Inversion of temperature occurs when warm air lies over cold air. Normally, temperature decreases with increasing altitude in the troposphere at an average rate of 6.5 °C per 100 meters but sometimes this normal trend of decrease of temperature with increasing heights is reversed under special circumstances i.e. temperature increases upward up to a few kilometers from the Earth’s surface. Thus, warm air layer lies over the cold air layer. This phenomenon is called temperature inversion.
Types of Temperature Inversion:
Inversion of temperature occurs in several conditions. Sometimes, it occurs at the ground surface while sometimes, it occurs at a greater height. Thus, temperature inversion classified into the surface and upper-air inversion based on relative heights from the Earth’s surface
(1.) Surface Inversion:
The inversion of temperature that occurs at ground levels.
(A) Radiation Inversion or Ground Surface Inversion, also called as Non-Advectional Inversion because it occurs in static atmospheric condition, characterized by no movement of air whether horizontal or vertical.
Such inversion normally occurs during the long cold winter nights in the snow-covered regions of the middle and high latitudes.
Surface inversion is caused due to excessive nocturnal cooling of the ground surface due to the rapid rate of loss of heat from the ground outgoing longwave terrestrial radiation.
Thus, the layer coming in contact with the cool ground surface also becomes cool while the air layer lying above is relatively warm. Consequently, temperature inversion develops because of the cold air layer below and the warm air layer above.
The ground inversion generally disappears with sunrise but sometimes it persists up to noon.
(B) Advectional Inversion develops where there is a horizontal movement of air occurs. Such inversion is caused when warm air invades the area of cold air, or cold air moves into the area of warm air because warm air being lighter is pushed upward by relatively dense cold air. Advectional inversions are usually short-lived (typically overnight) and shallow.
(C) Valley Inversion or Cold Air Drainage Inversion generally occurs in the mountainous valleys due to radiation and vertical movement of air.
The temperature of the upper parts of the valleys in mountainous areas becomes exceedingly low during winter nights because of the rapid rate of loss of heat from the surface through terrestrial radiation.
Consequently, the air coming in contact with the cool surface also becomes cool. On the other hand, the temperature of the valley floor does not fall considerably because of the comparatively low rate of loss of heat through terrestrial radiation.
Thus the air remains warmer than the air aloft and hence the warm and light air of the valley floor is pushed upward by the descending cold and heavier air of the upper part of the valley. Thus, there is warm air aloft and cold air in the valley floor and inversion of temperature is caused.
This situation is responsible for severe frost in the valley floors causing great damage to fruit orchards and vegetables and crops where is the upper parts of the valleys are free from frost. This is why the valley flowers are avoided for human settlements while the upper parts are inhabited in the mountainous valleys.
(2.) Upper-Air Inversion:
The inversion of temperature that occurs at well above the ground surface nearly always is the result of air descending from above.
(A) Ozone Layer Inversion is caused by the presence of ozone layer lying between the height of 15 to 35 kilometer in the stratosphere.
The ozone layer absorbs most of the ultraviolet rays radiated from the Sun and thus the temperature of this layer becomes much higher than the air layers above and below ozone layer.
This inversion of temperature occurs only when there is no vertical movement of air.
(B) Mechanical Inversion also called as Subsidence Inversion. The mechanical inversion of temperature is caused at higher heights in the atmosphere due to subsidence of air and turbulence and convective mechanism. Such inversion occurs in several ways e.g.
- Sometimes warm air is suddenly transported upward ( due to eddies formed by frictional forces) to the zone of cold air and thus cold air being denser lies under the warm air and inversion of temperature is caused.
- The descending air is warmed at the dry adiabatic rate of 10 °C per 1000 meters because of compression. This situation causes the existence of warm air above the cold air. Such mechanical inversion caused by the subsidence of air currents is generally associated with anticyclonic conditions. Such inversion of temperature is of very common occurrence in the middle latitudes where high pressures are characterized by sinking air.
- Trade Winds Inversion found in the tropics at heights up between 2000 m and 3000 m, caused by descending air of the Hadley Circulation. The heated air from the equator rises and takes a westerly course towards the north, around subtropics, it sinks and moves equatorward in the form of north-east trades producing an inversion layer between them.
In these cases, the frontal zone itself is converted into the inversion layer and through the inversion layer, there is an increase in temperature with increasing altitude.
(C) Frontal or Cyclonic Inversion is caused in the temperate zones due to temperate cyclone which is formed due to the convergence of warm westerlies and cold polar air and thus the warm air overlies the cold polar winds in the northern hemisphere. The warm air is pushed up by the cold air because it is lighter than the cold air. Thus, the existence of warm air above and cold air below reverses the normal lapse rate and inversion of temperature occurs.