients making up the remainder. The ratios of the major ions vary little throughout the ocean, and only their total concentration changes. The major nutrients, although not abundant in comparison with the major ions, are extremely important in the biological productivity of the sea. Trace metals are of specific importance for certain organisms, but carbon, nitrogen, phosphorus, and oxygen are almost universally important to marine life. Carbon is found mainly as bicarbonate, HCO3

-; nitrogen as nitrate, NO3

-; and phosphorus as phosphate, PO4:.

BUYS BALLOT LAWThe direction of winds around a pressure center depends on whether the center is north or south of the equator.

Cyclone, in strict meteorological terminology, an area of low atmospheric pressure surrounded by a wind system blowing, in the northern hemisphere, in a counterclockwise direction. A corresponding high-pressure area with clockwise winds is known as an anticyclone. In the southern hemisphere these wind directions are reversed. Cyclones are commonly called lows and anticyclones highs. The term cyclone has often been more loosely applied to a storm and disturbance attending such pressure systems, particularly the violent tropical hurricane and the typhoon, which center on areas of unusually low pressure.

Anticyclone, large rotating wind system centered around a region of high atmospheric pressure. Anticyclones are also called high-pressure areas, or highs, and they range in diameter from several hundred kilometers to several thousand kilometers. Anticyclones rotate in the clockwise direction in the northern hemisphere and in the counterclockwise direction in the southern hemisphere. They usually bring clear, fair weather.

Ridge, is a long area of high pressure in a weather system.

Trough is an elongated area of low atmospheric pressure that may be associated with a front.

Col is a region of low atmospheric pressure, a pattern of atmospheric pressure distribution that develops between two anticyclons and two depressions arranged alternately, characterized by light variable winds and often thundery weather in summer or foggy conditions in winter.

A more convenient form of barometer (and one that is almost as accurate) is the aneroid, in which atmospheric pressure bends the elastic top of a partially evacuated drum, actuating a pointer. A suitable aneroid barometer is often used as an altimeter (instrument measuring altitude), because pressure decreases rapidly with increasing altitude (about 25 mm/1 in. of mercury per 305 m/1000 ft at low altitudes).

CloudCloud, condensed form of atmospheric moisture consisting of small water droplets or tiny ice crystals. Clouds are the principal visible phenomena of the atmosphere. They represent a transitory but vital step in the water cycle, which includes evaporation of moisture from the surface of the earth, carrying of this moisture into higher levels of the atmosphere, condensation of water vapor into cloud masses, and final return of water to the surface as precipitation.

FORMATION AND EFFECTS

The formation of clouds caused by cooling of the air results in the condensation of invisible water vapor that produces visible cloud droplets or ice particles (see Meteorology). Cloud particles range in size from about 5 to 75 micrometers (0.0005 to 0.008 cm/0.0002 to 0.003 in). The particles are so small that light, vertical currents easily sustain them in the air. The different cloud formations result partly from the temperature at which condensation takes place. When condensation occurs at temperatures below freezing, clouds are usually composed of ice crystals; those that form in warmer air usually consist of water droplets. Occasionally, however, supercooled clouds contain water droplets at subfreezing temperatures (see Supercooling). The air motion associated with cloud development also affects formation. Clouds that develop in calm air tend to appear as sheets or stratified formations; those that form under windy conditions or in air with strong vertical currents have a towering appearance.

Clouds perform a very important function in modifying the distribution of solar heat over the earth's surface and within the atmosphere (see Solar Energy). In general, because reflection from the tops of clouds is greater than reflection from the surface of the earth, the amount of solar energy reflected back to space is greater on cloudy days. Although most solar radiation is reflected back by the upper layers of the clouds, some radiation penetrates to the surface of the earth, which absorbs this energy and reradiates it. The lower parts of clouds are opaque to this long-wave earth radiation and reflect it back toward earth. The result is that the lower atmosphere generally absorbs more radiative heat energy on a cloudy day because of the presence of this trapped radiation. By contrast, on a clear day more solar radiation is initially absorbed by the surface of the earth, but when reradiated this energy is quickly dissipated because of the absence of clouds. Disregarding related meteorological elements, the atmosphere actually absorbs less radiation on clear days than on cloudy days.

