Maarg Manthan · Topic 5.6

Inversion of Temperature

Geography › Climatology · Topic 5.6

In the troposphere, temperature normally falls with height at the normal lapse rate of about 6.5 degrees Celsius per km, because the air is heated from below. Temperature inversion is the reverse, when a layer of warm air lies over cooler air near the ground. It occurs on long, clear, calm winter nights and in valleys, and it traps fog, smoke and pollutants and creates stable weather.

6 min read · UPSC Prelims · Geography · Climatology

At a Glance

Normal lapse rate 6.5 degrees Celsius per km
Dry adiabatic rate About 9.8 degrees per km
Inversion Temperature rises with height
Best conditions Long night, clear sky, calm, dry air
Effects Fog, smog, frost, stable air
Exam link UPSC Prelims, Geography: Climatology

Where Does It Fit?

This topic completes the study of temperature by explaining the exception to the rule that the air cools with height. Inversion affects fog, smog and the siting of orchards and cities. Previous topic: Factors Controlling the Distribution of Temperature. Next topic: Atmospheric Pressure: Vertical and Horizontal Distribution.

How Does Temperature Change with Height?

In short: in the troposphere the air gets colder as you go up, by about 6.5 degrees Celsius every kilometre.

The vertical distribution of temperature describes how air temperature changes from the ground upward.

The atmosphere is heated mainly from below, as the ground absorbs insolation and warms the air that touches it, as described in Heat Balance of the Atmosphere: Conduction, Convection, Advection, Terrestrial Radiation, Albedo. The air near the surface is therefore the warmest, and temperature decreases with altitude. The rate of decrease is the lapse rate. The average lapse rate of the troposphere, called the normal lapse rate, is about 6.5 degrees Celsius per km, or about 1 degree for every 165 m. It is an average that varies with place and time. The temperature stops falling at the tropopause, and the changes in the layers above are in Structure of the Atmosphere: Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.

What Are the Different Lapse Rates?

Term	Meaning	Value
Environmental (normal) lapse rate	The actual change of temperature of the still air with height	About 6.5 degrees per km on average; varies
Dry adiabatic lapse rate	Rate at which a rising parcel of unsaturated air cools by expansion, with no heat exchange with the surroundings	About 9.8 degrees per km
Saturated (moist) adiabatic lapse rate	Rate of cooling of rising air in which condensation is occurring; slower because condensation releases latent heat	About 4 to 6 degrees per km

If a rising air parcel stays warmer than the surrounding air, it keeps rising, and the atmosphere is unstable, giving clouds and storms. If it becomes cooler than the surroundings, it sinks back, and the atmosphere is stable. These ideas explain the formation of clouds and rainfall, as in Types of Clouds: Cirrus, Cumulus, Stratus, Nimbus and Their Combinations.

What Is Temperature Inversion?

Temperature inversion is a reversal of the normal lapse rate, in which the temperature rises with height over some layer, so that a warm layer lies over a cooler layer of air near the ground. The inversion layer is very stable, because the denser cold air lies under lighter warm air, and so there is little vertical mixing. Inversions are usually shallow, a few hundred metres, and short-lived, but they have important effects.

The ideal conditions for a surface inversion are a long winter night, a clear sky with no clouds, calm air or very light wind, dry air and, often, a snow-covered ground. On such a night the ground loses heat quickly by terrestrial radiation, and cools the air layer in contact with it, while the air above stays warmer.

What Are the Types of Inversion?

A radiation inversion is the commonest kind and forms near the ground on clear nights, and a valley inversion forms where cold air drains into hollows.

Type	Cause	Where
Radiation (surface) inversion	Ground cools by radiation on a clear, calm night	Winter nights in mid and high latitudes, plains and valleys; Indo-Gangetic plain
Valley (orographic or drainage) inversion	Cold dense air drains down slopes at night and collects on the valley floor, while warmer air stays above	Mountain valleys, such as in the Himalaya, Kashmir and the Alps
Advection inversion	Warm air moves over a cold surface, such as snow or a cold sea	Coasts and areas with snow
Subsidence inversion	Air sinking in a high-pressure area warms by compression and caps the cooler air below	Subtropical anticyclones; California, deserts
Frontal inversion	Warm air rides up over a wedge of cold air at a front	Fronts of temperate cyclones

In mountain valleys, the pattern is the opposite of what one might expect: the valley floors are colder than the slopes, so that settlements and orchards are often placed on the middle slopes, called the thermal belt, which are safer from frost. This is a valuable fact for horticulture in hill areas.

