Wind: Small-scale and Local Systems

Monsoon: a regional circulation characterized by the seasonal reversal of prevailing winds such that winds blow from land to sea in winter and vice versa in summer over eastern and southern Asia.

Monsoon winds have sufficiently long trajectories and persist long enough to be influenced by the Coriolis effect such that surface winds are deflected to the right in Northern Hemisphere and to the left in the S. Hemisphere.

Monsoon circulation results wet summers and relatively dry winters imposed by insolation, topography, and land and water distribution. Summer monsoon rains over southern Asia can reach record amounts. For example, Cherrapunji, India receives an average of 1080 cm of rainfall each year mostly between April and October. The maximum amount of rainfall that fell in a city in one day was 944 mm in Mumbai, India.

Figure 9.38, page #252 (Ahrens)

Figure 9.39, page #253 (Ahrens)

Monsoon rainfall is neither uniform nor continuous. The rainy season typically consists of a sequence of active and dormant phases. The weather is cloudy and rainy during a active period, while the weather is hot and dry during a dormant period. The sequence of active and dormant phases is repeated about every 15 to 20 days during the wet season.

The monsoon rainfall vary from one year to another. The north-south shifts of the intertropical convergence zone (ITCZ) govern the seasonality and amount of rainfall. For example, northward shift of ITCZ in spring triggers convective rains in the sub-Sahara region, while southward shift of ITCZ in autumn brings rainy season to end. However, when ITCZ does not penetrate as far north as usual, or arrives late, or moves southward early, rainfall will be below average. A succession of such summers may result a drought as observed in the sub-Sahara region.

Sea (lake) breeze: a relatively cool mesoscale surface winds directed from sea toward land in response to differential heating between sea and land. It develops during the day reaching its maximum strength by mid-afternoon. The inland extent of the breeze varies from only a few hundred meters to many tens of kilometers.

Land breeze: a relatively cool mesoscale surface winds directed from land toward sea in response to differential heating between sea and land. It develops during the night reaching its maximum strength just before the sunrise but tends to be weaker than a sea breeze.

Sea and land breezes are shallow systems, generally confined to the lowest kilometer of the troposphere and develop by a localized horizontal pressure gradient in the absence or weak synoptic-scale winds.

Figure 9.19, page #242 (Ahrens)

Sea and land breezes typically develop and diminish so rapidly that they are not significantly influenced by the Earth's rotation. The Coriolis effect may shift the direction of a sea breeze through the course of a day in some cases.

A lake breeze often develops on the shoreline of lake Michigan in spring and summer when synoptic-scale winds are relatively weak and surface waters are cooler than adjacent land surface. A shallow mesoscale high develops over the lake and surface winds diverge from the high toward the shoreline. A lake breeze front may then form following convective clouds and precipitation on the leading edge of the front.

Figure 9.20, page #243 (Ahrens)

Lake-effect snow: a highly localized fall of snow immediately downwind from an open lake. It forms along the lee shore of a large lake when cold air flows across a relatively mild lake surface. The frequency and intensity of lake-effect snow depend on i) the temperature contrast between the mild lake surface and overlying cold air, and ii) the over water trajectory, fetch, of the advecting cold air. The lake shore topography also plays an important role in governing the intensity of lake-effect snow.

Snowbelt: the region to the lee side of a large lake that is subject to frequent lake-effect storms. The greatest potential for the snowbelts in the Great Lakes region is along the eastern and southern shores of the lakes. Utah's Great Salt Lake is also subject to the lake-effect snow but it is generally less intense than its Great lakes counterparts.

Figure 1, page #297 (Ahrens)

Figure 2, page #297 (Ahrens)

Heat island circulation: a local circulation between relatively warm urban areas and its relatively less warm surroundings. It develops when synoptic-scale winds are weak and transports air pollutants, mainly aerosols, in a dust dome over a city. The pollutants may become as much as a thousand times more concentrated over a urban area than in the air over the rural area. If local winds strengthen to more than about 15 km/hr (9 mi/hr), the dust dome elongates downwind to form a dust plume which spreads the urban pollutants to the rural areas.

The factors contribute the temperature contrast between the urban and rural areas: i) high concentration of heat sources in urban areas such as industry, furnaces, cars, ii) high rate of conduction in urban building materials, and iii) low rate of evapotranspiration with higher Bowen ratio (ratio of sensible heating to latent heating) in urban areas.

Katabatic wind: the downslope flow of cold and dry air under the influence of gravity. It usually originates over extensive snow-covered plateaus or other highlands.

Most katabatic winds are weak, usually under 10 km/hr (6.2 mi/hr), however, in some places where steeper slopes are present and/or katabatic winds are channeled by narrow valleys, the katabatic winds are sometimes accelerated to potentially destructive speeds such as gusts of 100 km/hr (62 mi/hr) observed along the edge of the massive Greenland and Antarctic ice sheets.

Figure 9.25, page#245 (Ahrens)

Mistral: a katabatic wind that flows from the Alps down the Rhone River Valley of France to the Mediterranean coast.

