UPSC Express

Environmental lapse rate (ELR) is the rate at which the air temperature changes with height in the atmosphere surrounding a cloud or a rising parcel of air.  Though the overall average rate decreases about 6.5°C/km, the rate varies greatly in different airstreams and different regions of the world and at different seasons of the year.

Inversion:

Where the lapse rate of temperature is negative (that is, when temperature increases with the height), a temperature inversion is said have occurred.

Types of temperature inversion:

There are four kinds of temperature inversions: ground, turbulence, subsidence, and frontal.

1) Ground inversion: A ground inversion develops when air is cooled by contact with a colder surface until it becomes cooler than the overlying atmosphere; this occurs most often on clear nights when the ground cools off rapidly due to radiation. If the temperature of surface air drops below its dew point, then the chances of fog appearing increases. Topography greatly affects the magnitude of ground inversions. If the land is rolling or hilly, the cold air formed on the higher land surfaces tends to drain into the hollows, producing a larger and thicker inversion above low ground and little or none above higher elevations.

2) Turbulence inversion: A turbulence inversion often forms when quiescent air overlies turbulent air. Within the turbulent layer, vertical mixing carries heat downward and cools the upper part of the layer. The unmixed air above is not cooled and eventually is warmer than the air below; an inversion then exists.

3) Subsidence inversion: A subsidence inversion develops when a widespread layer of air descends. The layer is compressed and heated by the resulting increase in atmospheric pressure, and as a result the lapse rate of temperature is reduced. If the air mass sinks low enough, the air at higher altitudes becomes warmer than at lower altitudes, producing a temperature inversion.  Subsidence inversions are common over the northern continents in winter and over the subtropical oceans; these regions generally have subsiding air because they are located under large high-pressure centres.

4) Frontal inversion: A frontal inversion occurs when a cold air mass undercuts a warm air mass and lifts it aloft; the front between the two air masses then has warm air above and cold air below. This kind of inversion has considerable slope, whereas other inversions are nearly horizontal. In addition, humidity may be high, and clouds may be present immediately above it. This type of inversions is generally found in northern Europe.

Temperature inversions are interesting and abnormal climatological phenomenon. They can also have adverse health impact when they occur near the surface of the earth – by trapping pollutants near the ground.