Winds
Introduction
The horizontal movement of air, parallel to the Earth’s surface is known as ‘Winds’.
The winds play an important role in the distribution of temperature and humidity in the atmosphere.
Winds are generated due to differences in pressure from one place to the other. Winds have a tendency to move from areas of high pressure to those of low pressure.
The direction of the winds is determined by the direction of the pressure gradient.
The direction of winds is identified by an instrument called ‘Wind vane’ or ‘Wind cock’.
‘Anemometer’ is used to measure the speed of the winds.
Wind Wane
Anemometer
Characteristics of Winds:
Difference in Pressure
Wind always blows from places of high pressure to the areas of low pressure. The speed of wind is directly governed by the ‘pressure gradient’. Steeper the gradient greater will be the speed, whereas gentle gradient causes slow movement of air.
The Rotation
Due to the rotation of the Earth the wind movement is towards its right and become a clockwise direction in the northern hemisphere. While in the southern hemisphere the wind movement is towards its left, which is anti-clockwise direction.
The direction of the wind
The direction of wind is generally identified from its source. On the basis of this, different types of winds are seen on the Earth. eg. In India during June to September wind blows from south west to north east direction and from September to December it blows from north east to south west.
Types of Winds:
Winds are influenced and supported by temperature and pressure. On the basis of origin, nature and features winds are classified into four main types:
Planetary winds
Seasonal or Periodic winds
Local winds
1. PLANETARY WINDS
Planetary winds are also called Permanent winds. These winds blow regularly from high pressure to low pressure belts in a fixed direction throughout the year. They play a major role in climate change, desert formation, guide navigation routes etc., There are three types of Planetary winds. They are:
Trade winds
Westerlies or Antitrade winds and
Polar winds.
Trade Winds
Trade winds are also called the ‘Tropical Easterlies’.
These winds originate and blow from Sub-tropical high-pressure belts to Equatorial low-pressure belt.
The word trade is derived from the Latin word ‘trado’ means, constant direction (Phrase-to blow trades).
There are two types of trade winds.
North East Trade (NET) winds
South East Trade (SET) winds.
Trade winds are also called
North East trade winds from the northern hemisphere and South east trade winds from southern hemisphere meet near the Equator.
This region is the converging zone of trade winds known as the ‘Inter-tropical convergent zone’ (ITCZ). This region has different characteristics-low pressure, variable winds and calm conditions and convergence of trade winds.
Antitrade winds
These winds originate and blow from Subtropical high-pressure belts to Sub-polar low-pressure belts.
These are from Southwest to Northeast direction in the northern hemisphere and Northwest to Southeast in the southern hemisphere. Therefore, they are called ‘
The direction of these winds are opposite to trade winds. Hence, they are known as ‘Antitrade winds’.
In the northern hemisphere because of the presence of vast landmasses with varied relief features large-scale disturbances are formed by Antitrade winds.
In the southern hemisphere they are more regular and blow with great velocity because of the vast expanse of ocean water.
These winds cause great problems to navigation in the southern hemisphere.
The important westerlies in the southern hemisphere are:
- ‘Roaring Forties’ found around 40º south latitude.
- ‘Furious Fifties’ found around 50º south latitude.
- ‘Shrieking sixties’ or ‘Screeching sixties’ or’ Screaming sixties’ around 60º south latitude.
Polar winds
These are extremely cold winds blowing from Polar high-pressure belts to Sub-polar low-pressure belts.
Polar winds are more regular and blow without much variation. However they are strong in the winter season (cold wave) than in summer.
They blow from Northeast to Southwest in the northern hemisphere and South east to Northwest in the southern hemisphere (similar to trade winds), as such they are also known as ‘Polar Easterlies’.
2. SEASONAL OR PERIODIC WINDS
The winds which change their direction in different seasons are called Seasonal or Periodic winds.
Monsoon winds are the best example of seasonal winds. They are primarily formed with the unequal heating of land and water bodies.
Seasonal winds are mainly found in the tropical region and sometimes extended towards temperate regions. They are mainly caused with thermal variation.
In India, South West monsoon winds blow from South west to North east direction during June to September and North East monsoon winds blow from North east to South west direction from late September to middle of December due to thermal variation and pressure difference in the land and water bodies.
3. LOCAL WINDS
The regular pattern of planetary and seasonal winds are affected with the local disturbances. Difference in temperature and pressure leads to the development of movement of winds, called as ‘Local Winds’. Several types of local winds are found in different parts of the world. Important among them are:
Land and Sea breezes
Mountain and Valley breezes
Land and Sea Breezes
The winds blowing alternatively during day and night from the sea and the land near the coasts are known as ‘Sea Breeze’ and ‘Land Breeze’ respectively. These are the best-developed local winds near the coastal regions.
Sea Breezes
During daytime land gets heated more quickly than the adjacent sea. So the air gets heated and rise upwards to produce a low-pressure region. At the same time, the pressure at sea is comparatively high. The warm air of the land, being light, rises upwards allowing the air from sea to enter in. Such incoming air from the sea is called ‘Sea Breeze’.
Land Breezes
At night the land loses its temperature quickly due to rapid radiation and high pressure is developed. As the sea water still retains temperature the air is lighter and rises upward, thus allowing the air from the land to move towards the sea. Such wind is called ‘Land Breeze’.
Mountain and Valley Breezes:
Mountain and Valley breezes are the best developed local winds in mountain regions.
Mountain Breezes
Mountain and Valley breezes are the best developed local winds in mountain regions.
During night the mountain peaks, (height) which are exposed, radiate and get cooled quickly, while the valleys still remain warm. So, the dense air of the high altitudes (high pressure) slowly moves down along the slopes towards the valley (low pressure). These are known as ‘Mountain Breeze’ or ‘Katabatic Winds’.
Valley Breezes
During the day the rays of the Sun strikes the peaks as well as their slopes and they get heated (low pressure) faster than the deep valleys (high pressure). The uprising air of the peaks and slopes allows the dense air of the valleys to move up. Such winds are known as ‘Valley Breeze’ or ‘Anabatic Winds’.
Other important local winds found in different parts of the world are:
Loo (India)
Aandhi (India)
Brickfielder (Australia)
Blizzard (High latitudes)
Sirocco (Sahara desert)
Harmattan (Western Africa)
Foehn (Northern Alps)
Mistral (France)
Chinook (USA) etc.,
