Transcript Air Masses and Fronts

Air Masses and Fronts
What is Air Mass?
• A huge body of air that has similar
temperature, humidity, and air pressure
at any given height.
Types of Air Masses
• Tropical – warm air masses that form in the
tropics and have low air pressure.
• Polar – high air pressure cold air masses that
form north of the 50° north latitude and south
of the 50° south latitude.
• Maritime – air masses that form over oceans
• Continental – air masses that form over land
*think: continent = land
Maritime Tropical Air Masses
• Warm, humid air masses forming over tropical
oceans.
– In the East, form in the Gulf of Mexico and bring
warm, humid air to the eastern United States
– In the West, form in the Pacific Ocean and bring
warm, humid air to California and the West Coast.
Maritime Polar Air Masses
• Cool, humid air masses that form over the icy
cold North Pacific and North Atlantic oceans.
– In the East, form in the Atlantic Ocean and are
often pushed out to sea by westerly winds.
– In the West, form in the Pacific Ocean and bring
cool, humid air to the West Coast.
Continental Tropical Air Masses
• Hot, dry air masses form mostly in the
summer over dry areas of the Southwest and
northern Mexico.
• Cover a smaller area than other air masses,
and bring hot, dry weather to the southern
Great Plains.
Continental Polar Air Masses
• Form over central and northern Canada near
the Arctic Circle and bring bitterly cold
weather with very low humidity.
– In the winter, bring cold, dry air to much of North
America.
How Air Masses Move
• Prevailing Westerlies – the major wind belts
over the continental US, push air masses from
west to east.
• Jet Streams – within the prevailing westerlies,
blow from west to east 10 km above Earth’s
surface, carrying air masses along their tracks.
• Fronts – The boundary where air masses
meet. Where storms and changeable weather
often develop
Cold Front
• A fast-moving cold air mass overtakes a slowly
moving warm air mass. The denser cold air
slides under the lighter warm air.
** think convection currents!**
• Clouds form here, and heavy rain or snow may
form. Cold fronts leave cooler weather behind.
Warm Front
• A fast-moving warm air mass overtakes a
slow-moving cold air mass. The warm air
moves over the dense cold air.
• This front brings cloudy and sometimes rainy
weather, and leaves warm, humid weather
when it passes.
Stationary Front
• When cold and warm air masses meet, but
neither can move the other.
*think stationary = stuck*
• Rain, snow, fog, or clouds form here, and can
bring several days of clouds and precipitation.
Occluded Fronts
• A warm air mass is caught between two cooler
air masses.
• Dense cool air masses move under a warm air
mass and push the warm air up.
– The warm air mass is occluded, or cut off, from
the ground.
• Clouds, snow, or rain may happen here, and
leave cooler temperatures behind.
Sometimes the boundaries between fronts
become distorted by land features or winds, and
bends develop, swirling the air.
Cyclones and Anticyclones
form here.
Cyclones
• A swirling center of low air pressure.
• As warm air at the center rises, the air
pressure decreases. Cooler air blows toward
the pressure from nearby areas where the air
pressure is higher.
• As air rises in a cyclone, the air cools, forming
clouds and precipitation.
• Cyclones and decreasing air pressure are
associated with clouds, wind, and
precipitation.
Anticyclones
• High-pressure centers of dry air.
• Winds spiral out from the center of an
anticyclone, moving toward areas of lower
pressure.
• Cool air moves down from higher in the
troposphere. As the cool air falls, it warms up,
and the relative humidity drops.
• The descending air in an anticyclone causes
dry, clear weather.