WEATHER

WEATHER

Air Masses and Weather• An air mass is a large body of air in the lower

troposphere that has similar characteristics throughout.– Can be several kilometers in diameter and several kilometers

high.– The temperature and humidity for the air mass depends on

where it originates and when it moves it takes the characteristics from its origin with it.

– As air masses travel, there characteristics can change based where it has traveled to and how long it sits in the new location.

– Earth’s topography can also contribute to changes in the temperature and humidity of an air mass as it travels.

Air Masses and Weather

• There are many different types of air masses. Meteorologists classify air masses based on where they originate.– Temperature of an air mass depends on whether

the air mass originates in the arctic, polar, or tropical region.

– The humidity depends on whether the air mass comes from land (continental) or ocean (maritime).

Air Masses and Weather• There are 5 types of air masses:– Continental Arctic (cA): originate in the arctic regions,

• Extremely cold and since cold air is incapable of holding moisture it is also very dry.

• Though they may warm slightly as they move southward they still cause extreme cold waves in regions they enter.

– Continental Polar (cP): originate over land areas of Alaska and Canada. • These air masses are somewhat warmer than cA there is only a slight

difference between temperature and humidity of these two types of air masses.

• Though this air is cold and dry it will create precipitation in some regions.

• One such region is when it passes over the Great Lakes in the fall, the warm water warms the air slightly that it can hold water vapor and then deposits the moisture downwind, creating the lake-effect snows.

Air Masses and Weather– Maritime Polar (mP): originates over the ocean at high latitudes

• These air masses are both cold and damp.• It is not as cold as cP air due to the contrast in temperatures between

land(colder) and oceans (warmer).• When mP air cools to its dew point, fog, clouds, or precipitation results.• mP air coming off of the Pacific causes a wet winter climate in the Pacific

Northwest.• mP air coming off of the North Atlantic brings heavy snowstorms, called

nor’esterns to the East Coast in the winter and cool clear weather in the summer.

– Maritime Tropical (mT): originates over a warm tropical ocean, acquires both warmth and moisture.• In the summer mT air rotates clockwise around the high pressure over

the Atlantic Ocean bringing heat and humidity to the Midwestern and Eastern United States

• Due to the large amounts of moisture, thunderstorms often develop during the heat of the day.

Air Masses and Weather– Continental Tropical (cT: originates over deserts

• This air mass is hot and dry.• Many times originates as a maritime air mass but becomes dry as it

passes over mountains.• In the summertime cT air produces tremendous heat waves in much

of the United States.• mT air usually as temperatures no

higher that 100oC, cT air can exceed 100oC

• Since cT air is dry it does not bring clouds or thunderstorms to cool the air

• If the dry air remains in agricultural regions too long it can cause serious damage to crops.

Fronts and Lows• Weather in the mid-latitudes is always changing. The

weather changes are due to the movement of low-pressure systems and their associated frontal systems.

• Front is the boundary between two opposing air masses. Air masses of different types do not easily mix.

Fronts and Lows– A front can range from 200 meters to 200 kilometer in width, can be as high

as 5 kilometers and as long as 2000 kilometers.– Fronts are most common at mid-latitudes where northward moving tropical

air masses and southward moving polar air masses often meet.– Air masses on either side of the front may differ in temperature, in humidity,

or in both.– Fronts can have steep slopes (ranging from 1/50 to 1/300).– Fronts usually bring precipitation.

• Weather associated with a particular front depends on the types of air masses involved and the speed at which the front is moving.

• Fronts are usually classified according to the temperature of the advancing front.

• There are four basic kinds of fronts:– Cold– Warm– Occluded– Stationary

Fronts and Lows• Cold Front: the boundary between an advancing cold air mass and the

warmer mass it is displacing.– Cold air is denser than warm air, so the cold air slides underneath the warm

air and forces it upward.– Cold fronts have steep slopes.– Weather a cold front brings depends on mostly on the type of air mass it is

displacing. – When cP displaces mT air, thunderstorms often form.– When cP displaces cT air, there will very little to no precipitation.– A passing front may cause no greater change than a shift in winds.– In the summer some cold fronts cause a change in humidity but little

change in temperature.– In the winter a cold front maybe marked by rain or snow.– Because a cold front has a steep slope the precipitation associated covers a

narrow band of ground. – Cold fronts move quickly so precipitation usually ends shortly after the front

passes.

Fronts and Lows• Warm Front: the boundary between an advancing warm air mass and the

colder air mass it is displacing.– The warm air rises above the denser cold air mass, the cold air mass retreats

slowly.– The slope of a warm front is more gradual than that of a cold front. – Weather changes associated with a warm front are less dramatic.– Signs of an approaching warm front are high cirrus clouds which may warn of

approaching precipitation more than a day before it arrives.– Cirrus clouds are followed by cirrostratus and lower and thicker stratiform clouds.

