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Air Masses and Frontsa) air mass- immense body of air, usually 1600 km or more across and several km thick, which ischaracterized by homogeneous physical properties (temp, moisture) at any given altitude-when this air moves out of its region of origin it will carry these temps and moisture conditions elsewhere eventually affecting a large portion of a continent-horizontal uniformity of an air moss is not complete-air mass weather – conditions experienced in an area as an air masses passes over it-weather will be fairly constant and may last for several days-Source regions- areas in which air masses originiate-must be an extensive and physically uniform area-area must be characterized by a general stagnation of atmospheric circulation so that air will stay over the region long enough to come to some measure of equilibriumb) classification of an air mass depends on the latitude of the source region and the nature of the surface in the area of origin – ocean or continent-classified by two letter codes -with reference to latitude air masses are placed into one of three categories-polar (P), arctic (A), or tropical (T)-lowercase letter m (maritime) or c (continental) is used to designate the nature of the surface in the source region and hence the humidity characteristics of the air mass-maritime forms over oceans – have a relatively high water vapor content compared to continental air masses that form over landmasses-mT – maritime tropical – warm and humid-mP – maritime polar – cool and humid-cA – continental arctic – very cool and very dry-cP – continental polar – cool and dry-cT – continental tropical – hot and dry-warming or cooling from below, the addition of loss or moisture, and vertical movements all actto bring changes in an air mass-when cA or cP moves over the ocean in the winter evaporation from the water brings large quantities of moisture to the once dry continental air – bc surface is warmer than mass the air gets warmed up and can change to unstable mP air mass-when air mass colder than surface is going to be warmed in its lower layers-causes greater instability that favors the ascent of the heated lower air and creates the possibility of cloud formation and precipitation (rain or thunderstorms)-visibility will be clear bc of stirring and overturning of air-air mass warmer than the surface its lower layers are chilled-a surface inversion that increases the stability of the air mass often develops-does not favor the ascent of air so it opposes cloud formation and precipitation-any clouds that do form will be stratus clouds and precip will be light to moderate-upward and downward movements induced by cyclones and anticyclones or topography can also affect the stability of an air mass. Such modifications are often called mechanical or dynamic-air mass drawn into a low – convergence and lifting occur and mass more unstable-the subsidence associated with anticyclones acts to stabilize an air mass-lifted over highlands – stability reduced; descends the leeward side – more stable-which air masses are dominant in the US during different seasons-cP and cA air masses -winter  both cP and cA are bitterly cold and very dry – strong persistent temperature inversion with the coldest temps near the ground-carries cold dry weather to the US -usually the last freeze in spring and the first freeze in autumn can be correlated with outbreaks of polar or arctic air-summer only cP air has any influence on our summer weather-summer cP air is warmer and higher moisture content than winter-still cool and relatively dry compared with air in areas further south-brings cooling relief and bright and pleasant weather -skies over great lakes exhibit long rows of dense white snow producing clouds-formed as cold dry cP air moved from land over water surface-causes lake effect snows and cause highly localized storms-account for a high percentage of the snowfall in many area adjacent to the lakes-mP air masses-form over oceans at high latitudes-north pacific and north western atlantic from newfoundland to cape cod-accompanied by low clouds and shower activity-orographic uplift can produce heavy rainfall or snow on the windward side of mountains-during summer the ocean is cooler than surrounding continent – low stratus clouds and fogs characterize much of the western coast-during summer moves inland and is heated at the surface over the hot and dry interior-acts to reduce the relative humidity in lo in lower layers and the clouds dissipate-in atlantic – weather associated with a wintertime invasion of mP air from the atlantic is known as a nor’easter - strong winds and near freezing temps, high RH and likelihood of precip-summer – brings pleasant weather-mT air masses-originate over warm waters of the Gulf of Mexico and adjacent waters-warm and humid and often unstable-export much heat and moisture to the cooler and drier areas to the north-winter – only occasionally enters central and eastern US-lower portions are cooled and stabilized – precip occurs only when mixed with cyclone-causes advection fog in the winter – warm humid air chilled as it moves over cold land-summer – mT from gulf is unstable as it moves inland over warmer land -heating of the sureface layer further increase the airs instability-bc RH high only modest lifting necessary – cumulus formed and tstorms form-in pacific responsible for large levels of precipitation causing mudslides etc. -cT air masses-only in summer do northen interior mexico and sw US produce cT air masses-unstable but bc very low RH barely any clouds are formed-can cause droughtc) Fronts-boundary surfaces that separate air masses of different densities – one of which is usually warmer and contains more moisture than another-fronts can form between ant two contrasting air masses-generally the air mass located on one side of the front moves faster than the air mass on the other side thus advances onto other front and collides with it-minimal mixing occurs – fronts maintain identity as one is displaced upwards over the other – no matter which air mass is advance it is always the warmer less dense air that rises-overrunning – process of warm air gliding up and over a cold air mass1. Warm front- warmer air invades territory formerly occupied by cooler air-on a map it is red lines with red semicircles protruding into the area of cooler air-east of rockies usually associated with mT air masses-as warm


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UD GEOG 220 - Air Masses and Fronts

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