Weather Patterns 1. Planetary Wind Belts 2. Air Masses and Fronts

Weather PatternsPaula Messina1. Planetary Wind Belts2. Air Masses and FrontsPlanetary Wind Belts (1)• Maximum insolation@ Equator• Hot air rises• Minimum Insolation@ Poles• Cold air sinks• Effect: a generalcirculation fromEquator to Poles1

Planetary Wind Belts (2)• Where air rises, itexpands, it coolsdown, it producesclouds• The Equator,therefore, is a zone ofLOW PRESSURE• (Low pressure isassociated withclouds/rain, hightemperatures)Planetary Wind Belts (3)• Where air sinks, itbecomes compressed.• Any liquid water in itevaporates, causing theair to be very dry.• The Poles, therefore, arezones of HIGHPRESSURE• (High pressure is associatedwith cold, dry air)Planetary Wind Belts (4)• Air rising at theEquator also sinksat latitudes of 30 o Nand 30 o S.90 o N• Therefore, inaddition to thePoles, there are highpressure zones ofsinking air found at30 o N and 30 o S.90 o S30 o N30 o S2

Planetary Wind Belts (5)• Air rises also atlatitudes of of60 o N and 60 o S.• Therefore, inaddition to theEquator, thereare low pressurezones of risingair found at60 o N and 60 o S.60 o N0 o60 o SPlanetary Winds (6)• So, Earth isdivided intozones, each 30 oof latitude thick,of alternatinghigh and lowpressure90 o N90 o S60 o N30 o N0 o30 o S60 o SPlanetary Winds (7)• Remember thatair moves fromareas of highpressure to areasof low pressure!90 o N60 o N30 o N0 o30 o S60 o S90 o S3

Planetary Winds (8)• Remember theCoriolis Effect!• Air is deflected tothe right in theNorthernHemispehere, and isdeflected to the leftin the SouthernHemisphere!90 o N90 o S60 o N30 o N0 o30 o S60 o SPlanetary Wind Belts (9)• Planetary Wind Belts• Polar easterlies• Prevailingwesterlies• Tradewinds• Zones of Convergence(and Low Pressure)• 0 o , 60 o N, 60 o S• Zones of Divergence(and High Pressure)• 30 o N, 90 o N, 30 o S, 90 o SPlanetary Wind Belts (10)• So, in the US, weather patterns migrate across thecountry generally from southwest to northeastHHH1004 mb1008 mb1012 mb1016 mb1020 mb4

Part 2: Air Masses and FrontsWhat is an Air Mass?• An air mass is a large quantity of air that has thetemperature & humidity characteristics of thearea over which it formed.Cool andmoistCool anddryWarmand dryWarmand moistAir Mass Nomenclature• mP=maritime Polar • cT=continental Tropical• cP=continental Polar • mT=maritime TropicalmPcPcTmT5

Cold Fronts (2)mPcT• Cold air wedges its way under warm air(because cold air is denser)• This causes the warm air to riseCold frontCold Fronts (3)• When air rises, it expands• When air expands, it cools downmPcT• When it cools to its dewpoint, clouds formCold frontCold Fronts (4)mPcT• Cold fronts are associated with thick (“cumuloform”)clouds, forming from rapidly rising air• Cold fronts produce heavy precipitation for shortperiods of time• Lightning is possibleCold front7

Warm Fronts (1)• When a warm air mass bumps into a cold airmass, a warm front forms at the boundary.cTcPWarm Fronts (2)cTcP• In cross section, this is what a warm frontlooks like:Warm frontWarm Fronts (3)• Since the cold air mass is denser, it stays close tothe ground.• The warm air mass gently slides over the cold airmass• So air is rising, but not significantly, as in thecase of a cold frontcTcPWarm front8

Warm Fronts (4)• The clouds that form are relatively flat(“stratiform”) clouds• Stratiform clouds are associated with lightrain or drizzle, for long periods of time.cTcPWarm frontCold Fronts vs. Warm Fronts• Cold fronts tend to move rapidly, so heavyprecipitation or showers last for shortbursts of time• Warm fronts move more slowly—and theirboundaries span more miles—so light rainmay persist for several days.Occluded Fronts (1)mPcTcP• A warm air mass may get “sandwichedbetween” two cold air masses9

mPcTcPOccluded Fronts (2)• Two cold air masses force a warm air mass totallyoff the groundOccludedfrontmPcTcPOccluded Fronts (3)• Clouds and precipitation are similar to those of awarm front, and a cold front in successionOccludedfrontStationary Front• A stationary front may be any type of front(cold, warm, or occluded) where the airmasses stop moving• Since the front does not move, precipitationmay persist for a long period of time.10

Weather Map Symbology (1)mPcTcTcP• Cold front (west); warm front (east)Weather Map Symbology (2)mPmPcT• Occluded front (west); stationary front (east)11