TEMPERATURE INVERSION

The temperature in the troposphere decreases with an increase in altitude. This vertical gradient of temperature is commonly referred to as the standard atmosphere or Normal Lapse Rate. 

However, this normal lapse rate varies with height, season, latitude, and other factors. Indeed, the actual lapse rate of temperature does not always show a decrease with altitude. 

Temperature inversion normally is a condition opposite to this Normal Lapse rate. Sometimes, the temperature in the lower layers of air increases instead of decreasing with elevation.

A temperature inversion is a layer in the atmosphere in which air temperature increases with height. An inversion is present in the lower part of a cap.

The cap is a layer of relatively warm air aloft (above the inversion). Air parcels rising into this layer become cooler than the surrounding environment, which inhibits their ability to ascend. 

This often happens in areas of high pressure, where the air high up often sinks towards the ground.

Inversions are most common in winter, when mist and fog become trapped in the cooler air low down, but inversions can happen all year round. This happens commonly along a sloping surface. Here, the surface radiates heat back to space rapidly & 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 phenomenon is especially observed in the intermontane valleys. In other words, the vertical temperature gets inverted during a temperature inversion.

CONDITIONS NECESSARY FOR INVERSION

Temperature inversion takes place only under certain specific conditions. These conditions are 

1. Long nights, so that the outgoing radiation is greater than the incoming radiation. 

2. Clear skies, which allow the unobstructed escape of radiation. 

3. Calm and stable air, so that there is no vertical mixing at lower levels. 

4. Snow covered ground surface, results in maximum loss of heat through reflection of incoming solar radiation.

TYPES OF TEMPERATURE INVERSION

Types of Temperature Inversion Depending on the nature of the underlying surface & reasons for the temperature contrast, there are four types of temperature inversion. 

1. Air Drainage Type of Inversion: This type of inversion occurs in a valley, where the dense, cold lower-level air slides or drains down a slope to settle down at the bottom of the valley. As a result, the valley floor has a lower temperature gradient than the upper layers, which are relatively warm. This situation is responsible for severe frost in the valley floors causing great damage to fruit orchards, vegetables & 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.

2. Surface Temperature Inversion: This is the most common type of temperature inversion and occurs even on relatively plain surfaces.

Rapid radiation of heat occurs in places where the air is still, clear and dry and the winter nights are long. The temperature, therefore, falls rapidly and temperature inversion takes place. This temperature inversion is not very deep and is destroyed as the sun rises. 

In the low latitude areas (tropical & subtropical areas) it occurs during winter nights only. The duration and height of surface inversion increase pole-ward. 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. 

3. Advectional type of Temperature Inversion: There are places where many types of air masses meet. The colder air mass, being heavy, settles down, while the warmer air mass, being light, rises over the colder air mass. This creates temperature inversion. This type of inversion is unstable and is destroyed as the weather changes.

4. Upper Surface Temperature Inversion: Thermal upper air inversion is caused by the presence of ozone layer lying between the height of 15 to 35 km (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. 

Sometimes massive upper layers descend and press the layers below them. As a result, the upper layers get warmed up and settle over the cold layers, and temperature inversion takes place. This keeps the atmosphere stable for a long time. Such a condition exists in a dry atmosphere. In the winters of mid latitudinal continental areas, stable high-pressure conditions are created, and create temperature inversion. This temperature inversion is called upper surface temperature inversion because it takes place in the upper parts of the atmosphere. 

EFFECTS ON VISIBILITY 

Temperature inversion, also called as thermal inversion, play an important role in determining cloud forms, precipitation, and visibility. An inversion acts as a cap on the upward movement of air from the layers below. As a result, convection produced by the heating of air from below is limited to levels below the inversion. Surface temperature inversions play a major role in air quality, especially during the winter when these inversions are the strongest.

The warm air above cooler air acts like a lid, suppressing vertical mixing and trapping the cooler air at the surface. As pollutants from vehicles, fireplaces, and industry are emitted into the air, the inversion traps these pollutants near the ground, leading to poor air quality. 

