When air moves, it is called wind.

Air moves from the region where the air pressure is high to the region where the pressure is low. The greater the difference in pressure, the faster the air moves.

In nature there are several situations, where warm air rises at a place. The air pressure at that place is lowered. The cold air from the surrounding areas rushes in to fill its place. This sets up convection in air, i.e. wind


Wind currents are generated due to Uneven Heating on the Earth. These situations are :

(a) Uneven heating between the equator and the poles.

Regions close to the equator get maximum heat from the Sun. The air in these regions gets warm. The warm air rises, and the cooler air from the regions in the 0–30 degrees latitude belt on either side of the equator moves in. These winds blow from the north and the south towards the equator. At the poles, the air is colder than that at latitudes about 60 degrees. The warm air at these latitudes rises up and the cold wind from the polar regions rushes in, to take its place. In this way, wind circulation is set up from the poles to the warmer latitudes.
The winds would have flown in the north-south direction from north to south, or from south to north. A change in direction is however, caused by the rotation of the earth.

(b) Uneven heating of land and water.

regions In summer, near the equator the land warms up faster and most of the time the temperature of the land is higher than that of water in the oceans. The air over the land gets heated and rises. This causes the winds to flow from the oceans towards the land. These are monsoon winds.
In winter, the direction of the wind flow gets reversed; it flows from the land to the ocean.
The winds from the oceans carry water and bring rain. It is a part of the water cycle.

 Activity 1:


Tin can crushed by atmospheric pressure

Take a tin can with a tight fitting stopper. First pour a little water into the tin can and heat it over a burner.
Keep on heating until steam starts forming so that it drives out most of the air from the tin can.
Remove the tin can from over the burner and fit the stopper at its mouth tightly.
Atmospheric pressure

Atmospheric pressure

Atmospheric pressure can crush a tin can

What happens to the tin can after sometime? We find that after sometime the tin can gets badly crushed. When the tin gets cooled, the steam inside the tin can condenses. Now, there is nothing inside the tin can to balance the outside atmospheric pressure. Therefore, the atmospheric pressure acting from outside crushes the tin can.

Caution: Do this activity only under the supervision of your teacher.

 Activity 2:

Pumping the air: we use air pump to fill the air into cycle tires and some times football. In pumping,air inside the pump cylinder get pressed by the moving the piston down and subsequently forced inside the cycle tire tube or football through a valve. If we just close the out let of pump with a finger, then it is very difficult to move the piston rod further down to other end as pressed air has no where to go and exert pressure on the piston thus preventing its further down movement.


 Activity 1:

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