The horizontal movement of air is called wind. Winds blow from high-pressure areas to low-pressure areas. The vertical movement of air is known as air current.
Factors affect the direction and speed of wind
Temperature: When the temperature of a place increases, the atmospheric pressure decreases. It is because an increase in temperature results in heating the air. Warm air expands. The molecules of warm air become less dense and hence exert less pressure. On the contrary, a decrease in temperature makes the air cool and dense. Cold air sinks creating a high-pressure area in a region.
Coriolis Effect: The Earth rotates on an inclined axis. Because of the rotation, winds get deflected to their right in the Northern Hemisphere and to their south in the Southern Hemisphere.
Altitude: The atmospheric pressure decreases with the increase in altitude. Air is compressible. As the gravity of the Earth pulls the molecules and gases towards the ground, the lowest layer of air gets densest. This dense layer of air exerts the greatest pressure.
Water Vapour: Moist air exerts more pressure as it contains fewer amounts of nitrogen and oxygen. Dry air, on the other hand, exerts less pressure as it has more nitrogen and oxygen and hence is denser.
Rotation of the Earth: The rotation of the Earth causes the air at the poles to be pushed away towards the Equator. In theory, air should have been accumulated at the Equator creating a high-pressure belt. However, this does not happen. On the contrary, the air gets heated at the Equator, gets warmer, and rises to create a low-pressure area.
Types of Wind
There are various types of wind. Winds that blow throughout the year from one latitude to the other are known as Permanent Winds. Certain winds reverse their direction periodically with seasons and are known as Periodic Winds. Some winds blow only over a small area for a specific period and have special characteristics. These winds are known as local winds.
There are three main permanent winds—Trade Winds, Westerlies, and Polar Easterlies.
- The trade winds blow in the tropics between the subtropical high-pressure belt and the equatorial low-pressure belt between 30°N and 30°S.
- Trade winds are warm winds, and hence, they pick up moisture and bring heavy rainfall on the eastern sides of the tropical islands.
- They are deflected to the right of their course in the Northern Hemisphere and to the south in the Southern Hemisphere. Thus, in the Northern hemisphere, they become North Trade Winds, and in the Southern Hemisphere, they are called Southeast Trade Winds.
- Trade winds blow at a constant speed and are regular.
- They are associated with constant depressions and cyclones.
- The Trade Winds are also known as permanent or constant winds. The only exception is that they are replaced by the monsoon winds in the Indian and Pacific oceans.
- They blow from Sub-Tropical High-Pressure Belts to Sub-Polar Low-Pressure Belts between 30° and 60°N of the Equator in the temperate latitudes.
- Because of the Coriolis Effect, they deflect to their right to become the South Westerlies in the Northern Hemisphere and deflect to their left to become the North Westerlies in the Southern Hemisphere.
- They are strong winds that are dominated by the movements of cyclones and anti-cyclones.
- They are responsible for carrying warm equatorial waters and winds to the western coasts of the temperate lands.
- Because of the shifting of the wind systems, not all the places located on the western coast of the temperate lands receive rainfall throughout the year.
- Polar winds blow from the polar high-pressure belt towards the Sub-Polar Low-Pressure Belt between 60°N to 60°S. In the Northern Hemisphere, they blow from the northeast and are known as the Northeast Polar Winds. In the Southern Hemisphere, they blow from the southeast and are known as the Southeast Polar Winds.
- Because they are deflected in the west in both hemispheres by the Coriolis Effect, they are known as the Polar Easterlies.
- These are cold winds as they blow from the ice-capped regions. However, they become warm when they blow over the oceans.
Periodic winds blow at regular intervals in regular cycles. These winds cause differences in temperature and pressure in the region over which they blow.
Land and Sea Breeze
The land is a bad conductor and a good radiator of heat. Therefore, it quickly gets heated during the day. This results in building a low-pressure area over the land and a high-pressure area over the sea. Because winds blow from the high-pressure area to the low-pressure area, winds from the sea blow towards the land. They are known as the sea breeze. During the nights, the land cools down rapidly creating high-pressure conditions. Thus, the land breeze blows from the land to the oceans.
- During summers, the land gets more heated than the sea. Because of high temperature, a low-pressure area is created over the land.
- The reverse conditions exist in the oceans. During summers, the air is cool, and hence, a high-pressure area develops. Because winds move from high pressure to low-pressure areas, winds blow from the sea to the land (from the Indian Ocean to the Indian mainland) known as the summer monsoon.
- In the summer months, the landmasses of the Indian subcontinents are heated by the vertical rays of the Sun. This creates a low-pressure area on the land. In the oceans, the high-pressure area develops as the air is cooled. So, the winds blow from the Indian Ocean northwards and in the northwestward direction bringing heavy rainfall to Southeast Asia.
- During winters, the conditions are reverse of summer. A high-pressure area develops over the landmass and a low-pressure area is developed over the seas.
- Because the winds blow from the land to the sea (winds move from high pressure to low-pressure areas), they result in cold dry weather and hardly produce any rainfall.
- These winds can bring rainfall only when they blow over the seas to the adjoining lands. The Coromandel Coast in Tamil Nadu and the Vietnamese Coast get winter monsoon. Because these winter monsoon winds blow to the northeast, they are also known as Northeast Winter Monsoons.
- Local winds blow over a particular place over a certain period of time. These winds may be warm or cold depending on the area from which they blow.
- Loo is a local wind that blows in the plains of northern Indian and Pakistan during hot summer. Its temperature ranges between 45°C and 50°C, causing sunstroke in people who are directly exposed.
- Chinook is the hot local winds that blow over the USA and Canada. Because of its warmth, the wind keeps the grasslands clear of any snow.
- Mistral is the local cold winds that originate over snow-capped mountains and blow towards the valleys. They are known by different names in different places. The most famous mistral is the wind which blows from the Alps over France towards the Mediterranean Sea. The winds bring down the temperature to a near freezing point.
- Cyclones are variable winds. In low latitudes, an intense depression with low-pressure conditions develops, known as tropical cyclones. These last only for few days. There are two chief types of variable winds—cyclones and anti-cyclones.
- Because of the Coriolis Effect, the cyclones blow in an anti-clockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere.
- Cyclones are associated with turbulent weather, cloudy weather, strong winds, and intense rainfall.
- A Tornado is a tropical cyclone that occurs over land. It is a funnel-shaped cloud extending downwards from the base of the thunderstorms. They may travel at a high speed of 500–800 km per hour. They are generally experienced over tropical waters in North America.
- Hurricanes and typhoons are some other tropical cyclones.
- Temperate cyclones are active over mid-latitudinal regions between 35° and 65° latitude in both hemispheres.
- Anti-cyclones represent calm weather. Winds blow gently in the clockwise direction in the Northern Hemisphere and in the anti-clockwise direction in the Southern Hemisphere.
- Jet streams are winds that blow horizontally from west to east at a high speed near the tropopause and the stratosphere. These are high-speed winds that influence the weather and climatic conditions of the region over which they blow.
- There are two types of jet streams that blow at a tropical belt of 300 north latitudes—the westerly jet stream and the easterly jet stream.
- A branch of the Westerly jet stream brings moderate to heavy rainfall followed by cold waves in the Indian subcontinent.
- The Easterly jet stream also helps the monsoon winds to blow into the greater part of India and bring monsoon showers.
Also, Read Pressure belts and types of winds