Air Masses & Fronts

Air Masses Large body of air with a uniform temperature and moisture content. Classified according to their source region. Maritime air masses – form over the oceans Continental air masses – form over the continents In the U.S. there are four main types of air masses

Maritime Tropical (mT) Form over the tropics. Usually warm air. Common across eastern U.S. Responsible for hot humid days of summer over the south and east.

Maritime Polar (mP) Cool and moist. Usually bring cloudy, damp weather to the USA. Form over the northern Atlantic and the northern Pacific oceans. Can form any time of the year and are usually not as cold as continental polar air masses.

Continental Polar (cP) Continental air masses form over continents. Cold and dry air. Usually present during the winter months in the US Responsible for cold spells in the winter and for clear and pleasant weather in the summer for the North.

Continential Tropical (cT) Hot and very dry air Usually form over the desert southwest and northern Mexico. Can bring record heat to the plains and the Mississippi valley during the summer. Don’t usually make it to the coast.

Movement of air masses Differences of air pressure at different locations create wind patterns. Air at the equator = warm Warm air rises Cold air from the poles fills in below the rising warm air. Generally, cold air moves from the poles to the equator.

Fronts When two air masses meet, they don’t mix because of different densities. A boundary forms between these two air masses called a FRONT. Typical fronts are several hundred km long. Fronts typically don’t exist in the tropics and poles. Why?

Cold Front A cold front is formed when a cold air mass overtakes a warm air mass. The warm air rises forming clouds. Creates storms along the boundary as the warm air rises. May be violent. SQUALL LINE – long line of heavy thunder storms along a cold front boundary.

Warm Front Occurs when a warm air mass overtakes a cooler air mass. Less dense warm air rises over the cooler air. The slope is very gentle. Clouds may form far ahead of the front due to this gentle slope. Generally produces precipitation over a large area.

Occluded Front Forms when a fast moving cold front overtakes a warm front. The warm air mass is lifted off the ground. The advancing cold front then comes into contact with cold air on the other side of the lifted warm front. Broad bands of bad weather occupy occluded fronts.

Isobars Connect areas of equal pressures on a map. Closely spaced isobars incicate a rapid change in pressure. Widely spaced isobars indicate a slow change in pressure.

High Pressure Areas High pressure areas consists of air descending. As the air falls, its temperature rises High pressure areas are usually associated with good weather. The temperature of the air is going away from the dew point which causes no precipitation.

Low Pressure Areas Low pressure areas consist of rising air. Rising air cools. As the air cools, it approaches its dew point causing the formation of precipitation. Low pressure areas are usually associated with poor weather.