In this article we will discuss about:- 1. Meaning of Temperature Inversion 2. Types of Temperature Inversion 3. Significance.
Meaning of Temperature Inversion:
Temperature decreases with increasing altitudes in the troposphere at an average rate of 6.5°C per 1000 metres (normal lapse rate) but sometimes this normal trend of decrease of temperature with increasing heights is reversed under special circumstances i.e., temperature increases upward upto a few kilometres from the earth’s surface. This is called negative lapse rate.
Thus, warm air layer lies over cold air layer. This phenomenon meteorologically is called inversion of temperature. 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 layer aloft through terrestrial radiation takes relatively longer period of hours.
Types of Temperature Inversion:
Inversion of temperature occurs in several conditions. Sometimes, it occurs at ground surface while some times, it occurs at greater height. Sometimes, it is caused in the static atmospheric conditions while some times, it occurs due to horizontal or vertical movement of air.
Thus, temperature inversion is classified into the following types on the basis of relative heights from the earth’s surface at which it occurs and the type of air circulation:
(1) Non-advectional Inversion:
(i) Ground or surface inversion or radiation inversion.
(ii) Upper air inversion.
(2) Advectional Inversion:
(i) Frontal inversion or cyclonic inversion.
(ii) Valley inversion due to vertical air movement.
(iii) Surface inversion due to horizontal air movement.
(3) Mechanical Inversion:
(i) Subsidence inversion, and
(ii) Turbulence and convective inversion.
Ground Surface Inversion:
Ground surface inversion also called as radiation inversion occurs near the earth’s surface due to radiation mechanism. This is also called as non- advectional inversion because it occurs in static atmospheric condition characterized by no movement of air whether horizontal or vertical (it may be noted that air is never static).
Such inversion normally occurs during the long cold winter nights in the snow-covered regions of the middle and high latitudes. In fact, surface inversion is caused due to excessive nocturnal cooling of the ground surface due to rapid rate of loss of heat from the ground through outgoing long-wave terrestrial radiation.
Thus, the air coming in contact with the cool ground surface also becomes cold while the air layer lying above is relatively warm. Consequently, temperature inversion develops because of cold air layer below and warm air layer above (fig. 34.1).
The ground surface inversion occurs under the following conditions:
(i) Long winter nights, so that the loss of heat by terrestrial radiation from the ground surface during night may exceed the amount of insolation received from the sun through incoming shortwave electromagnetic radiation waves and thus the ground surface becomes too cold.
(ii) Cloudless and clear sky, so that the loss of heat through terrestrial radiation proceeds more rapidly without any obstruction. Clouds absorb terrestrial radiation and hence retard loss of heat from the earth’s surface.
(iii) Presence of dry air near the ground surface, so that it may not absorb much heat radiated from the earth’s surface as moist air is capable of absorbing much of the radiant heat from the earth’s surface.
(iv) Slow movement of air, so that there is no transfer and mixing of heat in the lower layers of the atmosphere.
(v) Snow-covered ground surface, so that there is maximum reflection of incoming solar radiation. Snow, being bad conductor of heat retards the flow of heat from the ground surface lying below the snow- layers to the lower atmosphere.
Since such inversion of temperature occurs in the static atmospheric condition (very little movement of air) and hence it is also called static or non-advectional inversion. Ground inversion occurs in the low latitude areas (tropical and subtropical areas) during winter nights only and the inversion generally disappears with sunrise but sometimes it persists upto noon.
The inversion occurs up to the height of 30-40 feet in the low latitudes, a few hundred feet in the middle latitudes and half a mile in the high latitudes. It is apparent that the duration and height of surface inversion increase poleward. This inversion promotes stability in the lower portion of the atmosphere and causes dense fogs.
Upper Air Inversion:
Upper air inversion is of two types viz.:
(i) thermal upper air inversion and
(ii) mechanical upper air inversion.
The thermal upper air inversion is caused by the presence of ozone layer lying between the height of 15 to 35km (even up to 80km) 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 lying above and below ozone layer.
This inversion occurs only when there is no vertical movement of air (either ascent or descent of air). 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 a number of ways e.g.:
(a) 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,
(b) The descending air is warmed at the dry adiabatic rate of 10°C per 1000m 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 the anticyclonic conditions. Such inversion of temperature is of very common occurrence in the middle latitudes where high pressures are characterized by sinking air.
The poleward regions of trade winds are also characterized by high pressure caused by the subsidence of air and thus there is also frequent mechanical inversion of temperature. Since the temperature inversion causes stability in the atmosphere and thus there is dry condition. This is the reason that the poleward parts of trade winds are characterized by arid conditions.
Advectional inversion of temperature is also called as dynamic inversion because it is always caused due to either horizontal or vertical movements of air. Strong wind movement and unstable conditions of the atmosphere are prerequisite conditions for advectional inversion of temperature.
This type is further divided into 3 subtypes on the basis of the nature of air movements e.g.:
(i) Frontal or cyclonic inversion,
(ii) Valley inversion due to vertical movement of air, and
(iii) Surface inversion due to horizontal movement of air.
(i) Frontal or cyclonic inversion is caused in the temperate zones due to temperate cyclones which are formed due to the convergence of warm westerlies and cold polar winds in the northern hemisphere. The warm air is pushed up by the cold polar air and thus the warm air overlies 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 (decrease of temperature with increasing height) and inversion of temperature occurs.
It may be pointed out that contrary to other types of temperature inversion, the inversion layer associated with frontal or cyclonic inversion is always sloping because the coriolis force causes the boundary zone (front) between the warm westerlies and cold polar air masses to become slopy. It is also interesting to note that air moisture increases upward in frontal inversion of temperature while it decreases upward in other types of temperature inversion.
(ii) Surface inversion of temperature caused by horizontal movement of air occurs in several situations. 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 denser cold air.
When the warm air moves, such inversion is caused over the continents during winter and over the oceans during summer but when the cold air becomes active and invades the areas of warm air, such inversion occurs over the continents during summer and over the oceans during winter. Such surface inversion occurs generally in the low latitudes. The convergence of cold and warm ocean currents also causes such inversion of temperature.
(iii) Valley inversion generally occurs in the mountainous valleys due to radiation and vertical movement of air. This is also called vertical advectional inversion of temperature. The temperature of the upper parts of the valleys in mountainous areas becomes exceedingly low during winter nights because of 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 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 agricultural crops whereas the upper parts of the valleys are free from frost. This is why the valley floors are avoided for human settlements while the upper parts are inhabited in the mountainous valleys of middle latitudes.
Fig. 34.2 explains the mechanism of valley inversion of temperature:
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. The incidents of deadly urban smogs of December 1930 in Meuse Valley of Belgium (600 people fell ill and 63 people died), of October 26, 1948 at Donora in Pennsylvania (USA), and of 1952 in London (causing 4000 human deaths) tell the ordeal of urban smogs. 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 is 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). Fruit orchards were severely damaged in California in the year 1913, 1917 and 1923 due to severe frosts.
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.