WeatherWeather

OverviewOverview

Air MassesAir MassesmPmP, , cPcP, , mTmT, , cTcT

Lifting MechanismsLifting Mechanismsconvectional, convergent, convectional, convergent, orographicorographic, , frontalfrontal

MidMid--Latitude Cyclonic SystemsLatitude Cyclonic SystemsSevere WeatherSevere Weather

ThunderstormsThunderstormsTornadoesTornadoesTropical CyclonesTropical Cyclones

WeatherWeather

Weather vs. ClimateWeather vs. ClimateWeather refers to atmospheric conditions at any Weather refers to atmospheric conditions at any given place and timegiven place and timeClimate is a long term average of rainfall and Climate is a long term average of rainfall and temperature patternstemperature patterns

Weather is the product of different air masses Weather is the product of different air masses coming into contact through pressure induced coming into contact through pressure induced movement, such that atmospheric instability movement, such that atmospheric instability is produced through lifting.is produced through lifting.

Air massesAir massesLifting mechanismsLifting mechanismsPressure systems Pressure systems

Air MassesAir MassesThe characteristics of air are influenced The characteristics of air are influenced by the region of originby the region of origin

latitude (temperature)latitude (temperature)A, AA A, AA –– Arctic, AntarcticArctic, AntarcticP P –– PolarPolarMidMid--latitudes receive air masses from higher latitudes receive air masses from higher and lower latitudesand lower latitudesTT-- TropicalTropicalEE-- EquatorialEquatorial

surface, either oceans or continents surface, either oceans or continents (moisture)(moisture)

m m –– maritimemaritimecc-- continentalcontinental

For North American weather, important air For North American weather, important air masses include:masses include:

cPcP (continental polar): cold, dry, stable air(continental polar): cold, dry, stable airmPmP (maritime polar): cold, moist, unstable(maritime polar): cold, moist, unstablemTmT (maritime tropical): warm, moist, variable (maritime tropical): warm, moist, variable stabilitystability

mTmT Gulf/Atlantic Gulf/Atlantic –– warmer, very unstable (Gulf Stream)warmer, very unstable (Gulf Stream)mTmT Pacific Pacific –– less moist, stable to conditionally unstable less moist, stable to conditionally unstable (cold ocean currents)(cold ocean currents)

Influence of Influence of mTmT Gulf/Atlantic air mass on US rainfallGulf/Atlantic air mass on US rainfall

Lifting MechanismsLifting Mechanisms

For instability, adiabatic cooling, For instability, adiabatic cooling, saturation, condensation, cloud saturation, condensation, cloud formation and precipitation to occur, air formation and precipitation to occur, air must rise.must rise.

Convectional LiftingConvectional LiftingConvergent LiftingConvergent LiftingOrographicOrographic LiftingLiftingFrontal LiftingFrontal Lifting

Convectional LiftingConvectional LiftingLand masses heat up more quickly than oceansLand masses heat up more quickly than oceans

when maritime air masses cross onto land, they experience when maritime air masses cross onto land, they experience surface heatingsurface heatingcreates low pressure, and air risescreates low pressure, and air rises

Convectional lifting common overConvectional lifting common overUrban heat islandsUrban heat islandssurfaces with low surfaces with low albedoalbedo

Convergent LiftingConvergent LiftingWinds converge on an area of low pressureWinds converge on an area of low pressureCollision of winds force air upwardsCollision of winds force air upwardsCommon in the ITCZCommon in the ITCZProduces a high amount of precipitationProduces a high amount of precipitation

OrographicOrographic LiftingLiftingIf a pressure gradient crosses a If a pressure gradient crosses a mountain range, wind flow is forced mountain range, wind flow is forced over the mountainsover the mountains

Wind travels upslope (windward side), then Wind travels upslope (windward side), then drops back drops back downslopedownslope (leeward side)(leeward side)Cooling, saturation, condensation, cloud Cooling, saturation, condensation, cloud formation and precipitation on windward formation and precipitation on windward sideside

Removes moisture from the airRemoves moisture from the air

Dry wind drops down leeward side, Dry wind drops down leeward side, warming as it dropswarming as it drops

Rain shadowRain shadow

For islands, For islands, orographicorographic rainfall is the primary source of rainfall.rainfall is the primary source of rainfall.During times of high prevailing winds, a definite rain shadow During times of high prevailing winds, a definite rain shadow effect is present. One side of the island can experience a tropeffect is present. One side of the island can experience a tropical ical rainforest climate while the other experiences tropical monsoon rainforest climate while the other experiences tropical monsoon patterns. Atolls suffer especially, since the highest point of patterns. Atolls suffer especially, since the highest point of elevation rarely exceeds 5 m above sea level.elevation rarely exceeds 5 m above sea level.

