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UIUC ATMS 100 - Air Masses and Fronts

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Lecture 11Outline of Last Lecture I. Upper-Level Winds ReviewII. The Geostroph WindIII. The Geostrophic WindIV. Analyzing Upper-Air MapsV. Circular MotionVI. Curved Flow Around Lows Curved Flow Around HighsVII. Curved Flow Aloft: Gradient Wind BalanceVIII. Upper-Level MapsIX. NomenclatureX. Surface WindsOutline of Current Lecture XI. What Is An Air Mass?XII. Air MassesXIII. Air Mass ClassificationXIV. Air Mass Source RegionsXV. FrontsXVI. Frontal SymbologyXVII. Frontal MotionXVIII. Cold FrontsXIX. Warm FrontsXX. Warm Fronts: MixedXXI. Stationary FrontsXXII. Occluded FrontsXXIII. DrylinesXXIV. How to Find FrontsATMS 100 1st EditionXXV. ExamplesCurrent LectureXXVI. What is an Air Mass?a. A large Body of air with similar temperature and moisture concentra-tions in the horizontalb. form over flat, homogeneous regions of the earth’s surfacei. air acquires the characteristics of the underlying srufaceXXVII. Air Massesb. *note fronts and low pressure systems seperate areas of warm and cold air*c. centers of air masses are surface high pressure systemsd. air mass boundaries are along surface troughs of low pressurei. these are called frontsii. do not confuse these with upper-air troughsXXVIII. Air Mass Classificationb. classified based on moisture and temperaturei. two-letter classificationc. first letter refers to moisture, second letter refers to temperatured. moisturei. continental (c): dry (forms over land)ii. maritime (m): moist (forms over water)e. temperaturei. Tropical (T): warm/hotii. Polar (P): cool/coldiii. Arctic (A): frigidf. cP= Continental Polari. dry and coldg. cT= continental tropicali. dry and hoth. mP= maritime polari. moist and cooli. mT= maritime tropicali. moist and warmj. cA= Continental arctici. dry and very coldXXIX. Air Mass Source Regionsb. counter-clokwise flow around low pressure systems (cyclones) moves air massesc. air masses are modified by the surfaces over which they moveXXX. Frontsb. boundaries between air massesi. do not confuse fronts and air massesc. fronts are generally strongest at the surface and weaken with heighd. temperature gradients are found on the cold side of frontsXXXI. Frontal Symbologyb. symbols point in direction that front is movingc. triangles= cold frontd. half circles= warm fronte. half circle, triangle, half circle= stationary frontf. triangle, half circle, triangle=occluded frontXXXII. Frontal Motionb. what is “ahead of” and “behind” the front?c. triangles and semi-circles point in the direction the front is movingi. direction of movement variesd. **cold air is more dense than warm air**e. fronts are identified by the wind direction (relative to the front) in the cold airi. cold front: winds in cold air blow toward frontii. stationary front: winds in cold air blow parallel to frontiii. warm front: winds in cold air blow away from frontf. region south of warm front and head of cold front is called warm sec-tori. temperatures are warm; within warm air massXXXIII. Cold Frontsb. Cold air is advancingi. winds in teh cold air blow toward the frontii. typically have a steep vertical slopec. cold air is more dense and lifts the less dense warm aird. precipitation generally showeryi. brief but intenseii. thunderstorms possible, sometimes stronge. as front passes, temperatures and dewpoints decreasei. winds shift S or SW to N, NW, or W with passagef. cold fronts gernerally extend southward or westward from a low pres-sure systemXXXIV. Warm Frontsb. warm air is advancing (cold air is retreating)i. winds in cold air blow away from frontc. less dense warm air gradually overruns more dense cold airi. gradual vertical sloped. precipitation widespread, mainly north of fronti. intensity is light to moderateii. mixture of precipitation types (snow,ice) in wintere. temperature inversions common north of warm frontsf. as front passes, temperatures and dewpoint increasei. winds shift from e to s or SW with passageg. generally extends southward or eastward from a low pressure systemXXXV. Warm Fronts: Mixedb. snow farthest into cold airc. rain closest to warm aird. sleet and freezing rain found in betweene. snow, sleet, freezing rain, rainXXXVI. Stationary Frontsb. stationary fronts do not movei. flooding riskc. winds in the cold air blow parallel to the front (generally have an east-erly component)d. mixed precipitation often north of front in winterXXXVII. Occluded Frontsb. as a low pressure system strengthens, it wraps cold air around itselfi. when the cold front “catches up” to the warm front, the warm air is forced aloft and an occluded front fromsii. symptom, not causec. generally associated with widespread rain or snowd. temperatures may rise or drop slightly, but remain colde. occluded fronts always connect either a low with another low or a low with its warm and cold fontsf. triple point:i. intersection of cold, warm, and occulded frontsXXXVIII. Drylinesb. seperate mT air (warm/moist from Gulf of Mexico, to east) from cT air (hot/dry from deserts, to west)c. typically found in KS/OK/TX during spring and can trigger strong to se-vere thunderstormsXXXIX. How to Find Frontsb. Temperature Gradienti. large change in temperature with distanceii. tightly packed isothermsc. Dewpoint (moisture) gradienti. tightly packed isodrosothermsd. Wind Shifti. remember how to determine winds from surface isobars to helpe. Trough of low pressure at the surfacef. Lines of clouds or


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