General Awareness Atmosphere

Atmosphere

Atmosphere is a thick gaseous envelope surrounding the Earth from all sides and attached to Earth through the force of
gravitation.

Significance of Atmosphere :

Acts as a filter because it absorbs the various unwanted radiation and source to various gases. It supports life forms in biosphere.

Extent of Atmosphere :

Almost 97 % , of effective atmosphere confines within the height 29 km of the Earth's surface. So, the vertical distribution of the atmosphere is not uniform and even not homogeneous. It extends between 16-29000 km from the sea level.

Composition of Gases :

The atmosphere is composed of gases, vapours and particulates. Nitrogen, oxygen, carbon dioxide are present. Gases such as helium, ozone and hydrogen etc are present in traces. Ozone gas absorbs the ultraviolet radiations and save the biosphere from its adverse impact.

Layers of Atmosphere

`text(Troposphere)`

• It extends upto 16 km from the Earth's surface. Thickness varies from 8 km at the poles to 16 km at the equator. With every 165 m ascent, there is a drop of 1°C (or 6.4°C per km). This is called Normal Lapse Rate of Temperature.

• Tropopause separates troposphere from stratosphere. This layer accounts for practically the entire water vapour, all dust particles and most of the carbon dioxide contained in the atmosphere. Due to this all weather phenomena such as condensation, precipitation and storms, etc occur in the troposphere only.

`text(Stratosphere)`

• The stratosphere extends upto about 50 km, where stratopause separates it from the mesosphere.

• In this layer, the temperature increases with increase in height. This phenomenon is known as temperature inversion. The temperature rises in this layer from about -60 °C at the tropopause to 0 °C at stratopause.

• The part of the stratosphere, in which there is a concentration of ozone is often called ozonosphere. It absorbs ultraviolet radiation, which is harmful for us.

• Stratosphere is free from dust particles and also from atmospheric turbulence. Hence, this layer is considered ideal for flying of jet aircraft.

`text(Mesosphere)`

• Mesosphere extends above the stratopause upto a height of about 80 km. ln this layer, the temperature decreases with height like in the troposphere and it falls from about 0°C at its base to about -100°C at 80 km height.

• It is considered the coldest layer of the atmosphere. The upper limit of the mesosphere is marked by the Mesopause, a transitional layer separating it from the ionosphere.

`text(Ionosphere)`

• Ionosphere is located above the mesosphere and extends upto about 600 km. This layer is also called as ionosphere because it contains electrically charged ions that reflect the radiowaves back to the Earth thus, making radio communication possible.

• Absorption of solar radiation by ionised particles cause an increase in temperature with increasing height in the ionosphere.

• Due to large concentration of ionised particles in this layer the ionosphere acts as a protective layer against meteorites, that are burnt in this layer.

`text(Thermosphere)`

• The zone between the 85 km and 400 km above the surface is often called thermosphere. In this layer, the temperature increases with increasing altitude. The upper limit of the thermosphere, the thernopause is generally taken at an altitude of about 600 km.

• The day temperature at 600 km altitude exceed 1400°C while night temperature remain about 225°C. The upper part of the thermosphere contains only the lighter gases like helium and hydrogen.

`text(Exosphere and Magnetosphere)`

• The outermost part of the atmosphere of the Earth is called exosphere. This zone of the atmosphere extends up to a height of about 900 km.

• The upper limit of the exosphere is uncertain as this layer acts as a transitional layer between the Earth's atmosphere and the space. The outer part of the exosphere is called magnetosphere.

Atmospheric Pressure

Air is an extremely compressible gas having its own weight. The pressure exerted by air due to its weight is called atmospheric pressure on the Earth's surface. Atmosphere pressure is neither the same for all the regions nor the same for one region all the time. It is affected by various factors such as altitude, temperature and Earth's rotation.

Influence on the Atmospheric Pressure :

• Altitude Air pressure increases, when air descends due to the decrease in volume. When air rises its volume increases and the outward pressure of its molecule is spread over a larger area and its pressure decreases.

• Temperature The pressure of air rises, when its temperature falls. Low temperatures at the poles cause the air to contract-high pressure develops; whereas the high temperature along the equator cause the air to expand-low pressure develops.

Pressure Belts

The distribution of pressure is highly uneven and this is partly a result of variation in distribution of temperature. These are seven pressure belts on the globe. These pressure belts are discussed below

`text(Equatorial Low Pressure Belt)`

• It is located on either side of the geographical equator in a zone extending between 5 °N and 5 °S. Its location is not stationary and there is a seasonal drift of this belt with the Northward (summer solstice) and Southward (winter solstice) migration of the Sun.

• It is thermally induced because of the intense heating of the ground surface by the almost vertical Sun rays.

• It represents the zone of convergence of North-East and South-East trade winds. This convergence zone is characterised by light and feeble winds. And because of the frequent calm conditions this belt is called as a belt of calm or doldrums.

 
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