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Chapter 11 AIR MASSES AND FRONTS Air Mass an extremely large body of air whose properties temperature and humidity are uniform Essentially this is air that has all come from one place and retains the characteristics of that place Source Region the region where a given air mass originates Flat Uniform composition Light surface winds Usually dominated by high surface pressure Source region characteristics The longer the air stays in one place the more properties of the region it will acquire Ex icy arctic plains and subtropical oceans The mid latitudes are not good source regions because temperature and moisture vary greatly However they re good for weather producing interactions between air masses Air masses are names based on their temperature and humidity characteristics source region Arctic coldest Polar cold Tropical warm Temperature Humidity source Continental dry land Maritime moist water Continental Arctic Polar cA cP In the winter this air mass brings dry frigid air south into the U S This can produce lake effect snow when cold air blows over relatively warm lakes in early winter In the summer the source region experiences long days which melt snow and warm land The leading edge of a cA air mass moving southward is marked by a cold front Maritime Polar mP Because mP air masses come from water they are moist and polar regions they are cold they have high relative humidity Rain snowfall can be significant if this air lifted Various mountain ranges in the west or the Appalachians in the east will do this After crossing several mountain ranges cool moist maritime polar mP air from off the Pacific Ocean descends the eastern side of the Rockies as modified relatively dry Pacific air Maritime Tropical mT I e lots of water vapor content Air originating from tropical oceans is warm and humid This air easily produces cumuliform clouds in areas like the Gulf which contribute to daily afternoon showers and thunderstorms Continental Tropical cT Dry hot air originating from northern Mexico Low relative humidity means that condensation can t occur until after several kilometers of lift High pressure aloft remember warm air column produces sinking motion so condensation is even rarer yet Lake effect snow With the addition of water vapor from the lake snow storms form leeward of the lake Front is an example of modification of continental polar air masses Air blowing over a like warms significantly when the water is much warmer The rest of the air mass being cold the air becomes buoyant and rises Fronts often separate air masses with different temperature characteristics Fronts can also separate air masses with different humidity characteristics i e a dry line the transition zone between two air masses of different densities Temperature differences cause density differences Fronts are named for the air mass that replaces the other air mass Fronts are usually analyzed at the surface but they can have significant vertical extent A front is usually defined by sharp temperature gradients between air masses We consider its strength to be related to this Frontogenesis Frontolysis Lines are drawn on the map at the leading edge of the transition zone the formation or strengthening of a front i e increasing the temperature gradient the weakening or dissipation of a front i e decreasing the temperature gradient Cold air replacing warm air cold front Stationary Front Moves little or not at all stationary Winds generally blow parallel to stationary front but opposite directions on either side of it Separates different types of air masses Can be associated with dry weather or widespread precipitation Depends on the types of air masses that it is separating Cold Front Warm Front A cold front is a zone where cold air replaces warm air Warm air is lifted up and over the approaching cold stable air Because of friction cold fronts are vertically steep with strong upward forcing Heavy rain and thunderstorms are common along and just behind a cold front A warm front is a zone where cold air retreats and warm air advances Warm air rising up and over the cold air in this way is called overrunning Steady light to moderate precipitation is typically found ahead of or along a warm front Occluded Front when a cold front catches up and merges with a warm front when the warm front rides up and over the cold front The air behind cold front is colder Warm Occlusion when the cold front rides up and over the warm front Dry Line The air ahead of warm front is colder a front that separates air of different humidities Cold Occlusion Commonly found in Texas in the spring separating continental tropical and maritime tropical air Thunderstorms do not always develop along dry lines but when they do they are often severe Chapter 14 THUNDERSTORMS AND TORNADOES a storm containing lightning and thunder Thunderstorm Single cell ordinary non severe Types of thunderstorms Multi cell Supercell often severe Necessary ingredients for thunderstorm development Conditionally unstable or absolutely unstable atmosphere Sufficient moisture A trigger or forcing mechanism Warm air rising along a frontal zone Turbulent eddies Unequal heating at the surface leading to thermals Terrain upslope motion Converging surface winds Ordinary single cell Thunderstorms Also known as air mass thunderstorms pop up thunderstorms Characteristics Small scale 1 km wide Short duration Predictable life cycle Usually not severe Life cycle of an Ordinary Thunderstorm Cumulus growth Stage Only updrafts exist rising motion No precipitation lightning or thunder Cumulus cloud grows vertically Latent heat is released in the cloud Helps keep the cloud warmer than its environment Allows for air to continue rising As the cloud build dry air from outside the cloud mixes with the edge of the cloud entrainment The mixing causes some evaporation and cooling the cooler air descends downdraft Mature Stage Most intense stage with updrafts and downdrafts now present When downdraft hits surface air spreads out in all direction Precipitation thunder and lightning begins The leading edge of which is called a gust front The downdraft chokes off the updraft Dissipating Stage With no updraft the thunderstorm cannot sustain itself therefore it dissipates Light precipitation may fall Lower clouds begin to evaporate leaving only the cirrus anvil Multi cell Thunderstorms Tornadoes the ground arcus cloud associated with intense thunderstorms a multi cell thunderstorm complex that is oriented


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FSU MET 1010 - AIR MASSES AND FRONTS

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