Wind

• What is wind – Wind is the horizontal movement of air.
• Differences in air pressure within the atmosphere cause this movement.
• Wind under high pressure moves toward areas of low pressure.
• There are different types of wind, which we will learn about in this article.

Types of Wind

Primary Winds or Prevailing Winds or Permanent Winds

• These are the planetary winds which blow extensively over continents and oceans.
• The two most well- understood and significant winds for climate and human activities are trade winds and westerly winds.
• Trade Winds:
  • Trade winds blow from sub-tropical high-pressure areas towards equatorial low-pressure belt.
  • Confined to region between 30°N and 30°S on earth’s surface.
  • Flow as north-eastern trades in northern hemisphere and south-eastern trades in southern hemisphere.
  • Helped sea merchants sail ships due to their constant and regular direction.
  • Deflection explained by Coriolis force and Ferrell’s law.
  • Deflected to right in northern hemisphere and to left in southern hemisphere.
  • Descending and stable in sub-tropical high-pressure belt, become humid and warmer as they reach equator after picking up moisture.
  • Trade winds from both hemispheres meet at equator and cause heavy rainfall due to convergence.
  • Eastern parts associated with cool ocean currents are drier and more stable than western parts.
• Westerlies:
  • Westerlies blow from subtropical high-pressure belts (30°-35°) towards sub-polar low-pressure belts (60°-65°) in both hemispheres.
  • Blow from southwest to northeast in northern hemisphere and northwest to southeast in southern hemisphere.
  • Southern hemisphere westerlies are stronger and persistent due to vast expanse of water; northern hemisphere westerlies are irregular due to uneven relief of landmasses.
  • Northern hemisphere westerlies become complex and less effective during summer, more vigorous during winter.
  • Westerlies bring precipitation to western parts of continents by picking up moisture while passing over oceans.
  • Southern hemisphere westerlies become stormy and associated with gales due to lack of land and dominance of oceans.
  • Best developed between 40° and 65°S latitudes, often called Roaring Forties, Furious Fifties, and Shrieking Sixties – dreaded terms for sailors.
  • Poleward boundary of westerlies is highly fluctuating with seasonal and short-term fluctuations.
  • These winds produce wet spells and variability in weather.
• Polar easterlies:
  • The Polar easterlies are dry, cold prevailing winds blowing from north-east to south-west direction in Northern Hemisphere and south-east to the north-west in Southern Hemisphere.
  • They blow from the polar high-pressure areas of the sub-polar lows.

Secondary Winds or Periodic Winds

• Winds change direction with change in season.
• Monsoons are a large-scale modification of the planetary wind system.
• Periodic winds include land and sea breeze, mountain and valley breeze, cyclones and anticyclones, and air masses.
• Monsoons:
  • Monsoons are traditionally explained as large-scale land and sea breezes.
  • Monsoons are characterized by seasonal reversal of wind direction.
  • During summer, trade winds of the southern hemisphere are pulled northwards towards the Indian subcontinent, causing heavy rainfall due to over-saturation with moisture.
  • During winter, a high-pressure core to the north of the Indian subcontinent produces divergent winds, creating north-east or winter monsoons responsible for some precipitation along the east coast of India.
  • Monsoon winds flow over many countries in the region, including India, Pakistan, Bangladesh, Myanmar, Sri Lanka, Arabian Sea, Bay of Bengal, southeastern Asia, northern Australia, China, and Japan.
  • In eastern Asiatic countries like China and Japan, the winter monsoon is stronger than the summer monsoon.
  • 
• Land Breeze and Sea Breeze:
  • Land and sea absorb and transfer heat differently.
  • During the day, the land heats up faster and becomes warmer than the sea.
  • This leads to a low-pressure area over the land and relatively high pressure over the sea, creating a pressure gradient.
  • The wind blows from the sea to the land as the sea breeze due to this pressure gradient.
  • At night, the land loses heat faster and becomes cooler than the sea.
  • The pressure gradient reverses from the land to the sea, resulting in the land breeze.
  • 
• Valley Breeze and Mountain Breeze:
  • In mountainous regions, slopes heat up during the day and air moves upslope.
  • This creates a gap that is filled by air from the valley, resulting in the valley breeze.
  • During the night, slopes cool and dense air descends into the valley as the mountain wind.
  • Cool air from high plateaus and ice fields draining into the valley is called katabatic wind.
  • On the leeward side of mountain ranges, a warm wind called katabatic wind occurs.
  • Moisture in these winds condenses and precipitates while crossing mountain ranges.
  • When it descends down the leeward side, the dry air gets warmed up by the adiabatic process.
  • This dry air may melt snow in a short time.
  • 

Tertiary Winds or Local Winds

• Local differences in temperature and pressure produce local winds.
• Such winds are local in extent and are confined to the lowest levels of the troposphere. Some examples of local winds are discussed below.
• Loo:
  • Harmful Wind
  • A very hot and dry wind blows in the plains of northern India and Pakistan.
  • This wind blows from the west in the months of May and June, usually in the afternoons.
  • Its temperature usually ranges between 45°C and 50°C.
  • It may cause sunstroke to people.
• Foehn or Fohn:
  • Beneficial Wind
  • It is a hot wind of local importance in the Alps
  • Foehn is a strong, gusty, dry and warm wind
  • It develops on the leeward side of a mountain range
  • The windward side takes away whatever moisture there is in the incoming wind in the form of orographic precipitation
  • The air that descends on the leeward side is dry and warm (Katabatic Wind)
  • The temperature of the wind varies between 15°C and 20°C
  • Foehn wind helps animal grazing by melting snow
  • It also aids the ripening of grapes.
• Chinook
  • Beneficial wind in USA and Canada
  • Moves down the west slopes of the Rockies
  • Keeps grasslands clear of snow during much of the winter
• Mistral
  • Harmful wind in France
  • Blows from the Alps towards the Mediterranean Sea
  • Very cold and dry with high speed
  • Brings blizzards into southern France
• Sirocco
  • Harmful wind in North Africa and Southern Europe
  • Mediterranean wind that comes from the Sahara
  • Reaches hurricane speeds
  • Arises from a warm, dry, tropical air mass that is pulled northward by low-pressure cells
  • Causes dusty dry conditions along the northern coast of Africa, storms in the Mediterranean Sea, and cool wet weather in Europe