Air Masses

4.4

Air masses & Fronts• An air mass is a large body of air of similar

temperature, humidity, & air pressure.• Four major types of air masses influence

the weather in North America: maritime tropical, continental tropical, maritime polar, & continental polar.

• Tropical: warm air mass, forms in the tropics, low air pressure

• Polar: cold air mass, forms in the poles, high air pressure.

Air masses & Fronts

• Maritime: form over oceans, can be very humid.

• Continental: forms over land, can be very dry.

• In the continental US, air masses are commonly moved by the prevailing westerlies (pushing air masses from west to east) & jet streams (bands of high-speed winds about 10km above Earth’s surface).

Fronts-the boundary where the air masses meet.

• Cold front: a fast-moving cold air mass overtakes a warm air mass. (represented by triangles)

• Warm front: a warm air mass overtakes a slow-moving cold air mass. (represented by half circles)

• Stationary front: cold & warm air masses meet, but neither can move the other. (represented by half circles and triangles on opposite sides of a line)

• Occluded front: a warm air mass is caught between 2 cooler air masses. (represented by half circles and triangles on the same side of a line)

Air Masses & Fronts

• When a cold front passes through an area, colder, drier air moves in, often bringing clear skies, a shift in wind, & lower temps.

• After a warm front passes through an area, the weather is likely to be humid & warm.

Air Masses & Fronts

• If a stationary front remains stalled over an area, it may bring many days of clouds & precipitation.

• As the warm air of an occluded front cools and its water vapor condenses, the weather may turn cloudy & rain or snow may fall.

• http://www.fcgov.com/oem/oem-images/warm-front.jpg

Air masses & Fronts• A swirling center of low pressure is a

cyclone.

• Cyclones & decreasing air pressure are associated with clouds, wind, & precipitation.

• A swirling center of high pressure is an anticyclone.

• The descending air in an anticyclone generally causes dry, clear weather.

Anticyclones & Cyclones