The temperature inversions may be classified in the following types

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According to the processes that cause them and the relative heights from earth’s surface at which they develop, the temperature inversions may be classified in the following types:-

(1) Ground or surface inversions

(a) Radiation inversion

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(b) Advection inversion

(2) Upper-air inversions

(a) Subsidence inversion

(b) Turbulence and convective inversion

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(3) Frontal inversions.

Radiation inversion:

The surface inversion produced by radiational cooling of lower air is called radiational inversion. The inversion layer develops at an altitude of about 90 meters. Nocturnal cooling produced by the terrestrial radiation is the principal factor for this type of temperature inversion.

Since a land surface radiates more heat than the air, ground is cooled more rapidly than the air at great heights during night time. Consequently the coldest air lies at the ground and is overlaid by warmer air.

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The layers of air in close proximity to the earth’s surface are cooled by the processes of radiation and conduction more quickly than the upper layers of air. Thus, at a certain height (90 meters) the temperature increases with attitude, and this increase continues up to about 300 meters from the surface.

Temperature inversion near the surface may be produced under the following conditions: long and clear winter night, clear skies or skies with high clouds, relatively dry air, calm air, and snow-covered surface.

During long and cloudless winter nights the loss of heat by terrestrial radiation exceeds the amount of insolation received at the surface during day time. Therefore the surface gets ample time to get cooled.

When there are no clouds in the sky, radiation cooling after sunset proceeds more rapidly. On the contrary, cloudy nights check the loss of heat by terrestrial radiation which results in relatively higher temperature close to the earth’s surface.

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Similarly, dry air is incapable of absorbing much of the radiant heat from the earth’s surface, so that its temperature does not rise.

Temperature inversion is produced when there is little wind movement near the ground or wind movement is very slow, so that there is little mixing in the lower layers of atmosphere and the ground gets sufficient time to cool down adequately.

In higher latitudes where the ground is snow-covered, solar radiation falling on it is partly reflected back. Thus, the ground heats little by day. On the other hand, at night there is unretarded loss of heat by earth radiation.

Moreover, snow being a poor conductor of heat; it retards the outgoing radiation from the surface lying hidden under it.

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Therefore the air near the surface undergoes rapid cooling, and a temperature inversion is fully developed. These conditions are ideal for the occurrence of frost as well.

Surface inversion promotes stability in the lower layers of atmosphere. Smoke rising from near the ground, dust particles raised from the ground and smoke from the chimneys, all collect beneath the inversion layer and spread horizontally.

In the industrial cities and other factory towns where the smoke particles ejected from chimneys fill the lower strata of atmosphere, morning dense fogs are of common occurrence, especially during winter season.

The atmospheric turbulence produced near the surface transports water vapour to the base of the inversion layer which makes the lower air moister.

In these conditions, if there is light wind near the surface, fog is produced. If the inversion of temperature is sharper, then stratus clouds form beneath the upper boundary of inversion layer.

Over Polar Regions the temperature inversion is a common feature all the year round. Similarly, the snow-covered land masses in the temperate regions witness temperature inversions at night in winter. However, the inversion layer in the polar region is thicker than that in the middle latitude regions.

Generally, temperature inversion near the surface is found over the continents in winter and over the oceans in summer. Except the frozen Arctic Ocean, there is no trace of inversion of temperature over the oceans during the colder months.

While surface inversions of temperature are common on flat land topography, they occur more frequently in valleys in the mountainous regions.

It is a diagrammatic representation of temperature inversion in valleys. In long winter nights the mountain slopes are chilled by nocturnal cooling, so that air temperature in the adjoining layers drops to a considerable degree.

On the other hand, the air in free atmosphere at the same level is much warmer. The cold and denser air from slopes and hilltops tends to slip down in the valley bottoms. Thus, an inverted lapse rate is created up the slopes and in the free air over the valley floor.

That is why fruit growers always prefer gentle slopes to valley bottom for their orchards in countries like Switzer­land and France. The orange gardens in California (U.S.A.) are found on the slopes of hills rather than on valley floors.

