18 Microscale winds are the smallest scale of air motion May last for just a few seconds,chaotic in nature.
19 Dustdevils are another form of microscale winds.
20 Local Winds (These are mesoscale winds) Land and Sea BreezesMountain and Valley BreezesChinook (Foehn) WindsKatabatic (Fall) WindsCountry Breezes
21 Land and Sea BreezesTemperature contrasts (the result of the differential heatingproperties of land and water) are responsible for the formationof land and sea breezes.
22 Mountain and Valley Breezes Similar to the land and sea breeze in its diurnal cycle are the valleyand mountain breezes. Valley breezes occur in the day because airalong mountain slopes is heated more intensely than air at the sameelevation over a valley floor. Rapid radiational heat loss in theevening reverses the process to produce a mountain breeze.
23 Local winds can act together to create very strong winds Table Mountain in Cape Town, South Africa is an example of strong daily wind patterns because of the combination of afternoon mountain and sea breeze.
24 Chinook (Foehn) WindsThese winds are often caused by pressure systems on the leeward sideof mountains which pull air over the mountains. As the air descendsthe leeward slopes of the mountain it is heated adiabatically.Warm, dry winds sometimes move down the slopes of the Rockies,where they are called Chinooks, and the Alps they are called foehns.These naturally occurring winds can be very harmful to humanactivities.
25 Katabatic (Fall)Winds Cold air over highland areas is set in motion, gravity causes the air to rush over the edge of the highland like a waterfall. Katabatic winds are generally much stronger than a mountain breeze. There must be a strong temperature gradient with the colder air aloft.Diagram of Katabatic WindsSome Katabatic WindsmistralboraAntarctica is thewindiest place on earth. Wind speeds of 300 kilometres
26 Single-Cell Circulation Model George Hadley, in 1735, proposed that temperature contrastbetween the poles and the equator creates a large convection cell ineach hemisphere.Global circulation on anonrotating Earth. Asimple convection systemis produced by unequalheating of the atmosphereon a nonrotating Earth.
27 Three-Cell Circulation Model The zones between the equator and about 30 ° north and 30 ° south very much resemble the Hadley cell. Intense heating (high solar angle most of the year) results in upward motion. As the flow moves northward it begins to cool and subside.Recall the Coriolis force increases with increasing latitude. Thus, the area between ~20-35 ° is characterized by subsidence.
28 Three-Cell Circulation Model In the 1920’s a three-cell circulation model (for each hemisphere)was proposed.Features of thecirculation pattern:horse latitudetrade windsdoldrumsprevailing westerliespolar easterliespolar front
29 Observed Distribution of Pressure and Winds An imaginary uniform Earth with idealized zonal (continuous) pressure belts
30 Idealized pressure Belts Equatorial Low- warm air rising creates cell of low pressure.Intertropical Convergence Zone (ITCZ)- referred to as theconvergence zone because this region is where the trade windsconverge. Ascending air leads to cloud formation which makes thisregion clearly visible on satellite imagery.Subtropical Highs- These zones are caused primarily by Coriolisdeflection which restricts upper-level winds from moving poleward.Subsiding air and divergent winds at the surface cause warm, cloud-free weather (many large desert areas are located along thislatitudinal belt). Subtropical Highs tend to persist throughout theyear, with the center of the high migrating, and are regarded assemi-permanent pressure systems.
32 Idealized pressure Belts (cont.) Subpolar Low – located around 50 to 60 latitude. Associatedwith the polar front. The belt of low pressure is formed by theinteraction (convergence) of the polar easterlies and the westerliesPolar Highs – located over the poles! The process which producesthe polar highs is different than the process which produces thesubtropical highs. Surface cooling is the principle reason thepolar high.
33 Semipermanent Pressure Systems: Land Sea interactions and Topography complicatethe circulation patterns.We have viewed these pressure belts as continuous systems aroundthe earth up to this point. However, because the Earth is notuniform at most latitudes (more so in the northern hemisphere),the zonal belts are replaced by semipermanent cells of high andlow pressure.
34 Semipermanent Pressure Systems The real Earth with disruptions of the zonal pattern caused by large landmasses. These disruptions break up pressure zones into semi-permanent high and low pressure cells.
35 January Average Surface Pressure Systems and Associated Circulation Siberian High, Azores HighAleutian Low, Icelandic Low
36 July Average Surface Pressure Systems and Associated Circulation Bermuda High
37 Monsoons The seasonal reversal of wind direction associated with large continents, especially Asia. In the winter, the wind blows fromthe land to the sea; in the summer, it blows from the sea to theland.The Asian MonsoonThe Asian Monsoon is the result of a complex interaction betweenthe Siberian High (which is strongest in the wintertime), themigration of the ITCZ, and the topography of the region (i.e. theHimalayan mountain range and the Tibetan Plateau).
38 The Asian monsoon circulation occurs in conjunction with the seasonal shift of the ITCZ and development of the Siberian High.(dry, cool, continental air)
39 In the summer ITCZ migrates north and the Siberian High weakens which allows results in a reversal in wind direction.(moist, warm, maritime air)
40 Monsoons The North American Monsoon Extreme summertime heating over the desert Southwest, createsa low pressure system centered over Arizona that draws inwarm, moist air from the Gulf of California.