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Air Masses and Fronts What is an Air Mass? • Air masses are large bodies of air which have similar temperature and moisture characteristics. • Air masses form when air stays over a region (called the source region) for several days. • Air masses that form over water will be moist. • Air masses that form over land will be dry. Latitude determines temperature Moisture determined by whether it forms over oceans or continents c continental = dry A= Arctic P= Polar T= tropic m marine = moist Continental Polar does not move west Fronts – boundary that separates 2 air masses with different temperatures Cold Front a. short-lived thunderstorms b. move faster than warm fronts Warm Front – longer, steady precipitation Stationary Front • A stationary front occurs when the air masses on either side of the front are not moving toward each other. OCCLUDED FRONT Warm air mass gets caught between 2 colder air masses and is forced upward Occluded Front Winds spiral clockwise out of the High and counterclockwise into the Low Fronts are low pressure – lousy weather After the front passes – happy weather Remember this! • • • • Warm air = less dense Warm air rises! Cold air = more dense Cold air sinks! Weather Changes after Fronts Pass • 2 TOTALLY different air masses so a change in weather • Temperature changes • Wind direction changes • Wind velocity changes • Dew Point changes • Barometer falls when front approaches and rises after it passes BRRR! WINDS FLOW COUNTERCLOCKWISE IN TO THE LOW PRESSURE SYSTEM – RISING AIR WINDS FLOW CLOCKWISE OUT OF THE HIGH PRESSURE SYSTEM – SINKING AIR Hurricanes – counterclockwise low pressure systems (lousy weather) Weather in a High • Clockwise Rotation • Bright, Clear • Happy Weather LAKE EFFECT - SNOW Box 9.2 Thunderstorm Facts • At any given time there are an estimated 2000 thunderstorms in progress, mostly in tropical and subtropical latitudes. - About 45,000 thunderstorms take place each day • Annually, The U.S. experiences about 100,000 thunderstorms. • About 16 million thunderstorms occur annually around the world! • The lightning from these storms strikes Earth about 100 times each second Lightning • Within the thunderstorm clouds, rising and falling air causes turbulence which results in a build up of a static charge. The negative charges concentrate in the base of the cloud. • This first, invisible stroke is called a stepped leader. • As soon as the negative and positive parts of the stepped leader connect there is a conductive path from the cloud to the ground and the negative charges rush down it causing the visible stroke. Tornadoes • Area of quickly rotating air associated with a thunderstorm • Some contain winds of over 300 miles per hour • They can be as small as a hundred yards wide or larger than a mile wide