Cloudiness has considerable influence on human activities. Rainfall, which is very important for agricultural activities, has its genesis in the formation of clouds (see Rain). The marked effect of clouds on visibility at flight levels proved to be a major difficulty during the early days of the airplane, a hazard that was alleviated with the development of instrument flying, which permits the pilot to navigate even in the midst of a thick cloud (see Navigation). The sharp increase in consumption of electricity for lighting during cloudy days represents one of the major scheduling problems faced by the electric-power industry.

The first scientific study of clouds began in 1803, when a method of cloud classification was devised by the British meteorologist Luke Howard. The next development was the publication in 1887 of a classification system that later formed the basis for the noted International Cloud Atlas (1896). This atlas, considerably revised and modified through the years (most recently in 1956), is now used throughout the world.

CLASSIFICATION

Clouds are usually divided into four main families on the basis of their height above the ground: high clouds, middle clouds, low clouds, and clouds with vertical development that may extend through all levels. The four main divisions are further subdivided into genera, species, and varieties, which describe in detail the appearance of clouds and the manner in which they are formed. More than 100 different kinds of clouds are distinguishable. Only the primary families and most important genera are described below.

A High Clouds

These are clouds composed of ice particles, found at average levels of 8 km (5 mi) or more above the earth. The family contains three principal genera. Cirrus clouds are isolated, feathery, and threadlike, often with hooks or tufts, and are arranged in bands. Cirrostratus clouds appear as a fine, whitish veil; they occasionally exhibit a fibrous structure and, when situated between the observer and the moon, produce halo phenomena. Cirrocumulus clouds form small, white, fleecy balls and wisps, arranged in groups or rows. Cirrocumulus and cirrus clouds are popularly described by the phrase “mackerel scales and mares' tails.”

B Middle Clouds

These are clouds composed of water droplets and ranging in altitude from about 3 to 6 km (about 2 to 4 mi) above the earth. Two principal genera are included in the family. Altostratus clouds appear as a thick, gray or bluish veil, through which the sun or moon may be seen only diffusely, as through a frosted glass. Altocumulus clouds have the appearance of dense, fleecy balls or puffs somewhat larger than cirrocumulus. The sun or moon shining through altocumulus clouds may produce a corona, or colored ring, markedly smaller in diameter than a halo.

C Low Clouds

These clouds, also composed of water droplets, are generally less than 1.6 km (1 mi) high. Three principal forms comprise this group. Stratocumulus clouds consist of large rolls of clouds, soft and gray looking, which frequently cover the entire sky. Because the cloud mass is usually not very thick, blue sky often appears between breaks in the cloud deck. Nimbostratus clouds are thick, dark, and shapeless. They are precipitation clouds from which, as a rule, rain or snow falls. Stratus clouds are sheets of high fog. They appear as flat, white blankets, usually less than 610 m (2000 ft) above the ground. When they are broken up by warm, rising air, the sky beyond usually appears clear and blue.

CLOUDS WITH VERTICAL DEVELOPMENT

Clouds of this type range in height from less than 1.6 km (1 mi) to more than 13 km (8 mi) above the earth. Two main forms are included in this group. Cumulus clouds are dome-shaped, woolpack clouds most often seen during the middle and latter part of the day, when solar heating produces the vertical air currents necessary for their formation. These clouds usually have flat bases and rounded, cauliflowerlike tops. Cumulonimbus clouds are dark, heavy-looking clouds rising like mountains high into the atmosphere, often showing an anvil-shaped veil of ice clouds, false cirrus, at the top. Popularly known as thunderheads, cumulonimbus clouds are usually accompanied by heavy, abrupt showers.

An anomalous, but exceptionally beautiful, group of clouds contains the nacreous, or mother-of-pearl, clouds, which are 19 to 29 km (12 to 18 mi) high, and the noctilucent clouds, 51 to 56 km (32 to 35 mi) high. These very thin clouds may be seen only between sunset and sunrise and are visible only in high latitudes.

The development of the high-altitude airplane has introduced a species of artificial clouds known as contrails (condensation trails). They are formed from the condensed water vapor ejected as a part of the engine-exhaust gases.

Meteorology

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