Atmosphere – The gaseous envelope surroundings the earth is called earth’s atmosphere.
Chemical composition of the atmosphere
The spectrum of the radiations emitted by planet helps us to decide the nature of the gases present in the atmosphere of a planet. In the spectrum, dark lines correspond to particular elements present in the atmosphere of the planet and intensity of the line tells about the relative abundance of the element.
Atmosphere of the earth consists of 78% nitrogen, 21% oxygen with some carbon dioxide and water vapour.
It extends up to a height of 12km. In this portion, the temperature decreases with height from 290K to 220K. The density of air at 12km height reduces from ρ0 to ρ0 /10.
It extends up to a height of 12km to 50 km from the surface of earth. The density of air decreases with height from ρ0 /10 to ρ0 /103. The temperature of this region varies from 220 K to 280K. Ozone layer which protects the earth from high energy radiation coming from outer space and entering in our atmosphere lies at a height of about 30 km to 50 km from the surface of earth.
It extends from a height 50 km to 80 km from the surface of earth. The density of air decreases from ρ0 /103 to ρ0 /105. The temperature of this region varies from 280 K to 180K.
It extends from a height of 80 km to 400 km above the surface of earth. The density of air falls from ρ0 /105to ρ0 /1010. The temperature of this region varies from 180K to 700K. As temperature is this region rises to some extent hence it is called thermosphere. There are two layers in this region having large number of electrons called Kennely-Heaviside layer and Appleton layer, beyond 110 km and 250 km from the surface of earth which reflect the radio waves and thus are very useful for the propagation of radio waves, by using sky wave propagation.
Green house effect
In the day time, the radiations from the sun reach the earth. At night, the earth atmosphere prevents the infrared radiations of earth from passing through it and thus helps in keeping the earth’s surface warm. This phenomenon is called Green house effect.
The radio waves which are travelling directly following the surface of the earth are known as ground waves. These waves have frequency up to 1500 kHz. The ground waves can bend round the corners of the objects on earth and hence, their intensity falls with distance. That is why ground wave propagation becomes weaker as frequency increases.
The radio waves which are received after being reflected from the ionosphere are called as sky waves. Sky wave has frequency more than 1500 kHz but less than 40 MHz Sky wave propagation from short wave band.