Composition and Structure of the Atmosphere

The atmosphere is the thin envelope of air that surrounds the Earth, held in place by gravity. We rarely think about it. Yet it gives us the air we breathe, keeps the planet warm, shields us from the Sun's harmful rays, and makes weather possible. To understand climate, winds and rainfall, you first have to know what the air is made of. You also need to know how it is arranged in layers from the ground up to the edge of space.

The air is a mixture of gases, water vapour and tiny solid particles.

The gases are dominated by just two. Nitrogen makes up about 78 per cent and oxygen about 21 per cent. Together they account for about 99 per cent of dry air. The small remainder includes carbon dioxide, argon and others. Carbon dioxide is present in a tiny amount, but it is very important. It absorbs heat and helps keep the Earth warm. Ozone is another minor gas. It is found mainly in the upper atmosphere, where it filters out dangerous ultraviolet rays.

Water vapour is the gaseous form of water. Its amount varies greatly, from less than 1 per cent in the cold polar air to about 4 per cent over the warm, wet tropics. The reason is temperature. Warm air can hold more water vapour than cold air: the capacity of air to hold moisture rises as its temperature rises. Since temperature falls from the Equator towards the poles, the moisture content of the air also decreases with increasing latitude. Hot deserts are an exception of a different kind: their air is warm enough to hold much vapour, but there is little surface water to supply it. Water vapour is the source of all clouds and rain. It also absorbs heat.

Dust particles (tiny bits of soil, smoke, salt and ash) float in the lower air. They are not spread evenly over the globe. Their concentration is highest in the subtropical and temperate regions, because these zones have large stretches of dry surface and frequent dry winds that lift dust into the air. Equatorial and polar regions, by contrast, carry less dust: the equatorial belt is wet, and the poles are ice-covered, so little loose material rises. Dust particles matter because water vapour condenses around them to form clouds, and they scatter sunlight, giving us colourful sunrises and sunsets.

The lowest layer is the troposphere. It extends to an average height of about 13 kilometres, though higher over the Equator (about 18 km) and lower over the poles (about 8 km).

This difference in thickness has a clear cause. The Equator receives intense insolation, which heats the surface strongly. The hot surface sets off powerful convectional currents: columns of warm air that rise and carry heat and air to great heights. This vigorous vertical movement pushes the tropopause upward, so the troposphere is thickest over the Equator. At the poles the surface is cold, convection is weak, and the layer stays shallow.

This is the most important layer for life. All weather happens here: clouds, rain, storms and winds. A key feature is that temperature falls steadily with height. The average rate is about 6.5 degrees Celsius per kilometre, called the normal lapse rate. The thin boundary at the top, where the fall in temperature stops, is the tropopause.

The fall in temperature with height has two causes. First, the atmosphere is heated mainly from below, not from above. Sunlight passes through the air with little effect; the ground absorbs it and then warms the air above it through terrestrial radiation. Air near the surface therefore sits closest to the heat source, and air higher up receives less of this warmth. Second, the density of air decreases with height. The thin air of the upper troposphere holds fewer molecules, so it absorbs and retains less heat than the dense air near the ground. Together, distance from the radiating surface and falling density make the troposphere cool steadily upward.

Above the troposphere lies the stratosphere, reaching up to about 50 kilometres. It is calm and cloud-free, with very little water vapour. That is why jet aircraft like to fly in its lower part, where the air is smooth and stable.

The stratosphere contains the ozone layer, a concentration of ozone gas. It absorbs most of the Sun's harmful ultraviolet radiation and acts as a protective shield for all life on the surface. Because ozone absorbs this energy, temperature actually rises with height in the stratosphere. That is the opposite of what happens in the troposphere.

Higher still are the upper layers.

The mesosphere extends to about 80 kilometres, and here temperature again falls with height, reaching the coldest point in the whole atmosphere. Meteors usually burn up in this layer.

Above it is the thermosphere, where temperature rises sharply. It contains the ionosphere, a region of electrically charged particles. These particles reflect radio waves back to the Earth, making long-distance radio communication possible.

The outermost layer is the exosphere, which gradually thins out and merges into the emptiness of outer space.

The atmosphere is essential in many ways. It supplies the oxygen that animals breathe and the carbon dioxide that plants use. It acts like a blanket, trapping enough heat to keep the Earth warm at night and preventing the surface from overheating by day. Its ozone layer screens out harmful radiation. By holding water vapour, it also powers the water cycle that brings rain. Without this thin shell of air, the Earth would be as lifeless as the Moon.

A cloud is a mass of tiny water droplets or ice crystals formed when water vapour condenses on dust particles in the air. These droplets and crystals are extremely small, so their density is far lower than that of the surrounding air. That is why clouds float instead of falling, and gentle rising currents of air keep them suspended.

Clouds are classified mainly by their height and form.

• Cirrus: high clouds, found above about 6,000 metres. The air there is well below freezing, so they are made entirely of ice crystals. They look white, thin, fibrous and feathery.
• Cumulus: clouds that look like heaps of cotton wool, with a flat base and rising dome-shaped tops. They form at lower and middle levels, not at great heights.
• Stratus: low, grey, layered clouds that spread like a sheet across the sky.
• Nimbus: dark, thick rain-bearing clouds.

Clouds also control how much heat the Earth keeps or loses, and the effect depends on their height. High clouds like cirrus are thin. They let most sunlight pass through but absorb the infrared radiation rising from the Earth's surface and re-emit part of it downward. Their net effect is warming. Low clouds like stratus are thick and bright. They reflect a large share of incoming solar radiation back to space, so their net effect is cooling. Be careful: it is a common error to reverse these two roles.

The same blanket effect explains a familiar observation. On a cloudy night, clouds reflect the Earth's outgoing radiation back to the surface. The ground therefore stays relatively warm and does not cool to the dew point, the temperature at which water vapour condenses. So dewdrops do not form on cloudy nights, while clear nights, which let the surface cool freely, produce heavy dew.