1. Air Masses and Fronts

2. What is Air Mass? A huge body of air that has similar temperature, humidity, and air pressure at any given height.

3. 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

4. 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.

5. 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.

6. 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.

7. 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.

8. 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

9. 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.

10. 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.

11. 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.

12. 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.

13. Sometimes the boundaries between fronts become distorted by land features or winds, and bends develop, swirling the air. Cyclones and Anticyclones form here.

14. 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.

15. 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.