Precipitation

PRECIPITATION, STORMS AND OTHER WEATHER PHENOMENA; CLIMATE

The principal actions brought on by weather systems that affect both land and sea and the humans, animals, and vegetation thereon are winds and precipitation. The latter comes in a variety of forms as discussed below. Most weather of consequence to people occurs in storms. These may be local in origin but more commonly are carried to locations in wide areas along pathways followed by active air masses consisting of Highs and Lows. This chart shows a meteorological classification of weather systems at various scales; the two categories of most interest to us are the Mesoscale and the Synoptic Scale (sometimes called Macroscale).

The key ingredient in storms is water, either as a liquid or as a vapor. The vapor acts like a gas and thus contributes to the total pressure of the atmosphere, making up a small but vital fraction of the total, as seen in this diagram:

Maps of the vapor pressure alone indicate its variability over a wide (subcontinental) region.

Water vapor in the air will vary in amount depending on sources, quantities, processes involved, and air temperature. Heat, mainly as solar irradiation but with some contributed by the Earth and human activity and some from change of state processes, will cause some water molecules either in water bodies (oceans, lakes, rivers) or in soils to be excited thermally and escape from their sources. This is called evaporation; if water is released from trees and other vegetation the process is known as evapotranspiration. The evaporated water, or moisture, that enters the air is responsible for a state called humidity. Absolute humidity is the weight of water vapor contained in a given volume of air. The Mixing Ratio refers to the mass of the water vapor within a given mass of dry air. At any particular temperature, the maximum amount of water vapor that can be

contained is limited to some amount; when that amount is reached the air is said to be saturated for that temperature. If less than the maximum amount is present, then the property of air that indicates this is its Relative Humidity (RH), defined as the actual water vapor amount compared to the saturation amount at the given tempeature; this is usually expressed as a percentage. RH also indicates how much moisture the air can hold above its stated level, which, after attaining, could lead to rain. This diagram helps to explain RH, the most common way to indicated the moisture content of air.

The red curve indicates the saturation condition for any given condition; its corresponding water vapor content is directly related to its vapor pressure "e". At 30°C, e is 40 millibars and the RH = 100%. If at that temperature, e was 20 mb, the RH would be 20/40 x 100 = 50%.

When a parcel of air attains or exceeds RH = 100% condensation will occur and water in some state will begin to organize as some type of precipitation. One familiar form is dew, which occurs when the saturation temperature for some quantity of moisture reaches a temperature Td at the surface at which condensation sets in, leaving the moisture to coat the ground (especially obvious on lawns). This diagram set the condition for dew formation:

The term dew point has a more general use, being that temperature at which an air parcel must be cooled to become saturated. Dew frequently forms when the current air mass contains excessive moisture after a period of rain but the air is now clear; the dew precipitates out to coat the surface (noticeable on vegetation). Ground fog is a variant in which lowered temperatures bring on condensation within the near surface air as well as the ground.

The other types of precipitation are listed in the following table along with descriptive characteristics related to each type:

Precipitation requires development of some non-vapor form of water. A droplet of water does not necessarily begin its existence at precisely the saturation temperature, i.e., may require some overcooling, also called supercooling; the air is then referred to as supersaturated. Commonly, the first to form are tiny nuclei of ice, which may start to grow around foreign particles such as dust. This ice crystal process is the most common starting point. But under some conditions only small droplets of liquid water are the first product of condensation. As the process proceeds, individual water-coated ice particles or tiny droplets are moved around the condensing air mass (now a cloud) and collide repeatedly; this leds to growth by coalescence. The next figure shows the relative differences in size of the water bodies that form in this way

This diagram suggests a possible history of water drops in a cloud. When the size of a drop reaches a critical value, it may actually fall towards or to the ground/sea surface as rain. But updrafts may carry the particle upward for more growth or change. Or, the drop may evaporate before it reaches the ground.

