Planetary Winds Definition - The winds that flow throughout the year from one latitude to another latitude because of latitudinal differences in the air pressure are called planetary winds. They are also called prevailing winds. Planetary winds blow from a single direction over a specific area over the earth.
The areas in which the planetary winds meet are called convergence zones. They generally blow east to west rather than blowing north to south. This usually happens because the earth’s rotation generates the Coriolis effect. The Coriolis effect makes wind systems twist anticlockwise in the Northern hemisphere and clockwise in the southern hemisphere.
Classification of Winds
The wind is the movement of air. It is caused by the uneven heating of the Earth by the sun and the Earth’s own rotation. Winds have a vast range - from light breezes to natural calamities such as cyclones and hurricanes. Differences in atmospheric pressure cause winds. It is a general property of air that warm air rises up and dense and cool air moves towards downwards. At the equator, the sun heats water and land more than anywhere else on the globe. As a result, the warm equatorial air rises high into the atmosphere and migrates towards the poles. Similarly, the cool and dense air moves over the Earth’s surface towards the equator to replace the heated air.
The winds are classified based on global and local phenomena. On Earth, three types of winds exist - Primary Winds, Secondary Winds, and Tertiary Winds. Winds can either be permanent or temporary. Primary winds are the permanent winds. Planetary winds are the primary winds. Secondary winds are seasonal or periodic winds. Types of secondary winds are - The Monsoon Winds, The Sea and Land Breeze, and the Mountain and Valley Breeze. The tertiary or local winds cover only small areas. Different types of tertiary winds are - Blizzards, Chinooks, Santa Ana, Etesian, or Meltemi.
Types of Planetary Winds
In the planetary wind system, there are three main types of planetary winds - The Trade Winds, The Westerlies, and The Easterlies.
The Trade Winds
The trade winds blow in a belt that lies between 5°N to 30°N in the northern hemisphere and 5°S to 30°S in the southern hemisphere.
It is known to all that the wind travels from high pressure to low pressure. There is low pressure on the equator and high pressure on the subtropics. Hence, the air moves towards the equator from the subtropics. Because the earth’s rotation generates a Coriolis effect, the wind moves from the left side in the southern hemisphere to the right side in the northern hemisphere.
The latitude of 30°- 35°N and 30°- 35°S are the areas where the air is descending and is characterized by calm and light variable winds. These winds are comparatively dry and the weather condition is quiet and stable. This latitude zone is called Horse latitude.
The westerlies blow in the latitude belt of 35°- 60° N and 35°- 60° S.
The air streams that flow towards the poles from the subtropical high-pressure areas deflect towards the east in the northern hemisphere to form south-westerlies.
The air streams that flow towards the poles from the subtropical high-pressure areas deflect towards the east in the southern hemisphere to form north westerlies.
Contrary to the trade winds, the westerlies are very much variable in both force and direction, especially in the northern hemisphere.
In the southern hemisphere, the westerlies blow with great strength and are regular throughout the year over the ocean. In the southern hemisphere, between the latitude 40° - 50° S, the westerlies have got the name of Roaring Forties.
These winds sometimes give a roaring sound because of their high speed.
The Easterlies or the Polar Easterlies
They blow from the Polar high-pressure area to the temperature low-pressure area.
Towards the equator, they are deflected towards the west in the northern hemisphere to form north easterlies and in the southern hemisphere to form south easterlies.
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Monsoon Wind System
Monsoon is a seasonal change.
Monsoon winds always blow from cold and high-pressure areas.
Monsoon winds are a part of the year-long cycle of uneven heating and cooling of tropical and mid-latitude coastal regions.
Sea and Land Breeze
The sea and land breeze are formed in the coastal regions.
They are formed during the day due to the thermal and pressure gradient between sea and land.
Sea cools and heats slowly because of higher specific heat and land heats and cools rapidly because of low specific heat.
During the day, the wind blows from sea to land which is known as the sea breeze and during the night, the phenomenon reverses and it is known as a land breeze.
Mountain and Valley Breeze
In the mountain regions, the mountain and valley breeze resembles sea and land breeze.
In the daytime, the valleys are hot compared to the mountains, hence, the air flows upwards towards the slopes - from the valley to the mountains, known as Valley Breeze.
At nights, the air slides down from the mountain peak to the valleys which are called the mountain breeze.
Tertiary or the Local Winds
Tertiary or local winds cover small regions. They are formed due to changes in local temperature and pressure. One very common example of tertiary winds in India is loo in the hot regions.