Thermal inversion: Definition and description

Introduction

The increase in temperature in the atmosphere is known as inversion of the temperature or thermal inversion. Under normal conditions, with rise in air the temperature decreases. But sometimes the temperature of the air in some places rises up to some distance which causes the thermal inversion occurs.

When this condition occurs in the air, the air layer adjacent to the surface is cool and above it is a relatively warm. It can occur near the earth surface and in the upper layer of the troposphere.

It is an atmospheric phenomenon. Only at the layer of the troposphere can be seen different slopes of land such as mountains. Sometimes the temperature does not decrease but increases at the height increases. Then this atmospheric phenomena was created. Below is a discussion of the definition and description of thermal inversion (1) & (4).

What is Thermal inversion?

Definition

Under normal conditions, the temperature decreases by 6.5°C with an increase in altitude per km. But sometimes as the altitude of a place increases, the temperature of the air increases without decreasing. This phenomenon is called the thermal inversion (1).

Description

The temperature decreases as the altitude gradually rises from the surface. This is called normal lapse rate. But sometimes the temperature does not decrease or increase according to the height. This is usually seen in the calm climate of the hill valleys.

On a calm cloudless night, the air over the mountain quickly radiates heat and becomes very cold. And the force of gravity descends down the valley along the slope of the mountain. As a result, the lower part of the valley is much warmer than the upper part of the mountain. This is known as inversion of temperature or thermal inversion.

That is why in many parts of Europe, settlement and agriculture are seen on the slopes of mountains, not at the bottom of valleys. There are two types of air seen in the inversion of temperature. These are anabatic wind and katabatic wind (3).

Causes of thermal inversion of the troposphere

• Of all the layers of the atmosphere, only the troposphere has low relative heat and high radiation. Although the relative heat in surface air is low, the rate of radiation is high. Therefore, mainly during the long winter nights, a lot of heat is radiated, resulting in inversion of temperature.
• Ground slope is required to create thermal inversion. It is located only within the layers of this troposphere on earth. For this reason, the only troposphere that has the inversion of temperature is the atmosphere.
• Only at the level of the troposphere can you see different slopes of land such as mountains. And this change in altitude causes thermal inversion to change the density of different places in the air and the apparent daily rays of the sun.

Types

There are three types of thermal inversion, depending on the altitude, weather and process. These are-

1. Ground or surface inversion

The earth radiates heat faster than the atmosphere. So the surface cools down quickly. This also cools the air adjacent to the surface. But the upper air is not easily cooled due to low air transport capacity. That is why this type of Inversion of temperature occurs in the air layer adjacent to the surface.

Ground or surface inversion are three types, such as

• Radiation inversion

Radiation inversion occurs more in upper and middle latitudes. This is because such thermal inversion are required-

1. Long winter night
2. Cloudless skies
3. Snow-covered lands
4. Clam winds.

• Advection inversion

Such a type of inversion of temperature occurs for horizontal air circulation. The kind of inversion of temperature that occurs when warm air flows over a cold surface is called advection inversion.

• Air drainage inversion

Air drainage inversion occurs in the hilly areas of the temperate zone. The air layer adjacent to the surface of the mountain slope radiates heat rapidly. As a result, the katabatic wind cools and falls to the bottom of the mountain valley. The wind on the upper slope increases as compared to the lower slope. And air drainage inversion causes.

2. Upper air inversion

Thermal inversion sometimes occurs in the upper part of the troposphere. Such inversion of temperature is known as upper air inversion. It occurs in two ways. Such as-

• Subsidence

When a dense, dry wind stream descends at a higher altitude than the earth’s surface, it is heated at a rate of about 10°C per km as a result of increase in air pressure. The contrast heat generated by the formation of a warm air layer over the lower cool air layer is called subsidence thermal inversion.

Sometimes the wind blows from top to bottom. This increases the pressure and heat in the air. The rate of heat increase is 10°C per km. This is called the dry adiabatic rate. Such types of inversion are observed in the tropical zones.

