Winds Class 7 Geography Notes Maharashtra State Board
We feel the wind but cannot see it. When certain objects in our surroundings move, we experience wind. The movement of air is called wind.
Fold a piece of paper into two same-sized rolls. Keep both the paper rolls on one side of the table. You and your friend should select one roll each. Without touching the role of the paper or the table, what can be done to move the paper rolls to the other end of the table?
We have learned that the air pressure is not uniform across the earth’s surface. Air moves from high-pressure areas toward low-pressure areas in a horizontal manner. Winds are generated due to this movement. The extent of the difference in the air pressure affects the velocity of the wind. If there is less difference in the pressure, winds blow with less velocity. Winds blow with greater velocities if the pressure difference is greater. Wind speed also varies. Wind velocity is measured in the units of knots or kilometers per hour.
When we consider the earth as a whole, we find the direction of the winds is influenced by the rotation of the earth. In the northern hemisphere, winds get deflected towards the right of their original direction, whereas in the southern hemisphere, they get deflected toward the left. This direction has been shown by curved arrows in the figure. The rotation from west to east causes this change in the original direction of the winds.
Winds are known by the direction from which they blow. For example, Westerlies are winds that blow from the west. The direction of blowing, the duration, the regions covered, and the condition of the air determine the following wind types:
Some winds blow regularly on the earth from high to low-pressure belts throughout the year. These winds cover a large portion of the earth, hence these are called planetary winds. For example, the Easterlies, the Westerlies, and the Polar winds. In both the hemispheres, the winds blow from the high-pressure area between 25° and 35° parallels to the equatorial low-pressure belt. Due to the rotation of the earth, their original direction changes. In the northern hemisphere, they blow from the northeast to the southwest whereas in the southern hemisphere, they blow from the southeast to the northwest. These winds coming from both sides converge near the equatorial calm belt. These winds are known as the Easterlies.
In both hemispheres, winds blow from the mid-latitudinal high-pressure belt to the subpolar low-pressure belt near 60° parallel. Their original direction changes due to the rotation of the earth. In the southern hemisphere, these winds blow from the northwest to the southeast, and in the northern hemisphere, they blow from the southwest to the northeast. These winds are called Westerlies. In both hemispheres, winds blowing from the polar high-pressure belt to the subpolar low-pressure belt (55° to 65°) are called Polar winds. Generally, they blow from east to west.
Winds in the southern hemisphere blow with great velocities. The southern hemisphere is mostly occupied by oceans. In this hemisphere, the obstacle caused by the relief of the land surface is almost absent. As there is no obstacle; winds blow with greater velocities in the southern hemisphere as compared to the northern hemisphere. Their characteristics are as follows:
- Beyond 40°S, winds blow with tremendous velocities. These winds are called Roaring Forties.
- Around 50°S, winds are stormy. Hence they are called Furious Fifties.
- The stormy winds around 60° S make tremendous noise and hence they are called Screeching Sixties.
The winds that blow for a short time, originate in specific regions, and blow over a limited area are called local winds. Local winds affect the climate of the region where they blow. These winds are known by different local names in the different regions where they blow.
Characteristics of the Mountain Breeze:
- Mountains cool down quickly at night.
- The valley zone is comparatively warmer.
- Air pressure is greater in the mountains.
- Winds blow from the mountain towards the valley.
- The hot and light air from the valley is pushed upwards and the cool air rushes down into the valley.
- Mountain winds set in after sunset.
The region up to 5° north and 5° south of the equator remains calm for most of the year and winds do not blow in this region. It is called the equatorial calm zone or Doldrums. In the area near the Tropic of Cancer and Tropic of Capricorn, that is, between 25° and 35° north and south, there exists a high-pressure belt. This is also a calm belt. This belt is known as Horse Latitudes.
The land is made up of dense matter. Land is stable and opaque. As a result, heat is transferred at a greater speed and in a higher proportion. Hence, land gets heated quickly. The density of water is comparatively less. Water is transparent and unstable. Hence, water does not get heated quickly. As a result, the air pressure in land and water areas is different.
In the coastal areas, the land gets heated during the daytime. Hence, the air on the land also gets heated, and the air pressure on the land decreases. The sea water gets heated slowly hence the air is less heated and the air pressure remains high. Winds blowing from the sea towards the land are called sea breezes. At night, land cools down faster as compared to the sea and therefore has higher air pressure. Hence, the land breeze blows from land towards the sea. Besides these, winds blow under particular conditions in different areas. These are also called local winds. For example, Fohn, Chinook, Bora, Loo, etc.
