REVISION NOTES ON WINDS and REASONS BEHIND IT, CH 8, CLASS 7

 Winds - When the air moves in a flow in a particular direction it is called wind.

Air Exerts Pressure
Why a tin can with hot water when cooled down with freshwater distorts its shape?
Figure 1 Air Exerts Pressure
As fresh water is poured over the hot can the steam in the can condensers and converts into the water. This results in reducing the amount of air that is present in water. The pressure of the air outside the can becomes more than the pressure of the air inside the can. This results in compressing of the can. Therefore, it is proved that air exerts pressure.
HIGH SPEED WINDS ARE ACCOMPANIED BY REDUCING AIR PRESSURE
Flow of Air
When the speed of the wind increases it lowers down the air pressure. Therefore, we can say that air flows from a region of high pressure to low pressure.
The speed of Air/ Winds
The speed of air or wind depends upon the difference between the pressures of the two regions. If the difference between the pressures increases, the speed of the air also increases.
AIR EXPAND ON HEATING
Hot Air and Cold Air
When the air is heated it expands. As a result, it occupies more space and becomes lighter in weight. Therefore, we can say that warm air is light in weight and cold air is heavy.
Why does smoke move in upward direction?
Smoke is hot air. Hot air is lighter hence it moves in an upward direction.
How Convection in air occurs?
When the hot air in a region rises up, the pressure becomes low in that region. As a result, the colder air fills its place. This is how convection in air occurs.
Figure 2 Convection in Air
Generation of Wind currents due to uneven heating of the earth
There are two situations where winds on earth generate:
1. Uneven heating between the equator and poles causes the north-south winds
We know that the Earth is divided into circular lines called latitudes that specify the north-south position of a place on the globe.
Also, the equator is latitude that is perpendicular to the axis of the earth's rotation.
The regions that are close to the equator receive maximum sunlight on the earth.
Hence the air in these regions is generally warm. The warm air, therefore, rises above and the cold air from the latitude around the equator moves in that place.
Similarly, cold winds from North and South poles move towards the equator and the neighbouring latitude and the circulation of winds take place on the earth.
Figure 3 Flow of Winds on Earth
You might observe in the figure given above the direction of the wind is not exactly from north to south or from South to North. This diversion in the direction of winds occurs because of the earth's rotation.
2. Uneven heating of land and water causes monsoon winds on earth which bring rainfall
In the summer season, the land near the equator gets warmer and its temperature generally remains higher than the oceans.
As a result, what are above the land rises and the cold air from the oceans moves towards the land. These winds are called monsoon winds.
Since these winds come from the ocean they carry water with them. In a similar way, the winds in the winter season move from the land towards the oceans.
Figure 4 Direction of Monsoon Winds in India