Maarg Manthan · Topic 6.2

Structure of the Atmosphere

Geography › Atmosphere and Temperature · Topic 6.2

The atmosphere is divided into five layers by temperature: the troposphere, where weather occurs and temperature falls with height, the stratosphere with the ozone layer where temperature rises, the mesosphere, the coldest layer, the thermosphere, which contains the ionosphere and where temperature rises sharply, and the exosphere, which merges into space. The boundaries are called the tropopause, stratopause, mesopause and thermopause.

6 min read · UPSC Prelims · Geography · Atmosphere and Temperature

At a Glance

Troposphere 0 to about 13 km; weather; temperature falls
Stratosphere To about 50 km; ozone; temperature rises
Mesosphere To about 80 km; coldest; meteors burn
Thermosphere To about 600 km; ionosphere; very hot
Exosphere Above; merges with space
Exam link UPSC Prelims, Geography: Atmosphere and Temperature

Where Does It Fit?

This is the second topic of the category on the atmosphere and temperature. The previous topic covered the evolution and composition of the air, and this one describes how the atmosphere is arranged in layers. The next topic covers heat transfer and the heat budget. Previous topic: Evolution and Composition of the Atmosphere. Next topic: Heat Transfer and Heat Budget of the Earth.

How Is the Atmosphere Divided into Layers?

In short: the air thins with height and changes temperature in a pattern that marks five layers.

The atmosphere extends to a height of more than 1,000 km, but it gets thinner with altitude. Air pressure and density fall rapidly: about half the mass of the atmosphere lies below 5.6 km, and 99 per cent below about 32 km. Scientists divide it into layers by the way temperature changes with height. In some layers the temperature falls with height, and in others it rises. The thin boundary between two layers is named with the suffix pause.

Layer	Height (approx.)	Temperature trend	Key features
Troposphere	0 to 8 km at poles; 18 km at equator; average 13 km	Falls about 6.5 degrees Celsius per km	Nearly all weather, clouds, water vapour and dust
Stratosphere	Up to about 50 km	Rises with height	Ozone layer; dry; jet aircraft fly in it
Mesosphere	Up to about 80 km	Falls to about minus 90 degrees	Coldest part; meteors burn up
Thermosphere	Up to about 600 km or more	Rises sharply, to over 1,000 degrees	Ionosphere; auroras; satellites
Exosphere	Above about 600 km	Very high but very thin	Gases escape into space

Why Is the Troposphere So Important?

The troposphere is the lowest layer and contains about 75 to 80 per cent of the mass of the atmosphere and nearly all of the water vapour. All weather phenomena, such as clouds, rain, storms and winds, occur here, since the heating of the Earth’s surface drives turbulence and convection. Its height varies: it is thicker at the equator, about 18 km, because of strong convection, and thinner at the poles, about 8 km. The temperature decreases at the average rate of 6.5 degrees Celsius per km, called the normal lapse rate, as discussed in Vertical Distribution of Temperature and Temperature Inversion. The top of the troposphere is the tropopause, a zone of about 1 to 2 km thickness. It is the coldest at the equator, at about minus 80 degrees, and warmer at the poles, at about minus 45 degrees, which is a surprising reversal worth remembering. Jet streams flow near the tropopause, as in Jet Streams.

What Are the Stratosphere and Ozone Layer?

Above the tropopause lies the stratosphere, up to about 50 km. It is dry and nearly free of clouds, with stable air and no turbulence, so jet aircraft fly in its lower part. Here the temperature rises with height, because the ozone layer absorbs ultraviolet radiation from the Sun and converts it to heat. Ozone is concentrated at about 15 to 35 km. Its boundary with the mesosphere is the stratopause. Sulphur aerosols from volcanoes reach the stratosphere, where they linger and cool the climate, as in Effects of Volcanoes. Rare polar stratospheric clouds there help to destroy ozone over Antarctica.

What Are the Mesosphere, Thermosphere and Exosphere?