Similarly, vine yards in Alsace and Moselle valleys in France have strong preference for hillsides. In valley lowlands the crops that are not of hardier varieties are damaged by killing frost on any clear, calm and cool night during the winter.

So the coffee plantations in the states of Brazil and Columbia are found on the mountain slopes beyond a certain height to protect them from frost injuries.

The most serious climatic hazard produced by surface inversion is seen in the great industrial city of London during winter months. An abnormally large amount of smoke, dust and other atmospheric impurities present in the lower atmosphere produce a very dense fog there.

The pea-soup fogs of London have gained notoriety as the densest fog on earth. The cold and dense air collects into the Thames Valley on winter nights and produces a temperature inversion not much above the ground.

It results in the accumulation of a huge mass of smoke and dust into the air beneath the inversion layer, so that under favourable conditions a very dense and dirty fog develops in that city. The fog is sometimes so dense that the rays of sun cannot pass through it.

Advection inversion:

Advection of a thick layer of warm air over a cold surface produces an inversion of temperature in the lower layers of the atmosphere for the warm air is cooled by conduction.

Of course, this type of inversion occurring at a certain height, i.e. the height of the warmer layer of air, is called advection inversion. This type of stable inversion occurs when the warm air passes over a cold water surface.

Under these conditions, it may also occur over cold land surface or snow-covered ground. In the same way, during summer the oceans are cooler than the adjacent land masses.

So when a cool mass of air is transported to the land, the presence of a warmer mass of air aloft produces the same type of advection inversion. Warm and moist air masses coming from the oceans produce stable inversion over the vast snow-covered lowland of northern Eurasia and northern Canada.

Because of the greater frequency of temperature inversions in cold months of the year, the lapse rates are low in winter and steep in summer.

Subsidence inversion:

Subsidence inversion, as the name indicates, is produced well above the earth’s surface on account of subsidence of air currents. This type of upper-air inversion occurs in an air mass when a thick mass of air subsides.

The sinking air warms at the dry adiabatic rate of 10″C/km. In certain cases, the subsidence continues to a particular level where the air diverges horizontally above a lower layer.

Temperature inversion of this type generally develops in a layer separating the upper region of subsiding air and the lower region characterized by the absence of vertical motion.

Subsidence inversions are of common occurrence in regions of high pressure that are characterized by sinking air. There are circumstances when during the night two inversion layers may develop in the high pressure regions, one at the ground produced by radiational cooling and the other at considerable altitudes produced by subsidence.

There are semi-permanent high pressure regions in the Atlantic and Pacific Oceans in the latitude of 30°N, which are called the Azores High and the Pacific High respectively.

Subsidence in the eastern portions of these anticyclones being more pronounced leads to strong temperature inversions, usually at about 500 m. to 1000 m. above sea-level.

The subsidence inversion is of great environmental significance, for the pollutants from motor vehicles and other industrial sources become concentrated in the lower layers of the atmosphere and thus form a great environmental hazard to the inhabitants of industrial towns.

Trade wind inversion:

Well- developed subsidence inversions are found over the lower trade winds. The thickness of inversion layer varies from a few hundred to about a thousand meters, and increases as the trade wind proceeds towards lower latitudes.

These upper-air inversions of trade winds are associated with the warm anticyclones of tropical regions. The trade wind inversion layer has different values of temperatures in its upper and lower portions.

The difference in temperatures between the top and the bottom of the inversion layer may vary from about ten to only a few degrees. Trade wind inversion is best developed over the eastern parts of the subtropical anticyclones. Besides, the inversion is more pronounced during winter than during summer.

The trade wind inversion prevents the vertical movements of air. That is why precipitation in the trade wind areas is only modest. However, when atmospheric disturbances destroy or lift the inversion layer, the precipitation may be fairly large.

Near the inter-tropical convergence zone or the equator, because of the prevailing con­vergence, the inversion layer generally disappears.