These clouds form in the warm stable air sliding up the frontal surface.– The clouds may stretch 1500 kilometers ahead of the place where the warm front

touches the ground.– The lower thicker stratus clouds can also most screen out the sun and the moon. – Finally heavy nimbostratus clouds arrive, and steady rain or snow begins.– The area of precipitation can stretch hundreds of kilometers ahead of where the

warm front touches the ground.– Precipitation can last for a day or more.– Thunderstorms occasionally form but are not typical of a warm front.

Fronts and Lows

• Occluded front: when a cold front “catches up” to a warm front. This happens because cold fronts typically moves twice as fast as warm fronts.

• Stationary front: is a front that is not moving forward. This occurs because the approaching warm front is not strong enough to force the cold front to continue its forward motion. – If a stationary front remains stationary for too long, flooding can

occur.

Weather Associated with Pressure Systems• Weather you see from a low-pressure system will

depend on where the center of the low passes in relationship to your location.– If the low-pressure systems center passes to the

north of you, a warm front may move over your location, followed by a cold front.

– If the low-pressure systems center passes to the south of your location, no fronts by pass over you, but you may experience steady rain or snow.

– Low-pressure systems bring precipitation to areas that they pass over or nearby.

• Winds blow outward from a high-pressure system in a clockwise direction in the Northern Hemisphere, and in a counterclockwise direction in the Southern Hemisphere.– There is little to no wind in the center.– Air here takes on the temperature and humidity of the area. – Thus highs are where air masses generally form– A high represents one air mass

• Winds blow inward in a low-pressure system in a counterclockwise direction in the Northern Hemisphere, and in a clockwise direction in the Southern Hemisphere.– There is little to no wind in the center.– A low is surrounded by two or more air masses

Winds Associated with Pressure Systems

Thunderstorms and Tornadoes• Thunderstorms: storms with lightning, thunder, rain and

sometimes hail– The cumulonimbus clouds form in warm, moist unstable air.– Storms can attain heights up to 20 kilometers in the atmosphere.– Weather can consist of torrential rains, damaging winds,

lightning, thunder, hail, and tornadoes.– Can occur at any hour, they often occur during the afternoon

because of surface warming throughout the day causes air to become unstable.

– Consist of one or more convection cells, that can exist for an hour or less.

– Formation of new convection cells can extend the life of a thunderstorm.

– Some thunderstorms may cover a entire state and last for as long as a day.

• Thunderstorms (continued)– Most form along a large scale low-pressure system frontal

boundary which forces air to rise and can start and stop for days.

– Often occur in lines hundreds of kilometers along the frontal surfaces or ahead of the front in squall lines.

– Very large single-celled thunderstorms with particularly strong updrafts are called super cells.

• Lightning is a discharge of electricity from a thundercloud which can occur from– Cloud to ground– Cloud to cloud– Between clouds

Thunderstorms and Tornadoes

• Tornadoes is a violent rotating column of air that usually touches the ground– Destructive and often unpredictable so they are difficult to study.– Predicting the path is also difficult.– By studying the tornadoes from a distance scientists have

discovered that for a severe thunderstorm to produce a tornado a there must be a rotating updraft called a mesocyclone.

– Mesocyclone is an updraft that occurs when low-altitude winds are blowing at a different speed and in a different direction than the winds higher up.

– A mesocyclone may become visible at the base of a the storm and may lower to form a wall cloud.

– Only about a third (1/3) of the mesocyclones produce a tornado and some tornados form without a visible mesocyclone or wall cloud.

Thunderstorms and Tornadoes

– Often appear as a vortex or funnel-shaped cloud of flying debris. Some are rope shaped and others are wedge shaped with more than one vortex.

– Funnel-shape due to air pressure at the center being very low and air is sucked into the funnel expands and cools; water vapor in the air condenses, forming a funnel-shaped cloud. If the air is drier or the air pressure inside the tornado is higher, it may consist of dust and debris creating a loud roaring sound.

– They appear at the back edge of a thunderstorm and travel with the storm.

– Tornadoes can travel in any direction and often move erratically.

– Tornadoes can occur anywhere in the United States and at any time, however, they occur most often in Tornado Alley in the spring and summer.

Thunderstorms and Tornadoes

– Tornado Alley extends from Texas northward to North Dakota where conditions are more favorable to their formation.• The warming of the surface in the area in the spring and summer

along with wind conditions in the region can be ideal for the formation of mesocyclones.

• Warm, moist air may blow in from the south near the ground while cool, dry air may blow in from the southwest or west higher up in the atmosphere, setting conditions for tornadoes to form.

– Tornados are classified using the Fujita scale (F-scale) which indicates wind speed range and effects.