Mesosphere: from about 50 to 80 km; temperature falls to about minus 90 degrees Celsius at the mesopause, the coldest place in the atmosphere. Meteors burn up here and make shooting stars.
Thermosphere: from about 80 km to 600 km and beyond; the air is extremely thin, yet temperatures reach 1,000 to 2,000 degrees because the sparse molecules absorb intense solar radiation, although it would not feel hot, since there are too few molecules to transfer heat. The International Space Station orbits at about 400 km, in this layer. The boundary of space is commonly set at the Karman line, at 100 km.
Ionosphere: the part of the upper atmosphere, from about 80 to 400 km, where solar radiation ionises gases into charged particles. It reflects radio waves back to the Earth, which makes long-distance radio communication possible, and it is where auroras occur, as in Magnetosphere, Auroras and Van Allen Radiation Belts.
Exosphere: the outermost layer, where air is so thin that light gases like hydrogen and helium escape to space.

How Else Is the Atmosphere Divided?

Besides temperature, layers are named by composition and activity. The homosphere, up to about 80 km, has well-mixed gases, and the heterosphere above it has gases separated by mass, with the lightest at the top. The magnetosphere is the region influenced by the Earth’s magnetic field. As an analytical point, the lowest 10 per cent of the atmosphere by height carries most of its mass and nearly all of its weather, so climate change, pollution and storms are phenomena of a very thin skin of air, while the upper layers matter for communication, satellites and protection from radiation.

Why Is This Topic Important?

Layers Core Prelims content
Features Ozone, ionosphere, tropopause
Applications Aviation, satellites, radio

What Came Next?

The next topic describes heat transfer and the heat budget of the Earth. Next topic: Heat Transfer and Heat Budget of the Earth.

Key People and Terms

Troposphere Lowest layer with weather
Stratosphere Layer with the ozone layer
Mesosphere Coldest layer
Ionosphere Charged layer reflecting radio waves
Tropopause Top of the troposphere
Karman line Conventional edge of space at 100 km

Exam Corner

Points to Remember

Troposphere is 18 km at the equator and 8 km at the poles.
Lapse rate is 6.5 degrees per km.
Ozone is in the stratosphere.
Mesosphere is the coldest layer.
Ionosphere reflects radio waves.

Do Not Confuse With

Troposphere and stratosphere Weather layer versus ozone layer
Ionosphere and thermosphere Electrical region versus temperature layer
Mesopause and tropopause Coldest point versus top of weather layer

Memory Hook

Tropo has weather, strato has ozone, meso is cold, thermo is hot, exo is gone.

Mains Angle

Questions ask for layers with importance. These points add depth.

Weather Confined to the troposphere
Shield Ozone in the stratosphere
Communication Ionosphere and satellites

A Question You May Face

An original practice question, not a past paper question.

“Describe the layers of the atmosphere and explain the significance of each.”

How to Answer

Layers Five by temperature.
Features Height and trend.
Importance Weather, ozone, radio.
Contrast Equator and pole.

GS Relevance

Prelims: layers and features, ozone, ionosphere, aircraft and satellites. GS Paper 1: atmosphere.

Frequently Asked Questions

What are the layers of the atmosphere?

The atmosphere has five layers: the troposphere, up to about 13 km on average; the stratosphere, up to 50 km; the mesosphere, up to 80 km; the thermosphere, up to about 600 km; and the exosphere above. They are separated by the tropopause, stratopause, mesopause and thermopause.

Why does weather occur only in the troposphere?

The troposphere contains most of the water vapour and dust, and it is heated from the ground, which causes convection and turbulence. Clouds, rain and storms form there, while the stratosphere above is dry and stable, with almost no vertical mixing.

Where is the ozone layer?

The ozone layer lies in the stratosphere, at about 15 to 35 km above the surface. It absorbs most of the Sun's harmful ultraviolet radiation, and this absorption heats the stratosphere, so temperature rises with height in that layer.

What is the ionosphere?

The ionosphere is the part of the upper atmosphere, from about 80 to 400 km, where solar radiation ionises gases and produces charged particles. It reflects radio waves back to the Earth, enabling long-distance radio communication, and it is the region where auroras occur.

Why is the troposphere thicker at the equator?

The troposphere is about 18 km thick at the equator and about 8 km at the poles because strong heating at the equator drives powerful convection that lifts air to great heights, while the cold polar air has weak convection and cannot rise as high.

PYQ Practice — Statement Analysis

1 Temperature rises with height in the stratosphere.

True

Ozone absorbs ultraviolet radiation.

2 The troposphere is thicker over the poles than over the equator.

False

It is thicker over the equator.

3 Weather phenomena occur mostly in the troposphere.