Major Local Winds in the World
|Name of the Wind||Nature of the Wind||Characteristics and Areas of Influence|
|Loo||Hot and Dry||Blows in the North Indian plains during the summer season generally in the afternoons. They come from the hot Thar Desert.|
|Simoom||Hot, Dry, and Destructive||These blow with tremendous velocities from the Sahara and Arabian deserts. As these winds are quite strong they are destructive.|
|Chinook (Snow Eater)||Warm and Dry||These blow down the eastern slopes of the Rocky Mountains of North America. They cause the snow to melt and increase the temperature in the valleys.|
|Mistral||Cold and Dry||These blow in Spain, France, and areas around the Mediterranean Sea. They originate from the Alps. These cold winds reduce the temperature in the coastal areas.|
|Bora||Cold and Dry||These blow from the Alps Mountains towards the coastal areas of Italy.|
|Pampero||Cold and Dry||These blow around the Pampas grasslands in South America.|
|Fohn (foehn)||Hot and Dry||These blow along the northern slopes of the Alps.|
Seasonal Winds (Monsoon):
Monsoon winds are generated due to the uneven heating of land and water in the different seasons. During summers, Monsoons blow from the sea to the land, and in winter they blow from the land to the sea. Southeast Asia, East Africa, and North Australia are the regions where the pronounced effects of these winds are felt. The influence of monsoon winds is seen in the summer and winter seasons in the Indian subcontinent. Due to these winds, the Indian subcontinent experiences monsoon (rainy) and retreating monsoon seasons apart from summer and winter.
Monsoon winds are sea and land breezes blowing on a large scale. Most of the precipitation in the Indian subcontinent is the effect of monsoon winds. After crossing the equator, they blow from the southwest towards the Indian subcontinent during the period from June to September. They are called the southwest monsoons. They are full of moisture. From September to December, because of the low-pressure area developing near the equator, winds blow from the Indian subcontinent towards the equator. These are called the northeast monsoon winds. They are dry winds.
Cyclonic conditions are created when a low-pressure area is surrounded by high-pressure areas. In these conditions, winds start blowing towards the low-pressure area from the surrounding high-pressure areas. Due to the rotation of the earth, the cyclonic winds in the northern hemisphere move in an anticlockwise direction, whereas they move in a clockwise direction in the southern hemisphere. During a cyclone, the sky is cloudy, winds blow with a very high velocity and it rains heavily. The area affected by cyclones is limited. The duration, velocities, direction, and place of origin of these winds are unpredictable. See the image of a cyclone obtained by a satellite in the figure. On weather maps, the center of a cyclone is represented by the letter ‘L’. A cyclone system moves from one place to another.
Cyclones occurring in the western part of the Pacific Ocean along the coasts of Japan, China, Philippines, etc. are known as ‘Typhoons’. These storms arise in the months between June and October. Due to high-velocity winds and heavy rains, these prove to be destructive. The cyclones in the Caribbean Sea are known as hurricanes. These are also destructive. During these storms, the minimum velocity of the wind is 60 km per hour. Cyclones also originate in the temperate zone but they are not so powerful and hence are not destructive.
Under specific atmospheric conditions, the air pressure in a particular region increases, and the pressure in the surrounding areas remains low. In this situation, winds blow from the center toward the surrounding areas in a circular manner. In the northern hemisphere, these winds blow in a clockwise direction whereas in the southern hemisphere, they blow in an anticlockwise direction. During anticyclones, the skies are clear, winds blow with lesser velocities and the weather is pleasant.
Anticyclones generally last for a few days or a week. Such anticyclones originate in temperate zones. On weather maps, the center of an anticyclone is represented by the letter ‘H’. Anticyclones are vividly experienced in high-pressure belts. Winds in these regions are always moving out and hence the rainfall in such areas is quite low.
Naming the Cyclones:
Cyclones occurring in different parts of the world are assigned different names. A list of names is prepared for each of the oceans or its part. Names in the list are suggested by the countries in the region of that ocean. If the wind speed in the storm exceeds 33 knots (60 km/hour) the cyclone is assigned a name. Names make it easier for us to remember a particular storm.
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