Temperature Inversion

Temperature inversion, is a reversal of the typical behavior of temperature in the troposphere, in which a layer of cool air at the surface is overlain by a layer of warmer air. (Under normal conditions, temperature usually decreases with height).

Mechanism

In this meteorological occurrence, a layer of warm air sits on top of the cold air layer.
It is caused by static atmospheric conditions, but it can also be induced by horizontal or vertical air movement.
Temperature inversions are usually short-lived, but they are quite common.

Suitable Conditions for Temperature Inversion

Long winter nights: During the night, heat loss from the earth surface due to terrestrial radiation may exceed the amount of incoming solar radiation.
Dry air near the surface of the ground: It prevents radiating heat from the Earth’s surface from being absorbed.
The slow movement of air: There is no heat transfer or mixing in the lower layers of the atmosphere as a result.
Snow-covered the surface of the ground: It causes the greatest amount of heat loss by reflecting incoming solar light.
Clear and cloudless sky: Heat loss by terrestrial radiation is more rapid when there is no impediment.

Effects of temperature inversion

The inversion of temperature and its duration affects adversely the society and economy of the region of its occurrence. Some of the important consequences of temperature inversion are –

The occurrence of fog:

There develop clouds in contact with the ground (fog) with visibility usually restricted to less than 1km.

In the urban areas, the fog mixed with smoke takes the shape of smog.
While fog is injurious to crops, the smog is considered a health hazard.
In 1952, about 4000 people died of smog in London.
Breathing problems, asthma, and bronchitis, etc. are common problems in Delhi and big cities of northern India during the winter season.

Road accidents:

Visibility may be greatly reduced below the inversion due to the accumulation of dust and smoke particles.

The frequency of roads, railways, and air accidents increases during foggy conditions due to low visibility.
Trains and flights are often delayed.

Damage of crops:

the winter crops like wheat, barley, mustard, vegetables, chilies, potato, etc. are seriously damaged.

The sugarcane crop in the northern plains of India.
Especially in UP, Punjab, and Haryana develops the disease of a red rod which reduces the sugar content.

Vegetation: Orchards are closely influenced by the inversion of temperature. The lower valleys of Alps Mountains are almost without settlements, while the upper slopes are inhabited.

Clouds: In regions where a pronounced low-level inversion is present, convective clouds can not grow high enough to produce showers.

Diurnal Variations: Inversions also affect diurnal variations in temperature. Diurnal variations tend to be very small.

Ideal Conditions For Temperature Inversion

Long nights, so that the outgoing radiation is greater than the incoming radiation.
Clear skies, which allow unobstructed escape of radiation.
Calm and stable air, so that there is no vertical mixing at lower levels.

Types of Temperature Inversion

There are five kinds of inversions: ground, turbulence, subsidence, air drainage and frontal.

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 by radiation.
If the temperature of surface air drops below its dew point, fog may result.
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.

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.

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.

Air drainage type of Inversion

Sometimes, the temperature in the lower layers of air increases instead of decreasing with elevation. This happens commonly along a sloping surface.
Here, the surface radiates heat back to space rapidly and cools down at a faster rate than the upper layers. As a result the lower cold layers get condensed and become heavy.
The sloping surface underneath makes them move towards the bottom where the cold layer settles down as a zone of low temperature while the upper layers are relatively warmer.
This condition, opposite to normal vertical distribution of temperature, is known as Temperature Inversion.
In other words, the vertical temperature gets inverted during temperature inversion.
This kind of temperature inversion is very strong in the middle and higher latitudes. It can be strong in regions with high mountains or deep valleys also.

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 a considerable slope, whereas other inversions are nearly horizontal. In addition, humidity may be high, and clouds may be present immediately above it.

Temperature Inversion Effects

The temperature inversion helps in precipitation, forming clouds, and frost.
The inversion of temperature leads to the floating of dust particles in the air, thus, checking dust particles and smoke from dispersing on the surface.
Temperature inversion brings stability to the atmosphere by checking the downward and upward movement of the air.
At times the temperature inversion can become a matter of concern. It checks the convection clouds from rising higher, thus, pouring less rainfall.
Fog formed due to temperature inversion reduces visibility and is a concern for transportation.
Intense thunderstorms and tornadoes are associated with temperature inversions.

Economic Importance of Temperature Inversion:

Sometimes, the temperature of the air at the valley bottom reaches below freezing point, whereas the air at higher altitude remains comparatively warm. As a result, the trees along the lower slopes are bitten by frost, whereas those at higher levels are free from it.
Due to inversion of temperature, air pollutants such as dust particles and smoke do not disperse in the valley bottoms. Because of these factors, houses and farms in intermontane valleys are usually situated along the upper slopes, avoiding the cold and foggy valley bottoms. For instance, coffee growers of Brazil and apple growers and hoteliers of mountain states of Himalayas in India avoid lower slopes.
Fog lowers visibility affecting vegetation and human settlements.
Less rainfall due to stable conditions.