Local Winds over the Globe

Local Winds

Local Winds over the Globe

  • Local Winds over the Globe are winds that affect local weather.
  • Local winds usually affect small areas and are confined to the lower levels of the troposphere.

Some of the local winds are

Land and sea breezes

  • Land and sea breezes are prevalent on the narrow strips along the coasts or a lake.
  • It is a diurnal (daily) cycle, in which the differential heating of land and water produces low and high pressures.
  • During the day when a landmass gets heated more quickly than the adjoining sea or large lake; air expands and rises. This process produces a local low-pressure area on land.
  • Sea breeze then develops, blowing from the water (high pressure) towards the land (low pressure).
  • The sea breeze begins to develop shortly before noon and generally reaches its greatest intensity during mid-day to late afternoon. These cool winds have a significant moderating influence in coastal area.
  • At night, the land and the air above it cools more quickly than the nearby water body.
  • As a result, land has high pressure while the sea has comparatively a low pressure area.
  • Gentle wind begins to blow from land (high pressure) towards sea (low pressure). This is known as land breeze (see fig.)
Local Winds over the Globe

Mountain and valley breezes

  • Another combination of local winds that undergoes a daily reversal consists of the mountain and valley breezes.
  • On a warm sunny day, the mountain slopes are heated more than the valley floor.
  • Hence, the pressure is low over the slopes while it is comparatively high in the valleys below.
  • As a result gentle wind begins to blow from the valley towards the slopes and it assumes the name of valley breeze (see Fig.)
  • After sunset, the rapid radiation takes place on the mountain slopes. Here, high pressure develops more rapidly than on the valley floor.
  • Cold arid heavy air of mountain slopes starts moving down towards the valley floor. This is known as the mountain breeze (see Fig.)
  • The valley and mountain breezes are also named as anabatic and katabatic breezes respectively.
Local Winds over the Globe

Hot winds

Loo, Foehn, and Chinook are important hot winds of the local winds category.

Loo 

  • Loo are hot and dry winds, which blow very strongly over the northern plains of India and Pakistan in the months of May and June.
  • Their direction is from west to east and they are usually experienced in the afternoons.
  • Their temperature varies between 45°C to 50°C.

Foehn

  • Foehn is a strong, dusty, dry, and warm local wind that develops on the leeward side of the Alps mountain ranges.
  • Regional pressure gradient forces the air to ascend and cross the barrier.
  • Ascending air sometimes causes precipitation on the windward side of the mountains.
  • After crossing the mountain crest, the Foehn winds start descending on the leeward side or northern slopes of the mountain as warm and dry winds.
  • The temperature of the winds varies from 15°C to 20°C which helps in melting snow. Thus making pasture land ready for animal grazing and helping the grapes to ripe early.

Chinook

  • Chinook is the name of a hot and dry local wind that moves down the eastern slopes of the Rockies in the U.S.A. and Canada.
  • The literal meaning of chinook is ‘snow eater’ as they help in melting the snow earlier.
  • They keep the grasslands clear of snow. Hence they are very helpful to ranchers.

The Santa Ana

  • A hot, dry, strong, blustery, föhn-type wind which blows from the north-east or east over southern California and carries with it large quantities of dust.
  • It is most frequent in winter but may also occur in spring or autumn.
  • It may get its name from the Santa Ana Mountains or the Santa Ana Canyon but other possibilities are that it derives from santanas, meaning ‘devil winds’, or the Spanish Satanás, meaning Satan.
  • These winds can cause a great deal of damage.
  • As they are hot and dry, they cause vegetation to dry out, so increasing the risk of wildfires; and once fires start the winds fan the flames and hasten the spread of the fires. In spring, Santa Ana winds can cause considerable damage to fruit trees.

Sirocco

  • Sirocco is a Mediterranean wind that comes from the Sahara and reaches hurricane speeds in North Africa and Southern Europe.
  • It arises from a warm, dry, tropical air mass that is pulled northward by low-pressure cells moving eastward across the Mediterranean Sea, with the wind originating in the Arabian or Sahara deserts.
  • The hotter, drier continental air mixes with the cooler, wetter air of the maritime cyclone, and the counter-clockwise circulation of the low propels the mixed air across the southern coasts of Europe.
  • The Sirocco causes dusty dry conditions along the northern coast of Africa, storms in the Mediterranean Sea, and cool wet weather in Europe.

