Much of earth’s atmosphere is compressed into a relatively thin shell of dense air around the globe. Higher up the air is thinner and rarer. Solar radiations pass through atmosphere and heat earth’s surface. It is from earth’s crust that air derives most of its heat.
Lower layers, therefore, are warmer than upper layers. As warm air rises, lower pressures above cause it to expand and cool down. This brings it back towards earth’s crust to be heated again and an active churning of the contents of atmosphere is brought about.
The magnitude of reduction in the temperatures with increasing altitude or the temperature gradient in the atmosphere is largely determined by meteorological conditions of the locality. It is referred to as the Environmental Lapse Rate (ELR). It is the actual decline in the temperature per 100 metres of height covered. Under dull and cloudy weather earth’s surface may cool down rapidly, cooling the air nearby.
As upper layers are still at the same temperatures, the ELR is lowered than the DALR. Under conditions of bright sunshine the ELR may become significantly higher than the DALR. A parcel of the introduced gas on rising in the atmosphere may find different temperatures at different heights while its own temperatures vary at a steady rate of 1°C for a displacement of 100 metres. The following three situations may occur:
A. Neutral atmosphere:
In cases where ELR is equal to DALR, vertical movement of a mass of air, either downwards or upward, shall cause no variation in its temperature and the density with respect to the surrounding air. The atmosphere, therefore, shall neither resist nor promote any mixing (Fig. 1.3 A).
B. Stable atmosphere:
The term stable atmosphere is used to denote the set of conditions under which vertical mixing of pollutants is actively resisted. This happens when ELR is smaller than the DALR. Any displacement of a mass of air upward shall cool it faster than the surrounding air. Being cooler and, therefore, heavier, it shall show a tendency to move back to its original position.
Likewise any movement downward shall make it warmer and, therefore, lighter than the surrounding air. It shall have the tendency to move up to regain its earlier position. Thus any vertical movement shall be actively resisted and no mixing shall occur.
C. Unstable atmosphere:
It denotes a condition of atmosphere which promotes rapid mixing of contents of the air. This happens when ELR is greater than DALR. Any vertical movement upward of a mass of air shall cool it but the cooling shall not be as much as the cooling of the surrounding air.
The mass of air shall be warmer and hence lighter than the air around it. As a consequence it shall have a tendency to rise up on its own. Similarly, a movement of the air mass downward shall result in its being heavier than the surrounding medium. It shall show a tendency to sink down on its own.
3. Atmospheric inversion and pollution blankets:
It is the rapid and prolonged cooling of earth’s surface which causes conditions under which vertical mixing of the contents of the atmosphere is resisted. Cooling of atmospheric air near earth’s surface while the upper layers are still at the same temperature causes the ELR to be lowered.
Gaseous pollutants discharged in the atmosphere are unable to move upward and tend to accumulate in a thin sheet of air which expands in transverse direction covering the entire locality underneath. Little dilution is possible under such conditions and a blanket of waste gases, fumes and particulate material forms over earth’s surface. The canopy of gaseous pollutants thus formed is known as Pollution blankets.
With further cooling of earth’s crust an inverse temperature gradient is established in the atmosphere. Temperatures rise with height instead of decreasing as is the case under normal conditions. The blanket of pollution developed under stable conditions of atmosphere earlier becomes cooler and heavier. It sinks down bringing the entire load of gaseous wastes to earth’s surface.
Such a condition under which temperature gradient is inverted and upper layers of atmosphere tend to sink back towards earth’s surface is referred to as Atmospheric inversion. In fact, atmospheric inversion is the main cause of most of the air-pollution episodes so far recorded in the world.