Thunderstorms and Tornadoes

Storm or Tornado Watches and Warnings

• Storm or Tornado Watch: If conditions are favorable for a severe storm or tornado to form a WATCH is issued.

• Storm or Tornado Warning: If a severe storm or tornado has been seen and is approaching a WARNING is issued.

Hurricanes• Hurricanes are large storms that can last for a day or

more and affect an area hundreds of kilometers wide.• Hurricane is a large rotating storm of tropical origin that

has sustained winds of at least 119 km/h.– The air pressure in the center is extremely low and gets it

energy from the heat of surface ocean water.– The winds and rains are comparatively mild in the outer

edges and they increase in intensity the closer to the eye. – The strongest winds and rain occur in the eye wall.– Inside the eye winds are mild and there is usually little to no

rain.

• Formation of Hurricanes– There must be a supply of warm, moist air for extended periods

of time. – Hurricanes are born over warm water, generally between 5o and

20o of latitude (north or south of the equator).– Because hurricanes rely on the transfer of heat from the ocean,

they form only when surface ocean waters are sufficiently warm, which is usually June through November.

– Hurricanes weaken as soon as they make land fall or move over cold ocean water.

– Hurricanes are steered by global wind patterns.– Hurricanes that occur in the Atlantic Ocean north of the

equator initially move to the west/northwest and eventually north /northeast, however, there paths can vary considerably and erratic changes can take forecasters by surprise.

Hurricanes

• Hurricanes can cause tremendous amount of damage along the coast and inland. – Damage can include winds, inland flooding, large waves, and

storm surge.– Storm surge results from the strong winds of the eye wall which

blows water into a broad dome. The dome of water can raise the sea level several meters higher than it would be otherwise.

– When a storm surge and high tides coincide extremely dangerous sea levels can swamp low-lying areas. Storm surge is what cause the massive destruction and deaths in New Orleans from Hurricane Katrina.

Hurricanes

Hurricanes• Hurricanes are classified using the Saffir-Simpson Hurricane

Scale. – All hurricanes start as tropical depression then become tropical

storms and eventually can become a hurricane. – Once a hurricane it can change categories throughout its lifecycle. – A hurricane can change categories many times during its lifecycle

depending on wind speeds.• Hurricane Watches and Warnings

– Watches are issued to communities that could see the hurricane arrive in the next 24-36 hours.

– Warnings are issued to communities that could see the hurricane in less than 24 hours.

Winter Storms• Snowstorm: involves steady snow fall• Blizzard: a winter storm characterized by high

winds, low temperatures, and falling or blowing snow. Following three criteria must be met for a winter storm to be classified as a blizzard:– Winds must exceed 56 kilometers per hour.– Temperatures must be -7oC or lower. – Falling and/or blowing snow must reduce visibility.

Forecasting Weather• Meteorologist use large amounts of data for forecasting

the weather. • Source of these data come from:– Satellite images– Radiosondes: instruments attached to balloons– Weather stations – Weather radar– Airplanes– Ships

• Meteorologist create weather maps of the data collected from weather stations.

• There is a huge amount of data that needs to be on the compact map, meteorologist developed a station model to compact the data onto the map.

• Station model: includes information on the temperature, dew point, weather conditions, wind speed and direction, barometric pressure, and cloud cover.

• This is a standard model so that a meteorologist in any country of the world can read the weather map.

Forecasting Weather

• Surface maps give meteorologists the “big picture”• Surface maps are created by computers.– First the station models are shown– Then the sea-level air pressures – Then the computer draws isobars every four millibars– Identifies highs and lows

• Forecasters draw in the fronts.– They use three types of data from the station model to find

the fronts: temperature, wind direction and dew point.– They apply the following rules in locating fronts:

• Wind direction changes behind fronts• Temperatures changes sharply across fronts• Dew point changes sharply across fronts.

Forecasting Weather

• Modern-day weather forecasts are based on computer weather models.

• Weather models are large computer programs containing mathematical equations designed to simulate atmospheric processes. Many different models are used to make forecasts.

• Some models predict weather forecasts 2 days in advance while others predict 10 days in advance.

• All models start with the current weather data, such as temperature, humidity, and wind at various levels of the atmosphere.

• The computers transfer the data into their mathematical equations and predict the future. Predictions include temperatures, winds, sea-level pressure, precipitation, and configurations of the Jet Stream.

Forecasting Weather

• Although models are useful tools in the forecasting process, subtle factors not included in the models can have a large impact on the weather.

• These factors include such things as urbanization. Urbanization is the fact that cities are warmer due to buildings and other man-made materials that absorb and hold heat that vegetation does not. – Urbanization can cause heat waves that the computer models

cannot forecast– Urbanization can cause a cold rain in the city while surrounding

suburbs receive several inches of snow.• Forecasters use their knowledge of the local area weather

patterns to modify the predictions they receive from the computer models.

Forecasting Weather