Table 1 – Average composition of dry air

Nitrogen does not easily enter into chemical union with other substances, but it is an important constituent of many organic compounds. Atmospheric nitrogen acts as a reservoir pool for nitrogen cycle. Nitrogen fixing organisms such as Rhizobium use free nitrogen of the atmosphere to convert it to usable form such as nitrates.

Oxygen is an important part of the atmosphere and is necessary to sustain terrestrial life as it is used in respiration. It is also used in combustion. It is believed that first oceans got saturated with oxygen and after that it started flowing into the atmosphere. Source of oxygen is plants with photosynthesis. Mountain climbers sometime require oxygen cylinders due to low concentration of oxygen at greater heights.

Argon is an inert gas. Argon extracted from the atmosphere is used for industrial purposes such as bulb manufacturing, welding equipments etc.

Carbon dioxide is released from the earth’s interior, respiration, soil processes, deforestation, and combustion. Carbon dioxide is meteorologically a very important gas as it is transparent to the incoming solar radiation but opaque to outgoing terrestrial radiations. It absorbs a part of terrestrial radiation and reflects back some part of it towards the earth’s surface. It is largely responsible for the greenhouse effect.

Ozone is another important constituent of atmosphere. Ozone is made up of three atoms of oxygen when three molecules of oxygen gas convert into two molecules of Ozone using sun’s high energy radiations. It is found in very small quantity (0.00005 per cent by volume) in the upper atmosphere, 15-50km above the earth’s surface. Maximum concentration is found at the height of 15-35km. It protects the life on earth by absorbing ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth. In the absence of the ozone layer, high energy ultra-violet rays would have made earth unfit for habitation.

1.2. Water Vapour

Table 1 refers to the average constituents of dry air. The lower parts of the atmosphere, up to 10-15 km, contain in addition water vapour, which is largely derived by evaporation from water bodies on the earth and by transpiration from plants. It is one of the ‘most variable’ components of the atmosphere. It decreases with altitude and not found at great heights because mixing and turbulence is not sufficiently strong to carry it up very far. In the warm and wet tropics, it may account for 4% of the air by volume, while in the dry and cold areas of desert and polar regions, it may be less than 1% of the air. Water vapour also decreases from the equator towards the poles.

Water vapour, too, is capable of absorbing heat and acts like a blanket allowing the earth neither to become too cold nor too hot. Water vapour is fundamental to many essential meteorological processes, such as rain-making. It is source of all clouds and precipitation. In the condensation process, vast amount of energy is released in form latent heat of condensation, ultimate driving force for most of the storms.

The actual amount of the water vapour present in the atmosphere is known as the absolute humidity. It is the weight of water vapour per unit volume of air. The absolute humidity differs from place to place on the surface of the earth. The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as the relative humidity. It is greater over the oceans and least over the continents. The air containing moisture to its full capacity at a given temperature is said to be saturated. Moisture holding capacity of the air is directly proportional to its temperature.