Air Masses & Fronts

1. Air Masses & Fronts

2. 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

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

4. 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.

5. ContinentalPolar (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.

6. 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.

7. 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.

8. 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?

9. 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.

11. 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.

13. 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.

14. 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.

15. 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.

16. 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.