Class 7 Science
Air: Air is a mixture of gases.
Wind: Moving air is called wind. Some properties of air are given below.
Air exerts pressure. When you fill air in a balloon, the balloon becomes inflated because of air pressure.
High speed wind reduces air pressure. A storm blows away tin roof. This happens because high speed wind reduces air pressure. Low air pressure results in roof getting blown away.
Air expands on heating. You may have observed that when a bicycle is kept in intense sunshine for a long time, the tube of bicycle bursts with a bang. In fact, the air inside tube expands because of heating. When air tries to expand too much, pressure inside the tube increases manifold. Excess pressure results in tube getting blown up.
Warm air is lighter than cool air. You may have observed that smoke always rises up. Temperature of smoke is higher than that of air around it.
Different parts of the earth get different amount of heat from the sun. Sun rays fall in straight fashion on the equator. Hence, equator gets the maximum amount of heat. On the other hand, sun rays fall obliquely on poles. Hence, poles get the least amount of heat. That is why the temperature on equator is more than that on the poles. Uneven heating of the earth results in formation of air current.
The air around equator becomes lighter due to heating, and it rises up. This results in an area of low pressure near the surface. Air from 30° latitudes rush towards equator to fill this region of low pressure.
The poles are colder compared to 60° latitudes. This results in movement of air from the poles towards 60° latitudes.
The temperature difference, between 30° and 60° latitudes, results in air moving from 30° latitudes towards 60° latitudes.
You may be wondering that wind must be moving either towards north or south. But actual movement of wind is towards northeast or southeast or northwest or southwest. This tilt is because of a force generated by earth’s rotation on its axis. This force created by earth’s rotation is called Coriolis Force.
Uneven Heating of Land and Ocean Surface: You have read about land breeze and sea breeze in a previous chapter. You may recall that the land gets warmer than ocean surface during the day. Due to this, wind moves from ocean surface towards land in coastal area during the day. Opposite of this happens during night. The land cools down faster than ocean surface. Hence, wind moves from land to ocean surface in coastal area during the night.
During summer season, land becomes excessively hot while ocean surface remains comparatively cooler. Due to this, wind moves from ocean towards land. This wind is called monsoon. Monsoon brings moisture which causes rains.
This map shows air currents over Indian Peninsula during the month of July. The Indian Peninsula becomes hot in the month of July. Hot air moves up and creates a region of low pressure near the surface. To fill in the region of low pressure, moist air from ocean (situated towards south-west) moves towards land. That is why we get rains in the Indian Subcontinent in the month of July.
This map shows air currents over Indian Peninsula in the month of January. The temperature in northern part of the subcontinent remains low and it increases as we move south. Due to this, wind moves from the northeastern part of the subcontinent towards ocean. The air has less moisture. Due to this, there is scanty rainfall during winter.
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