• Turbulence

The lower warm air rises to the top and the upper cool air descends as heavier by convection currents. When this upward movement occurs in the atmosphere, the rapid upward movement of warm air occurs. And this kind of inversion of temperature occurs by temporarily warming the upper air layer above the lower air layer.

• Convective

Such inversion of temperature occurs if the air is turbulent and the convection currents are very active during mechanical processes in the upper atmosphere.

3. Frontal inversion

The inversion of temperature that sometimes occurs in the upper air as a result of the combination of air of two different temperatures is called frontal inversion (4).

Causes of thermal inversion

• Long night

If the night is longer than the day, the heat stored from the earth’s surface is completely radiated. So the air adjacent to the earth’s surface becomes extra cold. The result is a thermal inversion.

• Cloud-free sky

If there is cloud-free sky, the heat radiated from the earth’s surface goes directly into space without any obstruction. As a result, the lack of heat cools the air near the surface and causes inversion of temperature.

• Dry air

Normally air is dry in winter and due to low water vapor the heat radiated from the earth’s surface cannot be absorbed by the atmosphere. The heat goes directly into space. As a result the air layer adjacent to the earth’s surface cools and creates an inversion of temperature.

• Clam weather

When the weather is calm, the mixture in the upper and lower air is less. So the air below cools in contact with the earth’s surface, creating an inversion of temperature.

• Ice-covered surface

Ice is poorly conductive and can absorb very little sunlight during the day. At night the absorbed sunlight radiates very quickly. The result is an inversion of temperature.

• Topography

In the hilly region, especially at night, as a result of heat radiation, the air in the upper part of the mountain becomes extra cold, heavy and gradually descends to the adjoining valley. This phenomenon is called air drainage.

As a result, the area adjacent to the mountain valleys gradually cools down and the air in the mountain tops remains relatively warm. In this way the inversion of temperature is created (1) & (4).

Effects of Thermal inversion

The climatic and economic effect of inversion of temperature is immense. Cloud formation, precipitation and the atmosphere are entirely affected by inversion of temperature. Upward and downward movement of air currents, normal lapse rate, dry adiabatic lapse rate etc. causes the mixture at the air layer. Therefore, the effects of inversion of temperature are

1. The inversion layer of the atmosphere acts as a covering. As a result, smoke and pollutants cannot spread to any part of the atmosphere. Therefore, outbreaks of human lung diseases occur in urban areas.
2. Inversion of temperatures cause high levels of fog. Fog disrupts crop production and vehicular traffic.

Inversion of temperature reduces the amount of rainfall which disrupts agricultural work.

1. Daily temperature is affected by inversion of temperature. Inversion layers create clear boundaries between dry and humid air layers. Dry air at the top and humid air at the bottom. Inversion levels affect the distribution of moisture in different layers of the atmosphere.
2. In modern times the inversion of temperature plays an important role in the field of aviation. It controls the speed and direction of wind at different levels of the atmosphere. As a result, the smoothness of the aircraft is disrupted with changes in wind speed, direction and density. Inversion of temperature causes catastrophic conditions for all types of vehicles (2) & (4).

Q&A

1. What is thermal inversion?

Normally, as the surface rises, the temperature decreases at a certain rate, which is called normal lapse rate. But sometimes the temperature rises instead of decreasing as the altitude increases to create special atmospheric conditions. This is called inversion of temperature or thermal inversion.

2. What is a thermal inversion?

Sometimes as the altitude increases, the temperature does not decrease but increases. This atmospheric phenomenon is known as thermal inversion.

3. A thermal inversion, which can lead to serious pollution events, occurs when?

Cool air near the surface is covered by a layer of warm air that traps pollutants.

4. What causes thermal inversion?

Thermal inversion occurs when a warm, less dense air mass moves over a dense, cold air mass. When on a clear night the air near the ground quickly loses its heat. The surface cools rapidly when the air above it retains the heat of the earth’s surface during the day.

5. What is a major consequence of a thermal inversion?

Air pollutants become trapped and accumulate below the inversion layer.

Written By: Manisha Bharati

Reference

1. Savindra Singh. Climatology. Pravalika Publications, Allahabad. Chapter 2: Origin, composition and structure of atmosphere. Page No: 18.