Published on: 12/17/2020 3:11:04 AM

Temperature inversion, a reversal of the normal behaviour of temperature in the troposphere, in which a layer of cool air at the surface is overlain by a layer of warmer air. Such situation may occur near the earth's surface or at greater height in the troposphere but the inversion of temperature near the earth's surface is of very short duration because the radiation of heat from the earth's surface during daytime warms up the cold air layer which soon disappears and temperature inversion also disappears. On the other hand, upper air temperature inversion lasts for longer duration because the warming of cold air aloft through terrestrial radiation takes relatively longer period of hours.

Temperature inversion may be produced in five ways

1. When the earth's surface loses more heat by radiation than it gains by any of the energy transfer processes, as on a clear night or at high latitudes in winter, it cools and consequently lowers the temperature of the adjacent layer of air. Inversions due to radiational cooling (radiation inversions) develop best in calm air under clear skies over flat terrain and often are accompanied by ground fog or surface concentration of pollutants. They are not likely to form over water, which is slow to cool.

2. Because of its greater density, cold air from hilltops and slopes tends to flow downslope to collect in valley bottoms, creating an inverted lapse rate up the slopes as well as in the free air over the valley floor. Air drainage inversions are frequently associated with spring frosts in middle latitudes, and for this reason fruit growers prefer gentle slopes to valley bottoms for orchard sites.

3. When two air masses with different temperature characteristics come together, the colder air, being denser, tends to push underneath the warmer air and replace it. The boundary zones along which two air masses meet are called fronts, and the inverted lapse rate which results is a frontal inversion. Frontal inversions are not confined to the lower layers of the troposphere as are the types mentioned previously; they may form at upper levels wherever cold air underruns warm air or warm air advances above cold air.

4. Advection of warm air over a cold surface creates an inversion in the lower layers of the air mass as the warm air is cooled by conduction. This process is common when warm air passes over a cold-water surface, cold land, or snowfields, where fog may result.

5. Another type of inversion, the subsidence inversion, forms in an air mass when a large body of air subsides and spreads out above a lower layer. In the process the air heats dynamically more in the upper portion than at its base. Inversions of this type may develop at considerable altitudes. They are especially common in the region of the trade winds and in large, slow-moving high-pressure areas.

Significance of Temperature Inversion

Though inversion of temperature denotes local and temporary conditions of the atmosphere but there are several climatic effects of inversion which are of great significance to man and his economic activities.

1. Fog is formed due to the situation of warm air above and cold air below because the warm air is cooled from below and resultant condensation causes the formation of tiny droplets around suspended dust particles and smokes during winter nights. The smokes coming out of houses and chimneys intensify fogs and become responsible for the occurrence of urban smogs. When smog is mixed with air pollutants such as sulphur dioxide it becomes poisonous and deadly health hazard to human beings.

Fogs are also formed at the convergence of warm and cold ocean currents. For example, dense and extensive fogs are formed near Newfoundland due to convergence of warm Gulf Stream and cold Labrador current. Fogs reduce atmospheric visibility and thus they are responsible for several cases of accidents of air crafts while taking off and landing and ships in the oceans. Road and rail transport are also badly affected by the occurrence of dense fogs. Though generally fogs are unfavourable for many agricultural crops such as grams, peas, mustard plants, wheat etc. but sometimes they are also favourable for some crops such as coffee plants in Yemen hills of Arabia where fogs protect coffee plants from direct strong sun's rays.

2. Inversion of temperature causes frost when the condensation of warm air due to its cooling by cold air below occurs at temperature below freezing point. Frost is definitely economically unfavourable weather phenomenon mainly for crops because fruit orchards and several agricultural crops such as potatoes, tomatoes, peas etc. are totally damaged overnight. There are frequent cases of destruction of fruit orchards in the lower parts of valleys due to severe frost caused by inversion of temperature in California (USA).

The valley floors in the hills of Brazil are avoided for coffee cultivation because of frequent frosts. Alternatively, coffee is planted on the upper slopes of the valleys. The upper parts of the valleys are inhabited in Switzerland while lower parts are avoided. In fact, the upper slopes of valleys are characterized by fruit orchards, hotels and motels (for tourists), various resort centres etc. while the lower slopes are deserted.

3. Inversion of temperature causes atmospheric stability which stops upward (ascent) and downward (descent) movements of air. The atmospheric stability discourages rainfall and favours dry condition. The inversion of temperature caused by the subsidence of air resulting into anticyclonic conditions increases aridity. This is why the western parts of the continents situated between 20°-30° latitudes and characterized by anticyclonic condition represent most widespread tropical deserts of the world.