Clouds near the surface are called warm clouds and almost always produce liquid precipitation. Clouds that are higher or form in much colder air are called cold clouds and can produce ice nuclei that grow as more cold water precipitates on them (contact icing), yielding sleet (small ice droplets) or hail (ice bodies that can be centimeters in diameter). This is illustrated thusly:

The three general conditions in an atmospheric system that lead to local to widespread precipitation are 1) Convectional; 2) Orographic; and 3) Cyclonic.

Convectional precipitation is usually associated with thunderstorms. Warm, moist air rises as an unstable air mass and cools adiabatically. The rising parcel is commonly called a "thermal". As it reaches cooler air, and lower pressures, condensation begins and often yields numerous raindrops. These are eventually too heavy for the growing cloud (cumulonimbus) and fall to Earth in torrents. The updrafts of wind recirculate and windflow on the ground may be turbulent and violent.

These thunderstorm clouds are also described as supercells. Here is a sketch of this type of storm, and below it is an actual photograph of a supercell storm that displays a prominent anvil cloud at its top. which occurs when air of different properties is encountered..

Convective thunderstorms are the most common type of atmospheric instability that produces lightning followed by thunder. As seen in this image, lightning is one of the most spectacular phenomena witnessed in storms:

A typical lightning bolt can attain an electric potential up to 30 million volts and a current as much as 10000 amperes. It can cause air temperatures to reach 10000°C. But a bolt's duration is extremely short (fractions of a second). Although a bolt can kill people it hits, most can survive. A lightning bolt is the discharge of electrons (negative charges) that build up in a cloud. Both negative and positive charges accumulate from processes that derive them from the ground or by ionization of the air. With both charges present in a thundercloud (the thunder itself occurs as a sound wave as superheated air rushes back into the partial vacuum along the bolt's path), much lightning is discharged in and remains within the cloud. But if the Earth's surface is induced to have a surplus of + (positive charges), as when - (negative) charges are drawn off and carried upwards, the bolt may strike some spot on the ground if the potential difference is great enough. This diagram shows the sequence involved in forming lightning:

From Lutgens and Tarbuck, 1998

Orographic precipitation is a straightforward process, as depicted below. Moist air moves toward higher terrain - usually large mountain chains but smaller block mountains can also induce the effect. This wind-driven air is forced up the slopes to elevations where both P and T are reduced. At the lower temperatures, the moist air mass becomes saturated and precipitation ensues - usually as thunderstorms in the summer or as widespread snow storms in the winter. This air then becomes "dried out" - most of its moisture has precipitated in the pass over. The air mass that moves down the opposite slope is now drier and warmer. This side is said to be in a "rain shadow", i.e., general storms are infrequent and

arid conditions, with their characteristic vegetation, prevail. As it moves on, the air mass may gradually pick up moisture.

Clouds on the windward side of a mountain belt (before uplift) are distinctive, as these cirrus types demonstrate.

An example from space of cloud formation over most of a large, high mountain system - in this case, the European Alps - confirms this tendency for alpine topography to build up cloud cover.

On a much larger scale are the Mid-Latitude Cyclones which develop when polar air moves into latitudes largely within the 30 to 60° latitude range. Lows develop and compete with highs as both types of systems are moved around by Jet Streams and other contributors. These are the typical massive storms that affect Europe and Asia, North America, and to a lesser extent the southern continents throughout the year but are most noticeable and influential in the winter months. Some of the illustrations shown below are found on a Weather site maintained by the University of Arizona.

Before learning how these cyclonic systems form, lets look at one that shows up dramatically over the British Isles as seen from space:

The general characteristics of Mid-Latitude Cyclonic systems are summarized in this chart

The development of the cyclone follows a general sequence of stages beginning with the advance of an Arctic Cold Front into cooler air (to the south in the northern hemisphere) and often ending with an occluded phase. A 5 step sequence appears here; read the captions (click on image) for descriptions:

Stage 6 = weakening of the pressure gradient and system dissipation; not shown.