Since upper-air inversion of the type described above does not allow the upward movement of heat and moisture originating from the surface, the upper trades are found to be dry and more stable.

Atmospheric conditions above and below the inversion layer show a sharp contrast. Above the inversion layer the lapse rate is steep, approaching dry the adiabatic rate, but the air is dry.

On the contrary, below the inversion the vertical temperature gradient is steep, moisture content the air is high and there is a larger amount of cloudiness.

Trade wind inversion plays vital role in controlling the vertical circulation in the tropical atmosphere by restricting the vertical development of clouds. It virtually acts as a lid which effectively limits convection.

Turbulence and convective inversion:

This type of inversion is produced at altitudes above the surface by mechanical processes. Turbulence and convection are the contributory factors in causing this type of inversion.

Because of the frictional forces eddies form in the lower layers of atmosphere which transport lower air to higher levels and bring back the upper air to the lower levels. Convectional currents set up in the air near the ground are mainly responsible for the exchange of air between upper and lower levels of the atmosphere.

The phenomena of turbulence and convection cause a thorough mixing of the atmosphere in turbulent layers. However, the turbulent or convective mixing is limited to a certain height beyond which it does not and cannot penetrate.

It is at this height that the convective inversion is formed. In the process of vertical mixing the air carried upward is cooled adiabatically. Similarly the air brought downwards heated at the same adiabatic rate.

After a prolonged mixing in the atmosphere, the air at the maximum height of turbulent penetration becomes colder than what it was before, and that at the bottom of the turbulence layer will be warmer than what it originally was.

The transition from this cold upper part of the turbulence zone to the air above with its temperature unaffected by adiabatic cooling comprises a temperature inversion.

Clouds, if they ever form in this inversion layer, are of stratus or stratocumulus type. In certain situations, turbulence in association with heat from the ground leads to the formation of cumulus or cumulonimbus clouds.

Turbulence inversion may occur at a low level or it may form at very high altitudes. In case the inversion has formed at lower levels, smoke, dust particles and other pollutants are carried up to the inversion where they spread beneath the inversion layer and form distinct smoke or haze lines in clear weather.

On the other hand, the anvil-shaped upper portion of cumulonimbus clouds is the result of inversion at considerable heights.

It may be interesting to note that stratiform clouds appearing in the sky are indicative of the presence of an inversion layer above them. Sometimes the upper air inversion, by imposing a restriction on their vertical growth, makes the cumulus clouds stunted in appearance.

Frontal inversion:

The inverted lapse rate at the front is called frontal inversion, when differing air masses are brought together by converging movements; the warmer air being relatively higher tends to overlie the colder and denser air in a horizontal layer.

However, because of the Coriolis force the boundary zone between the air masses with contrasting physical properties are never horizontal; they are rather sloping. In fact, the frontal zone itself is converted into inversion layer in which the lapse rate is inverted.

In other words, at the frontal zone as one move up from the lower to upper layers of the atmosphere, there is an increase in temperature with increasing altitudes.

The following characteristics distinguish frontal inversion from other types of inversion: – (a) The inversion layer associated with fronts is sloping, while in the other types of inversion it is horizontal, (b) In frontal inversion the moisture content shows a marked increase with elevation, while in other types the temperature increases and humidity decreases with the increasing elevation.

That is why above the inversion layer clouds are generally seen. In other words, the frontal inversions show an increasing specific humidity in the inversion layer.

It is generally seen that along the fronts the inversion, in the strictest sense of the term, is never found. What happens is that the actual lapse rate becomes very low in the inversion layer. The reason is quite simple.

The warmer air masses ascend the retreating wedge of cold air and they cool by adiabatic expansion. Because of expansional cooling of the upward moving air at adiabatic rate, the frontal inversions are rarely observed beyond the height of 2 kilometers.

At greater heights there is a marked decrease in the lapse rate in frontal zone, whereas just above the top of the clouds formed by the rising warm air currents the inversion is always present. Recent investigations attach special significance to frontal inversions in the origin of extra tropical cyclones.

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