MSU GEO 203 - Lecture #13 wrap-up and Lecture #14

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GEO 203 Notes – 11/4/10 (Lecture #13 wrap-up and Lecture #14)Review: PGF drives windForce has two attributes: magnitude, directionPGF always points directly from high to low perpendicular to isobarMore Coriolis force as latitude increasesCentrifugal force is directly proportional to velocity²; inversely proportional to radiusFrictional force always in opposite direction of windNo Coriolis/centrifugal/frictional force if wind speed = 0Pressure gradient force + Coriolis force = geostrophic windWind under balance of PGF and Coriolis forcesPressure gradient force + Coriolis force + Centrifugal force = gradient windUpper-level wind that is parallel to curved isobarsPressure gradient force + Coriolis force + Frictional force = near surface windCounterclockwise, inward around low pressure centerClockwise, outward around high pressure centerLecture #14: Local WindsScales of MotionMicroscale: < 1kmLifetime: seconds to minutesi.e. turbulent eddiesMesoscale: 1km to 200kmLifetime: hours, up to one dayi.e. land/sea breezes, mountain/valley winds, thunderstorms, tornadoes/waterspouts/dust devilsDivided into mesoalpha/gamma/betascale based upon life spanSynoptic scale: 200km to 2000kmLifetime: days to weeksi.e. hurricanes, tropical storms, lows/highs, fronts, everyday weatherCovers region about the size of a statePlanetary scale: > 2000kmLifetime: weeks or longeri.e. longwaves in westerliesObstacles and surface friction creates eddies of whirling windsTurbulence is more intense (esp. in afternoon when atmosphere is most unstable)—larger eddies than normal (thermals)Dust DevilsTypical diameter: 10-300ft.Average height: 500-1000ft.Lifetime: a few minutesWind shear twists rising air (in hot dry environment), forming a rotating columnAlso occurs on MarsClear Air Turbulence (CAT): strong wind shears when there are large changes in wind speeds over a shortdistanceCan be dangerous for aircraft; see example in chapter of bookAir over mountains produces waves, can create vertical rotors/roll eddiesForms rotor cloudsEddies can also be produced by vertical wind shear when wind passes through an obstacle (i.e. strong wind over hill next to road produces rotor effect)Windmill should not be placed here (on PowerPoint, C is worst place for construction)B is best place for windmill construction—wind speed increases with heightConstruction also good over lakes; lower friction over water = higher wind speedRising warm air creates surface low, upper level highSinking cool air creates surface high, upper level lowWhen water is cooler than land during day, wind will move toward land (due to difference in pressure), forming sea breezeAt night, water is warmer than land; land breeze blows from land to waterMust occur over larger bodies of water so higher change in temperature/pressure occurs and pressure gradient can be formedSea breeze is stronger than land breezeBoth develop diurnallySea breezes can cause cool coastal communities, more humidity/haze/fog/thunderstormsExample: Florida (highest t-storm frequency in US due to sea breezes on both sides of peninsula)Clouds and thunderstorms also develop in UP of MI due to convergence of lake breezes from Lake Superior and Lake MichiganLower wind speed reduces Coriolis force—therefore, higher wind speed over water (due to less friction) increases Coriolis force, causing the wind to deflect to the rightIncrease in wind speed = divergence, sinking motionDecrease in wind speed = convergence, rising motionMore clouds/t-storms downwind of water surfaceMonsoon: seasonal phenomenon common in eastern/southern AsiaSimilar to huge land/sea breezesWinter: strong cooling = high pressure over SiberiaSummer: air over Asia heats/rises = convergence = lifting = heavy rain formationMexico Monsoon affects SW USDry westerly winds in spring/early summer replaced by moist southerly winds in summerLow-level moisture carried from Gulf of California into SW USAnnual MI precipitation: 35-40”Upslope flow during daytime, downslope flow at night (mountains)Daytime: air from plain to valley; switches at nightKatabatic wind in Antarctica (wind from mountaintop over lower valley)Chinook (Foehn) wind: warm, dry wind that descends leeward slopes of high mountainsAlso called “Snow Eater” due to warm temperatures and strong wind that causes snow to rapidly sublimate and disappearRelative humidity: <


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