Air Masses and Fronts

Chapter 11 Part 1

March 17, 2009 Air masses

• Air mass = large body of air that has similar temperature and humidity properties throughout • PtifiProperties of air masses, h owever are mo difidthdified as they move over areas outside where they originate • Source regions = regions where air masses develop their characteristics – need large areas with similar characteristic • Mid-latitudes not considered source regions for air masses as temppyyyerature, humidity vary considerably • Rather mid-latitudes are greatly affected by cold or warm, moist or dry air masses moving in Cold air mass over eastern, central US Air Mass Classification • Air masses have 4 major classifications • They are Polar (P) or Tropical (T), continental (c) or maritime (();m); extremely cold air masses are Arctic (A) • Winds aloft may move air masses from their source regions • Cold air mass moving over warmer surface warmed form below - instability- may have clouds, showers • Warm air mass moving over cold surface - stable- can have fog, stratiform coluds, pollution buildup Air Mass Classification Continental polar (cP) and Continental Arctic (cA) air masses • cP and cA air masses originate in northern Canada and Alaska and bring very cold air to much of the US at times in winter- following passage of cold fronts associated with m id-ltitdlatitude cyc lones • Long clear nights in winter allow for strong radiational cooling – thus these air masses are very cold- also very dry • Due to lack of terrain, air easily moves south through the Great Plains- warms a bit, but still dry so few clouds • Air mass stable- subsidence and warming aloft, cold air below- good for pollution buildup • Western US less influenced by cP or CA air masses as coldest air usually stays east of continental divide Air mass source regions and paths 2 very cold arctic outbreaks- including one setting records in Las Vegas Lake effect snow • Lake effect snows- cP or cA air mass moves cold air over relatively warm Great Lakes in early to mid winter • Air above lake warms and is saturated, cumulus clouds fditbliform and grow in unstable air- snow bibegins an did is enhanced by lifting of air over hills and convergence at surface due to slowing of air from increased surface friction • Lake effect snows greatest within first few kilometers of lklake, ex tendi ng up to 50k50 km ilinlan d • Need reasonable travel distance (fetch) over water (80 km or so) for significant lake effect snow • Can occur on lee side of other large lakes (e.g. Great Salt Lake) if land/lake temperature difference is large Formation of Lake Effect Snows

• Cold, dry air crossing the lake gains moisture and warmth from the water • The more buoyant air now rises, forming clouds that deposit large quantities of snow on the lake’s leeward shores Profiles of cP Air Masses

• cA air masses reaching south Texas and south Florida may cause large damages to fruit and veggpetable crops • Summertime excursions of cP air into the US are moderately cool and dry and give a break to aeras ithin the eas tern US USf from hot, humid weather Maritime Polar (mp) air masses

• Win terti me cP and c A a ir masses over nor thern Asia and the Arctic travels over the Pacific Ocean by the Aleutian Low and is modified by added warmth and moisture- changes into maritime polar air mass (mP) • Conditionally unstable- as it moves inland over mountains, orographic precipitation common • Dries out as it moves over series of mountain ranges in western US • Can have chinook winds develop • Brings moderate weather to east slope of Rockies when replacing cP or cA airmass mP Modification of mP airmass crossing western US mP airmass less common in northeastern US- gives cold , damp drizzly weather to NE US Hatteras lows and Nor’easters may develop Maritime Tropp()ical (mT) air masses

• Wintertime source for mT airmasses is subtropical east Pacific- warm, humid air coming from near Hawaiian Islands can produce very heavy rains and floo ding al ong wes t coast ( someti mes ca lle d “Pineapple Express”) • EtEastern US USTif mT air from G GlffMiulf of Mexico and Caribbean common in summer, less so in winter • Win ter – mTllfidtGlfCtFlidT usually confined to Gulf Coast, Florida • Briefly may move north, causing advection fog and llditlow clouds as it moves over co lddld ground • mT airmass pushed back to south from cold front bibring ing in c P a irmass mT airmass into California- heavy rain (Pineapple Express) mT eastern US , cP western US Summer mT Effects

• mT airmass frequent over eastern US in summer- clockwise circulation around Bermuda High • Summer mT airmass brings much moisture and conditional instability- thunderstorms common • Southwestern monsoon - warm moist air from Gulf of Mexico or Gul of Mexico Continental Tropical Air Masses

• cT source areas in northern Mexico and southwestern US during summer • Dry, hot, usually upper level high and subsidence • Sometimes moves into Great Plains cT airmass into central, western US Air mass summary

• Air mass i s l arge b o dy o f a ir w ith fa ir ly un iform properties of temperature and humidity • Air mass source regi ons t end t o b e fl at with hig h pressure and light winds • CtitliContinental air masses form over ldland, mar itime over water • PlPolar a ir masses from po lar reg ions; trop ica l a ir masses from warm, tropical regions •cPlddAP= cold, dry; cA=very co lddThtdld, dry; cT=hot, dry; mT=warm, moist; mP=cool, moist Fronts • Front = transition zone between 2 air masses of different densities (temperatures) and often humidity • Frontal location often identified by: 1. Sharp temperature difference over short distance; 2. Change in moisture content (e.g. dew point temperature); 3. Shift in wind direction; 4. Pressure and pressure changes; 5. Clouds and precipitation patterns • Lowest pressure at frontal boundary Example surface map showing pressure systems, fifronts, air masses, i ibiiisobars, precipitation Note kink in Example cold isobars and wind ftdfront and shift at front associated su rface weather

Pressure falling before frontal passage, rising after Cold Fronts • Cold, dry stable air replaces warm, moist unstable air • Blue line w ith blue tr iang les

• Clouds of vertical development • Thunderstorms, squall lines Vertical structure of cold front

Cold air wedges under warm air Front is steep near the ground at front, causing warm air to rise due to friction typical slope often prod uci ng sh owers or abt150tilbout 1:50 typical spee dbtd about thunderstorms 25 knots