What Are the Effects of Inversion?

Fog and mist: cooling near the surface leads to condensation, and the inversion keeps the fog from rising.
Smog and air pollution: an inversion acts like a lid and traps smoke, dust and gases near the ground. Winter smog over the Indo-Gangetic plain, including Delhi, is worsened by inversion layers, low wind speeds and stubble burning, and the 1952 Great Smog of London was a case of an inversion that trapped coal smoke and caused thousands of deaths.
Frost: cold air gathers in the hollows, harming crops.
Stable weather: clouds and rain do not form since the air cannot rise, and visibility falls, which disrupts aviation, road and rail traffic.

As an analytical point, winter pollution episodes in north India are not caused by inversion alone; inversion and calm winds only fail to disperse the pollutants, which come from vehicles, industry, dust and burning, so effective action has to cut emissions, and not just wait for the weather to change.

Why Is This Topic Important?

Concepts Lapse rates asked in Prelims
Pollution Delhi winter smog link
Hill agriculture Thermal belts and frost

What Came Next?

The next topic moves to atmospheric pressure and how it varies with height and across the globe. Next topic: Atmospheric Pressure: Vertical and Horizontal Distribution.

Key People and Terms

Lapse rate Fall of temperature with height
Adiabatic Without heat exchange
Inversion Temperature rising with height
Thermal belt Warmer middle slopes in valleys
Smog Smoke and fog
Stability Resistance of air to rise

Exam Corner

Points to Remember

Normal lapse rate is 6.5 degrees per km.
Dry adiabatic rate is about 9.8 degrees per km.
Inversion needs clear, calm, long nights.
Inversion traps pollutants near the ground.
Orchards in valleys are on middle slopes.

Do Not Confuse With

Normal and adiabatic lapse rates Environment versus rising parcel
Radiation and advection inversion Ground cooling versus warm air over a cold surface
Inversion and subsidence Layer reversal versus sinking air

Memory Hook

Clear night, calm air, cold ground, warm lid: inversion traps the smog.

Mains Angle

Questions ask about causes and effects of inversion. These points add depth.

Conditions Clear, calm, long nights
Valleys Cold air pools
Pollution Lid effect

A Question You May Face

An original practice question, not a past paper question.

“What is temperature inversion? Explain its causes and its effects on weather, agriculture and air quality.”

How to Answer

Define Normal versus inverted.
Causes Conditions and types.
Effects Fog, frost, smog.
Management Emission control.

GS Relevance

Prelims: lapse rate, inversion types, conditions and effects. GS Paper 1: climatology; GS Paper 3: air pollution.

Frequently Asked Questions

What is the normal lapse rate?

The normal lapse rate is the average rate at which air temperature falls with increasing height in the troposphere, about 6.5 degrees Celsius per km, or about 1 degree for every 165 m. It arises because the atmosphere is heated mainly from the ground.

What is temperature inversion?

Temperature inversion is the reversal of the normal decrease of temperature with height, in which a layer of warm air lies above cooler air near the ground. The layer is very stable and prevents air from rising, so fog, smoke and pollutants get trapped below it.

What conditions favour a surface temperature inversion?

A surface inversion forms best on a long winter night with a clear sky, calm air and dry air, and often snow-covered ground. The ground loses heat rapidly by radiation and cools the air next to it, while the air above stays warmer.

Why are orchards planted on mountain slopes and not valley floors?

In mountain valleys, cold dense air drains downslope at night and collects on the valley floor, producing a valley inversion with frost. The middle slopes, called the thermal belt, remain warmer, so orchards and settlements are placed there to avoid frost damage.

How is inversion linked to winter smog?

An inversion acts as a lid that stops pollutants rising and mixing into the upper air. In winter, with cold clear nights and calm winds, smoke, dust and gases are trapped near the ground, as over the Indo-Gangetic plain, producing dense smog and low visibility.

PYQ Practice — Statement Analysis

1 In a temperature inversion, temperature increases with height in a layer.

True

A warm layer lies over cool air.

2 Clouds and cloudy nights favour a surface radiation inversion.

False

Clear skies favour it.

3 The normal lapse rate is about 6.5 degrees Celsius per km.