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Warm-Up How are air masses named? What are the characteristics of a cT air mass? Which types of air masses have the greatest effect on much of the air conditions in the United States? Fronts Chapter 20, Section 2 Formation of Fronts When two air masses meet, they form a front, which is a boundary that separates two air masses Fronts can form between any two contrasting air masses In contrast to the vast sizes of air masses, fronts are narrow (~15-200 km wide) In the ideal case, the front would move in the same direction with warmer air overlying cooler air However, the distribution of pressure across a front causes one air mass to move faster than the other, causing it to advance into the slow air mass Warm Fronts A warm front forms when warm air moves into an area formerly covered by cooler air The surface position of a warm front is shown by a red line with semi-circles that point toward the cooler air The first sign of the approaching warm front is the appearance of cirrus clouds Because of their slow rate of movement and very low slope, warm fronts usually produce light-to-moderate precipitation over a large area for an extended period A gradual increase in temperature occurs with the passage of a warm front, as does a wind shift from the east to the southeast Warm Front Cold Fronts A cold front forms when cold, dense air moves into a region occupied by warmer air On a weather map, the surface position of a cold front is shown by a blue line edged with blue triangles that point toward the warmer air mass As a cold front moves in, it becomes steeper and advance more rapidly than warm fronts The forceful lifting of air along a cold front can lead to heavy downpours and gusty winds As a cold front approaches, towering clouds can often be seen in the distance The weather behind a cold front is dominated by a cold air mass, so weather clears shortly after the front has passed Cold Front Stationary Fronts Occasionally, the flow of air on either side of a front is neither toward the cold air mass nor toward the warm air mass, but almost parallel to the line of the front The surface position of the front does not move, and a stationary front forms On a weather map, stationary fronts are shown by blue triangles on one side of the front and red semicircles on the other Sometimes, gentle to moderate precipitation occurs along a stationary front Occluded Fronts When an active cold front overtakes a warm front, an occluded front forms The weather associated with an occlude front is complex: most precipitation is associated with the warm air’s being forced upward Remember, fronts, like all aspects of nature, do not always behave as we expect them to Formation of an Occluded Front Middle-Latitude Cyclones The main weather producers in the U.S. are middle-latitude cyclones, denoted by the letter L Middle-Latitude Cyclones – large centers of low pressure that generally travel from west to east and cause stormy weather The air in these weather systems moves in a counterclockwise direction and in toward the center of the low Most middle-latitude cyclones have a cold front, and frequently a warm front, extending from the central area Forceful lifting causes the formation of clouds that drop abundant precipitation MiddleLatitude Cyclone Model Cloud Patterns Associated With MiddleLatitude Cyclones The Role of Airflow Aloft Airflow aloft plays an important role in maintaining cyclonic and anticyclonic circulation The rotating wind systems are actually generated by upper-level flow Surface convergence must be offset by outflow somewhere higher in the atmosphere As long as the spreading out of air high up is equal to or greater than the surface inflow, the lowpressure system can be sustained More often than not, air high up in the atmosphere fuels a middle-latitude cyclone In an anticyclone, air spreading out at the surface is balanced by air coming together from high up Role of Airflow Aloft in Cyclonic Activity Assignment Read Chapter 20, Section 2 (pg. 564-570) Do Section 20.2 Assessment #1-9 (pg. 570)