Atmosphere

The atmosphere is a thick gaseous envelope which surrounds the earth from all sides and is attached the earth's surface by gravitational force. It filters the incoming solar radiation and thus prevents the ultraviolet rays to reach the earth's surface.

Composition of the Atmosphere

The atmosphere is composed of gases, water vapour and particulates. Among the gases, Nitrogen is in the highest amount, followed by Oxygen, Argon, Carbon dioxide, Neon, Helium, Ozone, hydrogen etc. in that order. Besides these gases, water vapor, dust particles and other particulates are also present in varying amounts.

• Nitrogen (78%): Among the atmospheric gases, it is the most important. It is fixed by the leguminous plants into nitrogenous nutrients.
• Oxygen (21%): This is the life-giving gas to the humans and animals. Green plants produce it during the process of photosynthesis.
• Argon (0.93%): This is a noble gas.
• Carbon dioxide (0.03%): This is a heavy gas. It is permeable for the incoming solar radiation but opaque for the outgoing terrestrial radiations. In this way, by trapping the heat, it works as a greenhouse gas. An International consensus is made to bring down its level by the Kyoto Protocol (1997).
• Ozone : Though it is present in very less amount,, it is an important constituent of the atmosphere. It acts as a filter and absorbs the harmful ultraviolet rays. If Ultraviolet rays reach the earth's surface, they may cause skin cancer and other diseases. Montreal Protocol (1987) was agreed upon to save the ozone layer from depletion.
• Water vapour : The water vapour content in the atmosphere ranges between 0-4% by volume. The water vapour content of the atmosphere decreases from the equator towards the poles, due to decreasing temperature. Climatically, water vapour is very important constituent of the atmosphere. The atmospheric water vapour is received through the evaporation of moisture and water from the water bodies, vegetation and soil covers. The moisture content of the atmosphere creates several forms of condensation and precipitation, e.g.- clouds, fogs, dew, rainfall, frost, hailstorm, ice, snowfall, etc.
• Particulates : The solid particles present in the atmosphere include dust particles, salt particles, pollen, smoke-soot, volcanic ashes, etc. Dust particles are hygroscopic nuclei that help in the formation of water droplets, clouds and various forms of condensation and precipitation.

Five Layers of the Earth's Surface

On the basis of the characteristics of temperature and air pressure, there are five layers of the earth's surface upwards.

1. Troposphere : It is the lower most and the most important layer of the atmosphere, as air all the weather phenomena occur in this layer. The average height of the troposphere is about 16-18 km. over the equator and 6-8 km. Over the poles. In this layer, temperature decrease with increasing height at the rate of 1°C/165 - or 6.5°C/1000 m. This rate of decrease temperature is called Normal lapse rate.
2. Stratosphere:Temperature remains stable at the beginning of this layer but it suddenly starts changing after the height of 20 km. This layer of the atmosphere is almost free from the weather disturbances, hence it is preferred by the pilots to fly their aeroplanes.
3. Mesosphere : This layer extends between 50 km and 80 km. Temperature again decreases with increasing height and reaches upto -100°C, which is the minimum temperature of the atmosphere.
4. Ionoshphere : It extends from 80 km to 640 km. Electrically charged or ionized particles are abundantly found in this layer and temperatures increases with increasing height. This layer reflects back the radio waves.
5. Exosphere : It represents the uppermost layers of the atmosphere. It extends beyond 640 km of height from the sea level. Electrically charged particles are found abundantly in this layer also and there are separate layer of N₂, O₂,He and H₂. The atmosphere becomes rarefied at the height of 1000 km, and it ultimately merges with the space beyond the height of 1000 km.