In the above five diagrams, read the captions for information that ties into the following: The beginning of a large-scale cyclonic development occurs as a northern cold air mass moves south (and often with an eastern component) against an air mass that is cooler, or even describable as warm; each air mass is moving. At this first stage, the boundary between air masses becomes stationary and air above it is in a pressure trough as air diverges horizontally. Air from the surface replaces the upper air and this leads to a pressure drop or a low along the front. At the surface, winds move towards to lower pressure centers and begin to circulate as a counterclockwise inspiral. The process is aided by imbalances in the jet stream where air is forced into uplift. The two fronts - cold and warm - are connected by an extratropical cyclone. This strengthens aloft and the process of cyclogenesis begins to produce stormy conditions.

With continued pressure drop, the cold front advances into the warm sector and the angle between air masses lessens. During this mature stage, prominent wave shapes are developed in each front (wave cyclones). If the storm tracks to the north of an observation point, that area will receive much rain if temperatures are warm or snow if the near surface conditions are cold. The faster moving cold front eventually overtakes the warm front, developing an occluded state, driving the warm air overhead. In time, the cyclone weakens as the storm moves more to the east. The horizontal pressure gradient diminishes, dissipating the front (frontolysis) and the dissolving stage is reached. After the passage of a mid-latitude low, a high usually follows: If the pressure gradient between air masses is high (steep), a period of strong winds usually results.

For the North American continent, the steering of this cold frontal system is controlled by the Westerlies (winds from the West) and storms in its path come usually from the Canadian West. In the U.S some storms tend to move notheastward along; these often originate in the Southwest, move east, and then track near the coast. Maritime polar air masses (see below) in the northwest Atlantic can produce east and northeast winds (along the northwest sector of a Low) causing severe storm conditions known as a "Noreaster" that can wreak havoc on the Atlantic coastline and inland.

During the Mature stage, this diagram indicates the relative conditions of air movement at the surface and aloft.

This diagram shows the late stage of maturity and beginning of occlusion when rainfall may be maximum.

There are other conditions leading to precipitation, e.g., from hurricanes, that are described below. For now, we will show first a general cross-section following a longitude line from pole to pole that indicates the most characteristic states of precipitation in the various zones previously named:

Note that for the United States the wettest region on average is the Southeast. Semi-arid conditions prevail from the western Great Plains over the Rockies and into the Great Basin. True desert conditions are not widespread.

This is a good time in this page to indicate the highly generalized degrees of precipitation proceeding from the equator to the poles. The deviations from ideal patterns are due mostly to the locations of continents and to the influence of oceanic currents and streams. These in turn are factors in differing climates (see below)

We turn now to some specific types of wind (and sometimes accompanying storm) action. Most of these are mesoscale types of wind flow. We start with sea breezes. During the day these form because the ocean pressures near the surface over cold waters cause wind to flow landward towards the lower pressures of the air warmed by the land. At night, the land ground is cooler than the ocean surface, reversing the relative pressures and causing air flow to be seaward.

On a larger scale, long period gravity waves can form in the upper atmosphere. This satellite image shows these regularly spaced linear clouds that represent the condensed moisture in a gravity wave train.

One mechanism of gravity wave formation is suggested in this diagram

Air can move up mountain slopes during the day when high pressure winds move against the mountain range that may have cooler air from the previous night's drop in temperatures, and down slopes, as cold air sinks, into valleys at night

The orographic effect can cause lifted air to lose moisture and become drier and warmer. This gives rise to the Chinook Wind effect (a U.S. term; called foehn winds elsewhere) involving hot winds coming off the mountains. When developed these can often be strong and may cause problems if fires occur in timber and brush on the mountains.

On a grander scale, warm air that forms beyond a mountain range cools as it rises to make a high and can then be driven over those mountains, cool more and descend to low lands beyond. In Southern California, these are called Santa Ana winds, which involve heating in the Mojave Desert to the north, passage of that warm air over the Transverse Ranges, and rapid descent (high winds) into the Los Angeles Basin. This was the main factor in the disastrous fires during October 2003 and since.