Bora: a katabatic wind that originates in Serbia and flows onto the coastal plane of the Adriatic Sea. Bora can produce gusts of 50 to 100 km/hr (31 to 62 mi/hr).

Columbia Gorge wind: a katabatic wind that originates Columbia plateau and flows westward through the Columbia River Gorge as a strong, gusty, and sometimes violent wind.

Chinook wind: the downslope flow of compressionally warmed and dry air. It usually originates over areas where surface temperatures often climb abruptly tens of degrees in response to compressional warming.

Chinook winds develop when the strong winds force a layer of stable in the lower troposphere to ascend the windward slopes of a mountain range and when the circulation about an anticyclone or cyclone situated well to the lee of a mountain range which will pull the air down the leeward slopes.

Figure 9.26, page #247 (Ahrens)

Foehn: a chinook wind that is drawn down the Alpine valleys of Austria and Germany.

Zonda: a chinook wind that is drawn down the Andes mountain valleys of Argentina.

Chinook winds are associated with downwind segment of a mountain waves. In a violent chinook, very energetic turbulent eddies that are generated aloft are transported downward into the foothills so that surface winds become very strong and gusty locally reaching destructive speeds such as gusts of 160 km/hr (100 mi/hr) observed along the foothills of the Front Range of the Rocky Mountains.

Figure 4, page#246 (Ahrens)

Santa Ana wind: a hot an dry chinook-type wind that blows downward from the desert plateaus of Utah and Nevada toward coastal southern California. It typically develops in autumn and winter when a strong high pressure center centered over Great Basin. Santa Ana winds sometimes gusts to 130 to 145 km/hr (80 to 90 mi/hr) and almost always cause some property damage, also forest and brush fires.

Figure 9.29, page #248 (Ahrens)

Desert winds: the regional gusty surface winds associated with vigorous convection (absolutely unstable) depending upon the intensity of the solar radiation such that the wind speed and gustiness usually peak in the early afternoon and during the warmest months.

Dust devils: swirling mass of dust caused by intense solar heating of dry surface areas. They are microscale systems, usually less than 1 m in diameter that typically last less than 1 minute, however, they occasionally exceed 100 m in diameter lasting for 20 minutes or longer. Such dust devils may be visible to altitudes topping 900 m (3000 ft.), but the rising air column may reach 4500 m (14,750 ft.). Dust devils can cause substantial damage when surface winds exceed 73 km/hr (45 mi/hr).

Figure 9.35, page#251 (Ahrens)

Figure 9.36, page#251 (Ahrens)

Sand and dust storms are produced by the larger scale winds of thunderstorms or migrating cyclones in deserts. Sand storms differ from dust storms based on particles size range: Sand, which cover a small fraction of desert terrain, consists of large particles (0.06 to 2.0 mm in diameter), while dust consists of very small particles (less than 0.06 mm in diameter).

Haboob: a dust storm caused by the downdraft of a desert thunderstorm. It may have more than 100 km wide and may reach altitudes of several kilometers. It is most common in Sudan of North Africa, but also occur in the southwest US. A haboob severely restrict the visibility.

The highest temperature ever recorded in the Western Hemisphere was at Death Valley, California, 57C (134F), just 1C (2F) less than the world record high temperature set at El Azizia, Libya.

Figure 9.34, page#250 (Ahrens)

Mountain and valley breezes: a local wind system of a mountain valley that blows downhill (mountain breeze) at night and uphill (valley breeze) during the day. Valley breezes are particularly developed on south-facing slopes of a mountain in summer, where sunlight is more intense in the Northern Hemisphere.

Figure 9.23, page #244 (Ahrens)

Local cold winds

Buran, a strong cold wind that blows over Russia and Central Asia.

Purga, a buran accompanied by blowing snow.

Pampero, a cold wind blowing from south to Argentina, Uruguay, and into the Amazon basin.

Burga, a cold northeasterly wind in Alaska usually accompanied by snow.

Bise, a cold north or northeasterly wind that blows over southern France.

Papagayo, a cold northeasterly wind along Pacific coast of Nicaragua and Guatemala.

Tehuantepecer, a strong northlerly or northwesterly wind funneled through the gap between Mexico and Guatemala.

Local mild winds

Levanter, a mild, humid, often rainy east or northeast wind that blows over the southern Spain.

Harmattan, a dry, dusty, but mild wind from northeast or east that originates over the coll Sahara in winter and blows over the west coast of Africa.

Local hot winds

Simoom, a strong, dry, and dusty wind that blows over the African and Arabian desets. If it is from Morocco to Atlantic Ocean, it is named as Leste . If it is from Algeria to Spain, it is called Leveche . If it blows from Tunisia and Libya to Central Mediterranean and Italy, it is called Sirocco . If it is blowing over Eygpt, it is called Khamsin , while over Israel, it is called Sharav .

Figure 9.37, page #252 (Ahrens)