Frontal LiftingFrontal LiftingThe leading edge of an advancing air mass is called The leading edge of an advancing air mass is called a a frontfront..Cold FrontsCold Fronts

Cold, stable air advances into warm air mass, causing Cold, stable air advances into warm air mass, causing abrupt and rapid uplifting and lowering of pressureabrupt and rapid uplifting and lowering of pressure

Vertically developed clouds, thunderstormsVertically developed clouds, thunderstorms

Warm FrontsWarm FrontsWarm air advances over cold air, less abrupt liftingWarm air advances over cold air, less abrupt lifting

Form a variety of Form a variety of stratiformstratiform clouds at various altitudes clouds at various altitudes Warm air creates an inversion layer for cold air beneathWarm air creates an inversion layer for cold air beneath

With cyclonic systems:With cyclonic systems:Stationary fronts: warm and cold fronts meet headStationary fronts: warm and cold fronts meet head--ononOccluded fronts: cold front overtakes warm front in Occluded fronts: cold front overtakes warm front in cyclonic systemcyclonic system

MidlatitudeMidlatitude Cyclonic SystemsCyclonic Systems

The movement and collision of air masses The movement and collision of air masses create moving centers of low pressure create moving centers of low pressure

Tend to move from West to East, creating paths Tend to move from West to East, creating paths called called storm tracksstorm tracksCyclonic rotation of air around the low brings cold Cyclonic rotation of air around the low brings cold air masses from the north and west and warm air air masses from the north and west and warm air masses from the southmasses from the south

Creates a rotation of warm and cold fronts around the Creates a rotation of warm and cold fronts around the low pressure centerlow pressure centerCold front tends to move faster than warm front, so the Cold front tends to move faster than warm front, so the fronts eventually become occludedfronts eventually become occluded

This pattern is called a This pattern is called a MidlatitudeMidlatitude Cyclone or Cyclone or a Wave Cyclonea Wave Cyclone

CycleCycleStorm tracksStorm tracks

Higher latitudes during the summerHigher latitudes during the summerGreatest frontal activity during the SpringGreatest frontal activity during the Spring

CyclogenesisCyclogenesisForm on polar front, eastern slopes of mountainsForm on polar front, eastern slopes of mountainsIntensification of low pressure, commencement of Intensification of low pressure, commencement of rotationrotation

Open StageOpen StageCold air masses drawn from North and West, Warm air Cold air masses drawn from North and West, Warm air from the Southfrom the South

Occluded StageOccluded StageCold front overtakes warm front, forming an occluded Cold front overtakes warm front, forming an occluded frontfront

Dissolving StageDissolving StageLifting energy is spent, weather patterns dissipateLifting energy is spent, weather patterns dissipate

Severe WeatherSevere WeatherThunderstormsThunderstorms

Occur wherever there is pronounced lifting by any of the Occur wherever there is pronounced lifting by any of the mechanismsmechanisms

Common along cold frontsCommon along cold frontsRapid condensation liberates large quantities of heat energyRapid condensation liberates large quantities of heat energy

Thunderstorms produceThunderstorms produceLightningLightning

charge separation due to ice crystals at top layer of cloudcharge separation due to ice crystals at top layer of cloudPositive charges migrate to top of cloud, electrons to bottom anPositive charges migrate to top of cloud, electrons to bottom and groundd ground

Hail Hail Strong updrafts hold rain drops aloft, Strong updrafts hold rain drops aloft, recirculatingrecirculating them and freezing them and freezing themthem

Atmospheric TurbulenceAtmospheric TurbulenceIncoming gusts from convergent windsIncoming gusts from convergent windsOutgoing downdrafts pulled along by falling precipitationOutgoing downdrafts pulled along by falling precipitation

TornadoesTornadoesAs faster upper level winds overtake slower As faster upper level winds overtake slower surface winds, horizontalsurface winds, horizontal--axis turbulence axis turbulence developsdevelopsThis horizontal rotating air mass can be lifted This horizontal rotating air mass can be lifted by the updrafts of thunderstorms into a by the updrafts of thunderstorms into a vertical orientationvertical orientation

Accelerated by the cyclonic uplift of the storm Accelerated by the cyclonic uplift of the storm (positive feedback)(positive feedback)Forms a Forms a mesocyclonemesocycloneSmaller turbulent rotations within the Smaller turbulent rotations within the mesocyclonemesocycloneform Funnel Cloudsform Funnel Clouds

A tornado occurs when a funnel cloud A tornado occurs when a funnel cloud reaches the groundreaches the groundFujita Scale (Fujita Scale (F scaleF scale))

Tropical CyclonesTropical Cyclones

Form in the TropicsForm in the TropicsCoriolisCoriolis force is insufficient near the equator for cyclone force is insufficient near the equator for cyclone formationformationEnergy comes from hot ocean waterEnergy comes from hot ocean waterLow pressure centers migrate across oceansLow pressure centers migrate across oceansWinds converge, uplift, reach Winds converge, uplift, reach tropopausetropopause, diverge, diverge

Divergence aloft accelerates updraft, decreases pressureDivergence aloft accelerates updraft, decreases pressureIf a jet stream is overhead, further acceleration occursIf a jet stream is overhead, further acceleration occurs

Heat energy released by condensation further fuels Heat energy released by condensation further fuels processprocessExtreme drop in pressure causes a steep pressure Extreme drop in pressure causes a steep pressure gradient, converging winds accelerate to high speeds, gradient, converging winds accelerate to high speeds, and a tight, spiral structure is formedand a tight, spiral structure is formed

StructureStructureEyeEye

An area of calm at the An area of calm at the centercenter

Eye wallEye wallBand of intense Band of intense thunderstorms thunderstorms surrounding the eyesurrounding the eyeMost intense activityMost intense activity

Rain bandsRain bandsalternating zones of alternating zones of rainfall in spiral rainfall in spiral structurestructure

Storm surgeStorm surgeLow pressure raises Low pressure raises sea level below centersea level below center