Besides trapping smog, inversions also trap sound waves. Because of this, the loud sounds of things like airplanes taking off will seem louder as the sound waves refract off the inversion layer and back down to the ground.  light can be bent by the inconsistency of the temperature. Intense thunderstorms and tornadoes are also associated with inversion of temperature because of the intense energy that is released after an inversion blocks an area’s normal convention patterns.

If the inversion is strong enough, it can cause far off objects to look like they are floating above the ground. If things are far enough they are past the curvature of the earth, they may become visible. This reverse or "superior" mirage is called looming. 

INVERSION IN OCEANS

Temperature inversion in the oceans is usually observed to coincide with the halocline, where higher salinity in the subsurface layer compensates stability loss due to lower Sea Surface Temperature and maintains the stable temperature inversion. Such a halocline is usually associated with significant freshwater flux from river runoff and precipitation. 

Martin et al. (1981) reported that large river runoff from five major rivers (Ganges, Brahmaputra, Irrawaddy, Krishna, and Godavari) and excess precipitation (P) over evaporation (E) are significant sources of fresh water to the Bay of Bengal & the Indian Ocean.  

Two occurrence regions of inversion i.e. the western & northeastern Bay, have proximity to the two low salinity zones ( 28–29‰) that are located adjacent to major freshwater inputs from the Ganges, Brahmaputra, Irrawaddy, Krishna, and Godavari rivers.

The characteristics of temperature inversion are believed to be dependent on topographical features and synoptic regime. Western Ghats mountains located in the west coast of India have a complex topography, and the temperature inversion features for this region remain unknown due to the complexity and non-availability of observational datasets. This study investigates temperature inversion characteristics in the troposphere using high-resolution radiosonde data (2012-2015). 

The results showed that Temperature Inversion occurs in all seasons, however, their frequency differs. Winter (Dec.-Feb.) shows the maximum number of inversions followed by pre-monsoon (Mar.-May), post-monsoon (Oct.-Nov.), and monsoon (Jun.-Sep.). In winter, the depth of inversion is higher; however, the inversion strength is weaker as compared to other seasons. 

Temperature inversions are important in agriculture, because the air layering effect they cause changes the anticipated dissipation of pesticide spray solutions used by agricultural producers.  Inversions also affect the movement of smoke from prescribed fires used by land managers. 

IMPACT ON AIRCRAFT PERFORMANCE

Amongst those cases, an increase in temperature can be met when the altitude increases. That is the temperature inversion.

Under such circumstances, an increase of altitude will bring a decrease of thrust that is substantial than usual, because the effect of pressure and temperature both contribute to the decrease. In the event of temperature inversion, the climb performance will be affected in the cases where the thrust is affected. However, to affect the aircraft performance, a temperature inversion must be combined with other factors.

Temperature inversions frequently occur in anticyclones, but are also common in depressions when air in the middle troposphere subsides. Inversions may occur at any height, but the large temperature inversions at the tropopause and mesopause are stable and permanent features of the Earth's atmosphere.

MONSOON AND THERMAL INVERSION

The formation of thermal inversions over the Arabian Sea is one of such phenomena that significantly regulates the moisture transport towards the Indian landmass (Colon, 1964; Ramage, 1966).  

Although a weak signal of inversion occurs over the Arabian Sea during the pre- and post-monsoon seasons, the monsoon lifts up the inversion layers laterally from east to west and confines the strengthened inversion to small regions over the Western Arabian Sea  (Narayanan and Rao, 1981; Ramaswamy et al., 2017). This process of strengthening the inversion layers that is confined over the narrow region above the western Arabian Sea is referred to as the monsoon inversion (Colon, 1964).

The Monsoon Inversion significantly influences the exchange of moisture and energy between the ocean surfaces (Narayanan and Rao, 1981), and is primarily characterized by the temperature gradient and inversion depth over the Arabian Sea (Dwivedi et al., 2016a). 

The formation of thermal inversions tends to inhibit the vertical growth of clouds from western to central Arabian Sea which, ultimately influence the rainfall over the Arabian Sea and adjoining coastal regions (Narayanan, 1981)

Narayanan et al., (2004) revealed that subsidence of upper tropospheric air and the advection of monsoonal winds are essential ingredients in the formation, strengthening and maintenance of the Monsoon Inversion during the break and active phases of the monsoon.