Like the solid Earth, the atmosphere is divided into distinct layers. Take a deep breath: you have just inhaled part of the troposphere, the lowest layer of the atmosphere. A little like the Earths crust below' your feet, the amount of troposphere above you varies with your location. Near the Equator it is about 16km deep, but at the poles it is only half as thick. The troposphere is a relatively thin layer, but because it is at the bottom, and has the weight of the rest of the atmosphere pressing down upon it, it contains three-quarters of the mass of the atmosphere. It is also where most of the action takes place, including the bulk of the weather. The word is derived from the Greek verb tropos, to turn, and refers to all the air turbulence that goes on there. (This is also the origin of the word tropic, signifying the point where the Sun turns back towards the Equator in its annual journey.)
The troposphere is defined by temperature. The average temperature at the Earths surface is about 14.5°C. While the exact temperature you experience depends on whether you live in Egypt or Norway, what happens higher up does not. As you measure the temperature through the troposphere, it drops with height. Eventually you reach a sharp join called the tropopause. By this point, the temperature has fallen to about -52°C. This means a steep fall in average temperature in the tropics, perhaps 5°C per kilometre, but much less above the poles.
The reason for the drop in temperature is that the atmosphere does not absorb a huge amount of incoming solar energy, but the oceans and the land at the Earths surface do. It is the heat that they radiate back again that is mostly responsible for warming the air. The amount of warming is at its peak near the Equator, and it is mainly air near the ground or sea that is warmed. So the temperature of the air falls steadily as you get higher.
The troposphere is only the start of the story. It contains most of the material that makes up the atmosphere, but, at a few kilometres in depth, it is thin by comparison with the layers above. Beyond the tropopause, we arrive at a layer called the stratosphere. Here something unexpected happens: the temperature starts to climb again, and goes on climbing until it has almost reached a balmy 0°C at the top of the stratosphere. That can mean only one thing – something is heating it up. The energy comes from the fact that this region contains the famous “ozone layer”. The ozone layer absorbs most of the ultraviolet radiation in solar energy, and this energy warms the stratosphere. The ozone layer also protects us from the worst effects of the ultraviolet light from the Sun, although enough still gets through to contribute to skin cancer and other diseases.
Ozone, in case you are wondering, is just another form of oxygen. Chemists would call the sort you just breathed in O2. When you breathe it in, you are not absorbing single oxygen atoms, but molecules of two oxygen atoms each. The same applies to the nitrogen that makes up most of the atmosphere. It is written as N2. But ozone has three atoms per molecule, making its formula O3. It is formed by a sunlight-powered reaction using two-atom oxygen molecules as its feedstock.
The one thing everyone knows about the ozone layer is that it has a hole in it. In fact, it has more than one, but lets start simple. The canonical ozone hole was discovered by Joe Farman, Brian Gardiner and Jonathan Shanklin of the British Antarctic Survey and exists over the Antarctic. You might think that the stratosphere is far too cold for a lot ot chemical reactions to go on there. But in fact, it is so cold that ice crystals form whose surfaces are an ideal template tor chemistry, especially in the summer, when there is a lot of solar energy around.
The ozone hole has arisen because of the use of chlorotluorocarbons, CFCs, which used to be widely used for purposes such as propelling deodorant from spray cans. Once in the atmosphere, they hang around there for many years. As they decompose under the influence of solar energy, a series of reactions generates chlorine which reacts with and depletes ozone.
The use of CFCs is on a steep downward slope, a rare and encouraging international success in environmental protection. But some other ozone depleters are still in use. In most years, the area of severe ozone depletion in spring is still larger than the continental United States. In addition, similar holes have been observed over the Arctic. On current assumptions, it will take about a century for them to heal over completely. While the troposphere contains 75 percent of the atmosphere, the stratosphere contains a further 24 percent and extends up to around 50km above sea level. There you arrive at the stratopause, another join in the system.
Above the stratopause is the mesosphere and here, although some solar energy is still being absorbed, the temperature is on the way down again. Indeed, it will fall to -90°C by the time the top of the mesosphere – the mesopause, inevitably – is reached at about 85km above sea level. This high in the atmosphere there is little water, but at and around the mesopause there are sometimes seen “noctilucent” – night-shining – clouds composed of ice crystals. These are among the most difficult sights to spot in the atmosphere. They are best seen at night in polar latitudes when the sky is dark but the Sun is near enough to the horizon to light them up, in other wrords near dawn or dusk.
The troposphere and tropopause are termed the lower atmosphere by meteorologists, while the stratosphere and mesosphere are the middle atmosphere. The upper atmosphere consists of just one layer, the thermosphere. Whoever thought of the term was a genius. The main thing in the thermosphere is heat. While its temperature rises to over 1700°C, there are almost no molecules here. Indeed, the thermosphere is generally regarded as the start of outer space. It is the region where NASA's Space Shuttle, the International Space Station and other crewed spacecraft fly, as well as a host of other satellites. The fact that there is some residual air present means that there is friction on spacecraft in orbit here. This eventually causes their orbits to decay unless they are pushed higher first.
It is almost a matter of choice where you think the thermosphere ends. At about 600km up it starts being called the exosphere. But this is really a rag-tag ot hydrogen and helium atoms from the solar wind that gradually fade away into outer space after an interlude of being trapped by the Earth.
As we have seen, most of the mass of the atmosphere is in the first few kilometres above sea level. Wrhile the pressure at the surface is one bar, it has declined to 200 millibar by the time you reach the tropopause. At the mesopause it is more like 0.01 millibar.
What are we breathing?
Just what is all this air? The troposphere is extremely homogeneous. All that wind and weather ensures that it is thoroughly mixed. Unless you are standing next to a live volcano or a busy chemical works, you can rely on about 78 percent of the air around you being nitrogen. Nitrogen is largely inert but some plants have the means to “fix” it into living matter. Most of the rest is oxygen, which makes up about 21 percent of the atmosphere. This leaves about 1 percent for all the other components. The main one is argon, an inert gas. It is one of the “rare” gases whose number also includes neon, krypton and helium. But when you think that it constitutes almost 1 percent of the air around you, perhaps terming it “rare” is a little harsh.
In addition, the lowest part of the atmosphere contains much smaller amounts of carbon dioxide, water vapour, various oxides of nitrogen, and other components. Despite being present only in small quantities, these substances play a vital role. They are the greenhouse gases that keep the Earth far warmer than it would be without them, by absorbing heat radiated by the Earth and preventing it from escaping into outer space. For the truth about whether we are changing the Earths temperature by adding to the supply of greenhouse gases, see Chapter 8.
The tropopause, stratopause and mesopause are marked by severe changes in temperature gradient. But they are also the points at which the atmosphere changes its composition subtly. In the stratosphere, there is almost no water vapour and therefore very few clouds. But this layer does contain almost all the ozone in the atmosphere. The artificial chemicals that can destroy ozone are being removed from use. But other chemicals find their way into the stratosphere from the troposphere, often in huge tonnages when they are injected by big volcanic eruptions. These include nitric and sulphuric acids and their compounds.
Despite these changes in composition, the troposphere, stratosphere and mesosphere are sometimes called the homosphere because there is enough vigorous mixing between them to keep their composition very similar. But above the mesopause, there is less mixing and gravitation can slowly take effect in the vanishingly thin air there, continuing the task of differentiation with which we ended the previous chapter. Above about 200km, nitrogen dies out and the atmosphere is dominated by atomic oxygen – single atoms made by solar energy splitting up ozone and normal diatomic oxygen. Beyond 1000km we are into the exosphere, with helium dominant from 1000 to 2000km and then hydrogen.