Air Masses and Fronts Chapter 17 Section 1

Air Masses and Fronts Chapter 17 Section 1

Air Masses and Fronts n Today’s weather can be influence by air from thousands of kilometers away n A huge body of air that has similar temperature, humidity, and air pressure at any given height is called an air mass n A single air mass may spread over millions of square kilometers and be up to 10 kilometers deep

Types of Air Masses n Scientists classify air masses according to temperature and humidity n Four major types of air masses influence the weather in North America: maritime tropical, continental tropical, maritime polar, and continental polar n Recall: Temperature affects air pressure

Types of Air Masses n n Cold, dense air has a high pressure, while warm, less dense air has a lower pressure Tropical, or warm, air masses from in the tropics and have low air pressure Polar, or cold, air masses form north of 50° north latitude and south of 50 ° south latitude Maritime air masses form over oceans n n Typically very humid air Continental form over land n Typically drier air

Maritime Tropical Warm, humid air masses form over the tropical oceans n In summer, typically bring hot, humid weather n Many summer showers and thunderstorms in Eastern United States caused by this n In winter, humid air mass can bring heavy rain/snow n

Maritime Polar n Cool, humid air masses form over the icy cold North Pacific and North Atlantic oceans n Affect West Coast more than East coast n Typically bring fog, rain and cool temperatures to the West in summer

Continental Tropical n Hot, dry air masses form mostly in summer over dry areas of the Southwest and Northern Mexico n Cover a smaller area than other air masses n Move Northeast bringing hot, dry air to the Great Plains

Continental Polar Large continental polar air masses form over central and northern Canada and Alaska n Bring bitterly cold weather with very low humidity n Bring cold, clear, dry air to much of North America in the winter n Storms may occur when continental polar air masses move south and collide with maritime tropical air masses moving north n

Types of Fronts Air masses are commonly moved by the prevailing westerlies and jet streams in the US n Colliding air masses can form four types of fronts: cold fronts, warm fronts, stationary fronts, and occluded fronts n Type is determined by the characteristic of the air masses and how they are moving n

Cold Fronts Occurs when a rapidly moving cold air mass runs into a slowly moving warm air mass n Since cold fronts move quickly, they can cause abrupt weather changes, including thunderstorms, clouds, and precipitation n After a cold front moves through, colder, drier air often moves in along with clear skies, a shift in wind, and lower temperatures n

Warm Fronts A fast moving warm air mass takes over a slow moving cold air mass n If warm air is humid, light rain or snow will fall n If warm air is dry, scattered clouds form n B/c warm fronts move slowly, it may be rainy or cloudy for several days n After a warm front moves through, the weather is likely to be warm and humid n

Stationary Front Occurs when warm and cold air masses meet, but neither one can move the other n Where the warm and cool air meet, water vapor in the warm air condenses into rain, snow, fog, or clouds n If it stays stalled over an area, it may bring many days of clouds and precipitation n

Occluded Front Most complex weather situation n Warm air mass gets caught between two cold air masses n Two cold air masses mix and the warm air mass is cut off n Becomes cooler or cold on the ground n

Cyclones The L on a weather map stands for “low” and indicates an area of relatively low air pressure n A swirling center of low air pressure is called a cyclone n Cyclones play a large part in the weather of the US n Cyclones and decreasing air pressure associated with clouds, wind, and precipitation n

Anticyclones n Opposite of a cyclone----high pressure centers of dry air n Marked on a weather map with an H n The descending air in an anticyclone generally causes dry, clear weather