Like all gases, air constantly moves. Masses of air, warm or cool, wet or dry, move across land and sea and bring about weather changes. During this process, one air mass replaces another. When air is heated, it expands. Hot air is less dense than cold air. For this reason, it rises and leaves behind an area of low pressure. Unlike hot air, cold air has a large density.

Instead of rising, it presses heavily on the earth’s surface. Therefore, it produces an area of high pressure. Since gases always try to move from high to low pressure, winds are caused by the flow of cold air which tries to replace the rising hot air.



Why is there such a difference in the temperature of the air at various places on earth?

There are two major global air patterns on Earth. One is from the poles towards the equator and the other is from the equator towards the poles. On the earth’s surface, the poles are always cold and the equator hot. Cold air comes down from the polar regions. Since the distance from the poles to the equator is so great, the cold air from the poles warms up on the way.

Similarly, the hot equatorial air becomes cooler on its way to the poles and this is what causes the difference in temperature. These winds do not blow in the north-south direction, but they are diverted. The rotation of the earth is the cause of this change in direction. These two major global air patterns cover thousands of kilometres. Besides these air patterns, there are smaller cycles which cover hundreds of kilometres. These smaller air patterns form because of smaller changes in temperature.

For example, the air above the ground is heated by the ground whereas the air above the sea is colder. As a result, the cool air moves from the sea to the land, forming a ‘sea breeze’. During the night, the land is cooler than the sea (since water heats up and cools down more slowly) and the breeze blows from the land to the sea. This wind is called a ‘land breeze’. Winds that blow very powerfully can develop into storms, which can turn into hurricanes.


Actually, no one knows why some of the storms become hurricanes and others do not. A hurricane forms over tropical seas, it moves, and when it reaches the land or a colder part of the sea, it slowly diminishes, dies out. A hurricane can be 1000 kilometres in diameter. The centre of the hurricane is called the ‘eye’. The speed of the wind in a hurricane can range from 150 kph. (kilometres per hour) to 300 kph.


All hurricanes originate close to the equator. Hurricanes in the Pacific and Indian Oceans are known as  ‘typhoons’. Sometimes storms can also develop into tornadoes. These resemble hurricanes but form over land. Tornadoes can occur anywhere on Earth but are mostly observed over the central United States. A tornado, like a hurricane, is a strong wind spinning and turning around  a core. Unlike a hurricane, it contains a partial vacuum. The wind speed of a tornado is about 300 kph., but sometimes it can reach 800 kph.


Scientists do not know exactly how tornadoes form. It is thought that when warm moist air meets the cold air from the north, it causes clouds to form and storms to develop. This brings  about an uprush of warm air, which is known as a tornado. When a tornado passes over a house, for example, the low pressure at the centre causes the air in the house to expand suddenly and, as a result, the building explodes.

A. What do the following refer to?
1. ‘it’ (line 5):
2. These’ (line 41):
B. Mark the best choice.


1. Line 20, ‘diverted’ probably means .

a) directed b)changed c) blown d) rotated

2. Hot air rises because it .

a) leaves behind an area of low pressure           b) is not as dense as cold air

c) produces areas of high pressure            d) has a large density

3. Winds form due to the .

a) flow of cold air into a low pressure area

b) fact that hot air presses on the earth’s surface

c) flow of hot air into a high pressure area

d) fact that air is a gas

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