Composition And Structure Of Atmosphere

Nitrogen constitutes about 78-79% of the total gases present in the atmosphere. Oxygen constitutes 20-21% and other gases constitute 1%.

The troposphere layer is the lowermost layer of the earth’s atmosphere. It contains all the gases to a wide extent i.e. oxygen and is very important to the human beings.

Sea salts, pollen, ash, smoke soot, fine soil together form the dust particles. They are very small and accumulate to form dust.

The density of oxygen decreases with increase in height and so it will be in a negligible quantity at the height of 120 km from the earth’s surface.

Carbon dioxide in the earth’s atmosphere is transparent to the incoming solar radiation and opaque to outgoing terrestrial radiation. So it large accounts for greenhouse effect.

(i) Atmosphere is an envelope of gases surrounding the earth or any other planet. Nitrogen, oxygen, and other gases are present in the earth’s atmosphere. It is divided into a number of layers.

(ii) The elements of weather and climate are temperature, pressure, winds, humidity, clouds and precipitation. These elements are subject to change and influence human life on Earth.

(iii) The atmosphere is composed of gases, water vapour and dust particles. Nitrogen constitutes about 78-79%, oxygen constitutes about 20-21% and other gases like carbon dioxide, helium, hydrogen, methane constitute about remaining 1%.

(iv) Troposphere is the most important of all the layers of the atmosphere because-

• Most of the weather phenomena take place in this layer. This troposphere contains almost all the water vapour and most of the dust particles.

• All biological activities take place in this layer.

(i) The atmosphere is a mixture of gases, dust particles and water vapour. The following table illustrates the percentage composition of the gases

Gases like nitrogen and oxygen cover the maximum portion of the gases and other are covered by dust particles like smoke, pollen ash, soot etc. and water vapour. The density of gases decreases with increase in height. At above 120km, the oxygen density becomes negligible whereas carbon dioxide and water vapour remains upto 90 km from the surface of the earth. Among the gases the percentage of carbon dioxide is increasing day by day. Carbon dioxide is a greenhouse gas that allows ultraviolet rays to reach the earth and prevent the extra terrestrial rays from escaping. The water vapour absorbs the part of the insolation from the sun and prevents the earth’s radiated heat. It acts like a blanket allowing the earth neither to be too cold nor too hot.

(ii)

The atmosphere has a multilayered structure consisting of the following layers: Troposphere, Stratosphere, Mesosphere, Ionosphere, and Exosphere.

• Troposphere- The word troposphere is derived from the Greek word ‘tropos’ and it means turbulence and mixing. Most of the weather phenomena take place in this layer. This troposphere contains almost all the water vapour and most of the dust particles. Temperature decreases with height at an average rate of 1°C per 167 m of height above the sea level.

• Tropopause- The upper limit of the atmosphere is called the tropopause. The temperature stops decreasing at this level. It may be as low as -35°C

• Stratosphere- Above the troposphere lies the stratosphere which extends up to 50km above the surface of the earth. The stratosphere is free from water vapour and dust particles. There are no clouds or turbulence in the air. The temperature increases due to absorption of heat of the ultraviolet rays of the sun by Ozone present in this layer. It absorbs the harmful ultraviolet rays from the sun which would severely harm all the living beings on Earth.

• Mesosphere- The Mesosphere is the layer which extends to a height of 80 km above the stratosphere. It has he coldest temperature in the atmosphere. The temperature is around -100°C.

• Thermosphere- The ionosphere, also known as the Thermosphere extends from 80 km to 480 km. Amazingly, temperature in this layer rises to a very high degree, i.e. 1480°C sue to molecules in this layer absorbing the Sun’s deadly x-rays and its shot wave ultraviolet radiation of x-rays and gamma rays.

• Exosphere- The topmost layer of the atmosphere is called the exosphere. The atmosphere is in a rarefied form and pressure is extremely low. Temperature increases with height in this layer. The lack of gravity in exosphere allows traces of gas molecules such as oxygen, nitrogen, argon, and helium to be present.

Ozone layer prevents the ultraviolet rays from reaching the Earth. In absence of the ozone layer the rays will reach to us and damage our skin causing skin cancer and other deadly diseases.