Chapter 9 Air Masses and Fronts Continental tropical cT Dryline Front Maritime polar mP Maritime Tropical mT Occluded front Overrunning Static stability Stationary front Warm front Key Terms Air mass Air mass modification Blizzard Cold front Continental polar cP Summary An air mass is a large body of air with similar temperature and moisture properties The movement of air masses generates fronts and causes changing weather conditions These changes are sometimes welcomed while other times they are dreaded because of the weather hazards they bring Air masses adopt the characteristics of the source regions in which they form Cold air masses are referred to as polar air masses P because they usually form in the polar regions where the surface is cold Warm air masses are of subtropical or tropical origin both are referred to as tropical T air masses that form over water are referred to as maritime m whereas those generated over continents are referred to as continental c Maritime air masses are usually cooler and moister than continental air masses formed at the same latitude Mixing and matching these categories leads to the four basic air masses types cP cT mP and mT Some classification schemes also use the letter A to denoted bitter cold Arctic air masses An air mass eventually moves and exchanges heat and moisture with the ground in migrates over in a process known as air mass modification When a cold air mass moves over a warm surface lower layers of the troposphere warm decreasing the stability of the air mass This favors rising motion which increasing the possibility of condensation and precipitation Conversely when the warmer air mass moves over a cold surface it is cooled increasing stability of the air mass and opposing the formation of clouds and precipitation Fronts form when and where air masses collide The doler air mass pushes or lside under the warmer air mass When a cold air mass replaces a warm air mass the boundary between the two air masses is called a cold front Cold fronts are often associated with a narrow band of clouds and intense precipitation A warm front occurs when the warm air mass replaces a cooler air mass Warm fronts are often associated with a large deck of steadily lowering and thickening clouds Moderate precipitation occurs as a warm front near A stationary front occurs when neither air mass is advancing Occluded fronts are said to form when a cold front catches with and overtakes a warm front warm type occlusions are most common Drylines are moisture fronts that help trigger thunderstorms over the south west US General conclusions about fronts 1 Fronts form at the boundaries between air masses of different temperatures and moisture amounts 2 Warmer air always slopes upward over colder air 3 Clouds and precipitation form as a warm air mass rises over more dense colder air 4 The front always slopes upward over the cold or more stable air 5 Pressure drops as a front approaches 6 In the Northern Hemisphere wind direction near the ground shifts clockwise as the front passes
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