In large, high land masses in colder regions of the world, e.g., Greenland, the air risen adiabatically above the topographic highs can become quite cold. It then,

being heavy, moves off the pressure High to lower areas which may also be cool. The descending air is said to create katabatic winds.

Air can be made to swirl at microscales as eddies (similar to gyres). This circularlike behavior is often produced by small obstructions such as buildings or hills, and can be turbulent (remember the experience in walking through a city's downtown on a windy day, or watching leaves in Fall become lifted in a spiral updraft).

Eddies can also have a vertical component as seen in this figure:

Eddies are also one of the factors responsible for air turbulence which most have encountered during air flights. This is often due to wind shear, an airflow condition that develops between two distinct air layers differing in velocity and/or direction of movement. This is shown diagrammatically here, and beneath that diagram is a panel which further indicates how airplanes are affected.

Wind shear conditions often occur near ground surface and can affect a landing or taking off of an airplane with sufficient turbulence and downdraft to cause the plane to veer off course or drop too rapidly for the pilot to maintain control. Several major airline disasters in recent years are attributed to wind shear.

Swirls of air, and clouds formed therein, at small mesoscales (50 - 100 km) have been described as eddy-like. Here is an example of the coast of Norway:

We come now to two types of very severe storms - much feared and uncontrollable - that accompany extensive precipitation associated with very high winds.

The first is a mesoscale phenomenon associated with thunderstorms or strong Mid-Latitude Cyclones: in America it is called a tornado (sometimes referred to as a "cyclone", as Dorothy called it in "The Wizard of Oz). When over water, the resulting funnel cloud is known as a "waterspout"; if developed over a desert, it is said to be a "dust devil" (usually small and non-destructive). Here is a ground photo of an advancing tornado:

Below is a radar image (side-looking) of a tornado embedded in its host cloud, not necessarily destined to reach the ground.

Tornadoes are columns of rotating air that may be as thin as a few tens of feet or as wide as a mile. A tornado that forms a distinct cloud produces a vortex. Its interior pressure may be as much as 20% lower than external air. As air is drawn in, it swirls and spirals upwards - it is thus a very pronounced local low pressure center.. Wind speeds in the swirl can exceed 250 mph (400 km/hr) and those strong winds together with the lower interior pressures can exert great forces on objects encountered, such as buildings. Air is sucked from structures hit by tornadic winds and in a sense this produces an implosion. The tornado cloud is usually a mix of condensed water and dust and debris picked up from surfaces it traverses

The next diagrams are variants designed to show how tornadoes form. They commonly develop along the squall line that marks the Frontal Boundary of an advancing Mid-Latitude Cyclone. The favored condition is a clash between very cold maritime polar air and very warm, moist maritime tropical air moved into

temperate zones. In the upper diagram, a typical severe thunderstorm is depicted. Within it rain moves downward pulling air with it. But within the forming vortex rising air is carried upward in a counterclockwise swirl to establish the tornadic winds. In the lower diagram, a cutaway sketch of a thunderstorm cloud, the tornado results where warm moist air from outside the cloud circulating counterclockwise is joined by cold air within the cloud that also becomes entrapped and itself circulates ccw.

This plan view indicates the wind circulation in a thundercloud capable of developing a tornado.

Most tornadoes in the U.S. develop east of the Rocky Mountains. The flatness of the terrain in the Great Plains eastward means that there are few obstructions to hamper wind circulation. In typical years 500 to 1000 tornadoes are either observed or detected by radar in that region. Specialized radar is used to pick up a forming tornado and, if there is enough time, alarms are sounded and people alerted via radio and TV. Many tornados do not touch down. Most that do have paths of destruction of only a few miles length (and normally less than a mile in width) but some create paths up to 100 miles long. This is a map that shows the frequency of occurrence of tornadoes in the U.S.

