Geography Chapter 04: Air

Class 07 Geography NCERT

The earth is surrounded by a huge blanket of air called atmosphere. Nitrogen and oxygen are two gases which make up the bulk of the atmosphere. Carbon dioxide, helium,
ozone, argon and hydrogen are found in lesser quantities.

Our atmosphere is divided into five layers starting from the earth’s sur face. These are Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere.

Weather is this hour-to-hour, day to day condition of the atmosphere. However, the average weather condition of a place for a longer period of time represents the climate of a place. The degree of hotness and coldness of the air is known as temperature.

An important factor that influences the distribution of temperature is insolation. Insolation is the incoming solar energy intercepted by the earth. The amount of insolation decreases from the equator towards the poles.

Air pressure is defined as the pressure exerted by the weight of air on the earth’s surface. As we go up the layers of atmosphere, the pressure falls rapidly. The air always moves from high pressure areas to low pressure areas. 

The movement of air from high pressure area to low pressure areas is called wind. Winds can be broadly divided into three types: (i) Permanent winds, (ii) Seasonal winds, and (iii) Local winds.

When water evaporates from land and different water bodies, it becomes water vapour. Moisture in the air at any time, is known as humidity. Precipitation that comes down to the earth in liquid form is called rain.