Climatology: Types of Wind

Permanent Winds or Primary Winds or Prevailing Winds or Planetary Winds

What is it?
- Planetary or permanent winds blow from high-pressure belts to low-pressure belts in the same direction throughout the year.
- They blow over a vast area of continents and oceans
Types
1. Easterlies
  - The winds that blow from sub-tropical high pressure areas towards equatorial low pressure areas called trade winds.
  - These are confined to a region between 30°N and 30°S throughout the earth’s surface.
  - They flow as the north-eastern trades in the northern hemisphere and the south-eastern trades in the southern hemisphere.
  - As the trade winds tend to blow mainly from the east, they are also known as the Tropical easterlies.
  - Trade winds are descending and stable in areas of their origin (sub-tropical high-pressure belt), and as they reach the equator, they become humid and warmer after picking up moisture on their way.
  - The trade winds from two hemispheres meet at the equator, and due to convergence they rise and cause heavy rainfall.
2. Westerlies
  - The winds that move poleward from the sub-tropical high pressure in the northern hemisphere are deflected to the right and thus blow from the south west.
  - These in the southern hemisphere are deflected to the left and blow from the north-west, so these winds are called westerlies.
  - The westerlies are best developed between 40° and 65°S latitudes. These latitudes are often called Roaring Forties, Furious Fifties, and Shrieking Sixties.
3. 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

Periodic Winds

These winds change their direction with change in season.
- Monsoon Winds
  - The word ‘Monsoon’ has been derived from the Arabic word ‘Mausim’ meaning season.
  - The winds that reverse their direction with the change of seasons are called monsoon winds.
  - During summer the monsoon winds blow from sea towards land and during winter from land towards seas.
  - India, Pakistan, Bangladesh, Myanmar(Burma), Sri Lanka, the Arabian Sea, the Bay of Bengal, South-east Asia, North Australia, China and Japan are important regions where monsoon winds are prevalent.

- Land Breeze and Sea Breeze
  - Sea Breeze
    - This process takes place for the duration of the day.
    - Both the sea and the land surface are heated up by the sun.
    - The sea heats up slower than the land because it has a much higher heat capacity.
    - Thus, the temperature over the land surface increases, in turn, heating up the surrounding air.
    - Expansion occurs in the less dense warm air and an area over the land having low pressure is developed.
    - At the same time on the top of the sea, a high-pressure area develops.
    - Due to the difference in pressure, the air flows from the high pressure over the sea to the low pressure over the land.
    - This flow of air from the sea to the land is termed as the sea breeze.

- - Land Breeze
    - This process takes place for the duration of the night and the above-mentioned process gets reversed.
    - Both the land and the sea start cooling down when the sunsets.
    - As the heat capacity of the land is different from the sea it cools down quicker.
    - Thus, a low-pressure situation develops over the sea as the temperature above it is higher when compared to the land.
    - Due to this, the air flows from the land to the sea which is termed the land breeze.
- Valley Breeze and Mountain Breeze
  - Valley Breeze
    - During day time valley region receives more insolation and radiates more terrestrial radiation heating up the air present on the valley surface.
    - Air on heating loses its density and moves up the mountain surface causing valley breeze.
  - Mountain Breeze
    - During night, air on the mountain surface cools, gains density and seeps down to the valley floor causing mountain breeze.

Local Winds

Hot Winds

Hot Wind	Features
Loo	Blow very strongly over the northern plains of India and Pakistan in the months of May and June.
Foehn	Foehn is a strong, dusty, dry and warm local wind which develops on the leeward side of the Alps mountain ranges. They are making pasture land ready for animal grazing and help the grapes to ripe early.
Chinook	Chinook is the name of a hot and dry local wind which 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.
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.
Sirocco	Sirocco is a Mediterranean windthat comes from the Sahara and reaches hurricane speeds in North Africa and Southern 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	It is a warm and dry wind blowing from north east and east to west in the eastern parts of the Sahara desert. Harmattan is known as ‘doctor’ in the Guinea coastal area of western Africa.

Cold Winds

Cold Wind	Features
Mistral	They originate on the Alps and move over France towards the Mediterranean Sea through the Rhone valley.
Bora	The bora blows from the Dinaric Alps down to the Adriatic Coast in Europe.
Blizzard	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.
Pampero	The name given to severe line squalls in Argentina and Uruguay, particularly in the Rio de la Plata area.