The Great Plains states are most susceptible to tornadoes, followed by the Upper Midwest and Florida. They are rare, but not unknown, further west or in New England. A system has been devised known as the Fujita scale (named after a meteorologist at the University of Chicago) to rate tornadic severity and expected damage, as presented in this Table

Most tornadoes are lower intensity events; a Category V tornado is rare and unbelievably destructive. The frequency of occurrence of each category is shown in this pie chart:

Dust devils form as a swirling updraft under sunny conditions during fair weather. Also called whirlwinds, they are common in desert climates, and are almost always harmless.

Waterspouts closely resemble continental tornadoes. Many are in fact tornadic winds and are associated with storm clouds (as in the figure below). Others are non-tornadic and occur beneath cumulus clouds. A waterspout usually is light-colored owing to the water it has sucked up from an ocean or lake.

Now we will consider another potentially violent storm that affects very large areas over which its path crosses. This storm is developed almost exclusively in the Trade Winds zone 30° north and south of the Equator. Called a Hurricane when formed in low latitudes of the Atlantic, the same phenomenon is known as a Cyclone in much of the Pacific. A more general term is "Tropical Cyclone". Here is a satellite view of Hurrican Isadore in the Gulf of Mexico.

Hurricanes are impressive storms when seen from the ground - and even more impressive when any individual is caught within one. Here are two ground photos:

One of the hallmarks of hurricanes often is the appearance of small, puffy cumulus clouds that build up before the hurricane arrives; sometimes this type is a predecessor of a tornado:

Hurricanes are more extreme members of tropical disturbances that, in the Atlantic, tend to begin of the coast of equatorial Africa. In the next two charts, the properties of Hurricanes are listed in the first figure using the Saffir-Simpson scale and the amount of damage expected from each category is given in the second table.

In the next sketches of a developing hurricane, its growing size and resultant wind patterns are depicted. Note that there are three stages of development: Tropical Disturbance (winds less than 35 kph [22 mph]); Tropical Depression (max. winds < 61 kph [38 mph]); Tropical Storm (max winds < 115 kph [71 mph]; Hurricane (when its central winds exceed 115 kph [71 mph]):

This diagram shows a side view of the temperature, moisture and pressure conditions that lead to a hurricane.

A cutaway diagram of a hurricane locates the rain cells and the central eye, within which the winds tend to be much lower ("calm of the Eye") where warm air is spiraling downward in counterclockwise motion. Note that outer winds aloft appear to drive the swirls in a clockwise motion.

Atlantic hurricanes usually begin off the African coast in subtropical waters that are very warm in summer - often in excess of 28° C (however, in the southern Atlantic, wintertime waters prevent hurricanes from forming there). Ocean water is vaporized by the Sun's rays, producing warm air that rises as shown above. Its moisture then condenses with upward cooling, releasing great amounts of latent heat that drive the disturbance until, if conditions persist, it graduates into a hurricane. This rapid upward deployment also reduces surface pressures at the base which further accelerates the rise. Some of the rising air spills outward, some inward to flow down around and in the Eye.

Hurricanes in the Atlantic tend to average from about 5 to 15 events per year. Worldwide, between 80 and 100 normally occur. Most are Categories 1 to 3. Nevertheless, if they make landfall on the North American continent or pass over Atlantic or Carribean islands, the damage they wreak can be huge - hurricanes in the 1990s caused billions of dollars in homes, businesses and infrastructure wiped out or damaged beyond practical repair. In recent decades, Early Warning Systems - built on hurricane tracking programs - have kept loss of life low. But in 1906 a Carribean hurricane struck Texas around the Galveston coastline, killing more than 9000 there. Monsoonal cyclones in the Indian Ocean often take thousands of lives, especially in low-lying parts of the Bangladesh coastal zones where the Ganges River Delta has built up. Here is a global map showing general pathways of hurricanes developed in tropical climes of the different oceans

Driven by westward flowing upper level trade winds, and powered by their extreme internal energy derived from hot air condensation, hurricanes move across the Atlantic toward North and South America. Depending on interactions with air masses on the continents or the open ocean, hurricanes will frequently be blocked or deflected northward and may or may not make landfall. This diagram, applied to the September 2004 Hurricane Jeanne, shows how the circulation of a mid-continent low and Atlantic offshore high determined how Jeanne was deflected north after leaving Florida. The low generates a counterclockwise wind, the high clockwise, together leading to convergence.

