Composition and Structure of Atmosphere

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (Fundamentals of Physical Geography), Chapter 7.

Why can't you breathe on top of Mount Everest?

You can go weeks without food and days without water, but only minutes without air. Climbers on Everest are not short of oxygen in a percentage sense: the air up there is still about the same mix as at sea level. The problem is that the air is much thinner, so each breath holds far fewer molecules. That one fact, that air thins quickly as you go up, explains most of this chapter. The atmosphere is a blanket of gas wrapped around the earth, held there by gravity, and it is packed tightly at the bottom and spread very thin at the top.

The 99 per cent rule. Gravity pulls gas towards the surface, so air is densest near the ground and thins with height. About 99 per cent of the total mass of the atmosphere lies within 32 km of the surface. That is a very thin shell compared with the earth's radius, and it is the reason weather, life and nearly all human activity are confined to the lowest layers.

What the atmosphere is made of

The atmosphere is a mixture of gases, water vapour and dust particles. The proportions of gases change high up: oxygen becomes almost negligible at about 120 km, while carbon dioxide and water vapour are found only up to about 90 km. Air is colourless and odourless and can only be felt as wind.

Carbon dioxide: little gas, big effect. Sunlight arrives as short waves, and carbon dioxide lets them through. The warmed earth sends heat back as long waves, and carbon dioxide absorbs part of these and sends some back towards the surface. That one-way filter is the greenhouse effect. The amount of every other gas is steady, but carbon dioxide has been rising over the past few decades, mainly because of the burning of fossil fuels, and this has raised the temperature of the air.

Ozone: the sunscreen. Ozone, found between 10 and 50 km, absorbs the sun's ultraviolet rays so they do not reach the surface. Without this filter, harmful ultraviolet radiation would reach every living thing on the surface.

Water vapour: the variable gas. It decreases with altitude and also decreases from the equator towards the poles. It absorbs part of incoming sunlight and holds in the heat radiated by the earth. It works like a blanket that stops the earth from becoming too hot or too cold. It also contributes to stability and instability in the air, which decides whether clouds and storms can build.

Dust: the seed of every cloud. Dust stays mostly in the lower layers, although convection currents can lift it high. There is more of it over the subtropical and temperate regions than over the equatorial and polar regions, because of dry winds. This surprises most students, who guess the equator or the poles. Dust and salt particles are hygroscopic nuclei: water vapour condenses around them to make cloud droplets. Without them, clouds could hardly form.

Pause and predict: water vapour and carbon dioxide are both found only in the lower atmosphere, with limits of about 90 km. Which one varies much more from place to place, and where is it highest?

Check your answer: water vapour. It can reach about 4 per cent of the air in the warm, wet tropics and fall below 1 per cent in dry deserts and polar regions, and it declines with height and from the equator to the poles.

Beyond the textbook. For the exam: carbon dioxide is the gas responsible for the greenhouse effect. In reality: water vapour also traps heat and, being far more plentiful than carbon dioxide, contributes a larger share of the natural greenhouse effect. The difference is that humans have increased carbon dioxide directly, while the amount of water vapour in the air mostly follows temperature. In CUET, write carbon dioxide when asked which gas is transparent to incoming and opaque to outgoing radiation.

Five layers: a stack with a zig-zag temperature

The atmosphere is divided into five layers on the basis of temperature, and density falls in every layer as you go up. The temperature falls, rises, falls and rises again as you climb.

The pattern to remember is down, up, down, up: the troposphere cools with height, the stratosphere warms, the mesosphere cools and the thermosphere warms. A geographer's interest lies mainly in the first two layers, the troposphere and the stratosphere.

Why the troposphere is thickest at the equator. The equator is hot, so strong convection currents carry heat and air to great heights and push the top of the troposphere up to about 18 km. At the poles there is little such heating, so the layer is only about 8 km thick. This means the equatorial tropopause is both higher and, perhaps surprisingly, colder (about −80°C) than the polar one (about −45°C). The air at the top of a very tall, steadily cooling column has had much longer to cool.

Pause and predict: the surface at the equator is far hotter than at the poles. Why is the tropopause above the equator colder than the one above the poles?

Check your answer: because the tropopause over the equator lies much higher (about 18 km against 8 km). Temperature falls through the troposphere at 1°C per 165 m, so by the time you reach the top of the taller column, the air is colder.

What each layer does for you.

Beyond the textbook. For the exam: the ozone layer lies between 10 and 50 km, in the stratosphere, and it is the reason temperature rises in that layer. In reality: ozone is densest in the lower and middle stratosphere rather than evenly across the 10 to 50 km range, and the energy it absorbs from ultraviolet light is what warms the stratosphere. Give the 10 to 50 km range in the exam, and treat it as the zone where ozone is found rather than a layer with sharp edges.

Beyond the textbook. For the exam: the thermosphere is where temperature rises with height, and it contains the ionosphere. In reality: the temperature of a gas there is very high, but the air is so thin that it has almost no heat to give you. An astronaut outside a spacecraft would not feel "hot" because there are too few particles to transfer heat. Do not link "temperature rises" with "feels hot" in an exam answer.

Elements of weather and climate

The elements of the atmosphere that change and influence life on earth are temperature, pressure, winds, humidity, clouds and precipitation. The next chapters study them in turn, beginning with temperature in Solar Radiation, Heat Balance and Temperature.

Exam traps

60-second revision

Where this chapter connects

• Previous: Landforms and Their Evolution finishes the study of the lithosphere before the book turns to the air above it.
• Next: Solar Radiation, Heat Balance and Temperature builds on carbon dioxide, water vapour and the troposphere to explain how the earth is heated.
• Related: Water in the Atmosphere uses hygroscopic nuclei and water vapour to explain clouds and rain, and World Climate and Climate Change returns to the greenhouse effect.