Harmattan

  • The warm and dry winds blowing from northeast and east to west in the eastern parts of the Sahara desert are called harmattan.
  • These winds become ex­tremely dry because of their journey over the Sahara desert.
  • While blowing over the Sahara desert these winds pick up more sands mainly red sands.
  • The western coast of Africa is warm and moist and hence the weather becomes unpleasant because the weather con­ditions characterized by high temperature and high relative humidity become injurious to human health.
  • The weather becomes suddenly dry and pleasant at the arrival of harmattan as the relative humidity of the air is remarkably reduced due to the high temperature and hyper-aridity of harmattan.
  • This is why Harmattan is known as the ‘doctor’ in the Guinea coastal area of western Africa.
  • In fact, harmattan is very dusty and stormy wind blowing with so gusty speed that trees are up­rooted.
  • These winds are usually associated with dust storms resulting into marked reduction in the visibil­ity.
  • Harmattan becomes more vigorous during summer months.
  • In fact, harmattan is a special type of north­east trade wind.
  • It becomes extremely warm wind because of the hot and dry desert of the Sahara.
  • Similar warm, dry, very strong, and dust-laden winds arc called ‘brick-fielder’ in Victoria province of Australia, ‘black-roller’ in the Great Plains of the USA, ‘shamal’ in Mesopotamia and the Persian Gulf, and ‘norwester’ in New Zealand.

The Khamsin

  • A hot, dry, dust-laden, southerly wind over Egypt, the Red Sea, and eastern parts of the Mediterranean Sea ahead of eastward-moving depressions.
  • It occurs during the period February to June, being most frequent in March and April.
  • The Khamsin is a Sirocco wind, whose name comes from the Arabic word for ‘fifty’, which is approximately the length of time the wind blows. In the nineteenth Century, the plague was worst in Egypt when the Khamsin was blowing.

Cold Winds

The cold local winds originate in the snow-capped mountains during winter and move down the slopes towards the valleys. They are known by different names in different areas.

Mistral

  • Mistrals are the most common cold local winds.
  • They originate on the Alps and move over France towards the Mediterranean Sea through the Rhone valley.
  • They are very cold, dry, and high-velocity winds.
  • They bring down the temperature below freezing point in areas of their influence.
  • People in these areas protect their orchards and gardens by growing thick hedges and building their houses facing the Mediterranean Sea.

Bora

  • A cold winter wind that occurs in eastern Europe is called the “bora.”
  • The bora blows from the Dinaric Alps down to the Adriatic Coast.
  • This blustery wind travels from the north or northeast, like the mistral, and can also reach speeds greater than 100 mph (160 kph).

A “Papagayo”

  • A “Papagayo” is a strong, north-easterly wind that affects the Pacific coast of Central America, from Guatemala to the Gulf of Papagayo in Costa Rica.
  • The Papagayo winds are produced by a cold air mass that travels down through the Central American mountains. It brings weather that is cold and blustery, yet clear.

Blizzard

  • A blizzard is a violent stormy cold and powdery polar wind laden with dry snow and is prevalent in north and south Polar Regions, Siberia, Canada, and the USA.
  • The visibility becomes remarkably low because of snow and ice crystals. The velocity ranges between 80-96 km an hour.
  • The arrival of these winds causes a sudden drop in air temperature to a subfreezing level, a thick cover of snow on the ground surface, and the onset of cold waves.
  • These winds reach the southern states of the USA because of the absence of any east-west mountain barrier.
  • They are called ‘norther’ in the southern USA and ‘burran’ in Siberia.

Pampero

  • The name is given to severe line squalls in Argentina and Uruguay, particularly in the Rio de la Plata area.
  • They are associated with marked cold fronts and are usually accompanied by rain, thunder, and lightning, a sharp drop in temperature, and a sudden change of wind direction from northerly or northwesterly to southerly or south-westerly.
  • They are most likely to occur during the period June to September.

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