An the Atlantic, hurricanes move westward but usually start to move towards the North as they meet continental airflow. Many then swerve to the northeast. whether on land by then or if they are deflected into the Atlantic. There they

encounter cooler waters that deprive the waning hurricanes of the energy needed for sustenance.

Here is a map of the Atlantic Hurricane season of 1995; note that the hurricanes were referred to then by popular human first names (male one year; female the next; now in 2004 the male-female appellations are alternated during a single hurricane season).

It is instructive to look at the map below which plots the worldwide distribution of the paths of major cyclones (hurricanes) during the last 150 years.

There are several distinctive patterns revealed in this map. First, the regions susceptible to the most frequent and intense storms are the Philippines in the western Pacific, off the Mexican coast, and the Caribbean-Gulf of Mexico off the southeast U.S. Second, there appears to be a dearth of cyclones around the equatorial latitudes. That is because of the minimal influence of the Coriolis effect, near zero around the equator (maximum at the poles), which thus prevents unstable air that moves to the right north of the equator and to the left in the southern hemisphere from acquiring the spin needed to organize and maintain coherence of cyclonic air masses in this zone. Third, note the near absence of cyclones in the South Atlantic and Southwest Pacific, where ocean currents (Peru Current in the Pacific and Benguela Current off Africa) break up the formation of cyclones because of the cool waters they introduce. Fourth, the tendency of hurricanes/cyclones to start to swing poleward and even into trajectories towards the east is evident; this results from the action of the westerly winds in both hemispheres.

We close this mini-tutorial covering the rudiments of Meteorology with a brief discussion of Climate. Climate can be defined in two ways: 1) the general characteristics of temperature ranges, amounts of precipitation, frequency of cloud cover, and seasonal variations of these conditions that are typical of extensive regions on land and sea at various latitudes and longitudes; and 2) the prevailing weather conditions locally where one lives or moves about. Those who have experienced the year-round climatic conditions in San Francisco and in Pittsburgh, PA will readily recognize the differences seasonally between the two places. In the United States, we often speak of a New England climate, an Atlantic Coastal climate, the Mid-Atlantic climate, Florida's climate, climate around the Mississippi, the Great Plains (north notably different than the south), a Rocky Mountain climate, an Arizona climate, Seattle's climate, and the West Coast climate. People often choose a particular climate as favorable to their health and life style. Within any climate, there will be day-to-day weather changes, but these are just the variants that intrude on a region at different times of the year.

The three factors that most influence what a locality's or region's climate will be are shown in this chart:

In addition to latitude, terrain and ocean currents, one could also cite the types and entry locations of air masses. For example, the next figure shows that in the summer months two offshore air mass Highs have a big role to play.

Climates have been categorized and described by various classifications. The one most often used is the Koppen Classification. Two charts show the principal categories, listed by combinations of one to three letters (defined in the upper chart) which describe temperature and rainfall condition and other relevant factors.

Using this classification, a general worldwide map of the major climate types is shown as the final figure; refer to the letters in the previous chart to find the description of the climate in a particular region.

So there you have it - the basic principles, ideas, and applications of the Science of Meteorology - Weather and Climate - reduced to four (longish) pages. We hope that those unfamiliar with these concepts will learn enough by perusing (best repetitively) these four pages before exploring the rest of Section 14.

Primary Author: Nicholas M. Short, Sr.