Of course, Earth is not the only planet to have an atmosphere. But we do have the atmosphere, and it's slightly more important than the others because we need it to survive. It is a layer of mixed gases (collectively called air, if you want to be technical) around Earth, held there by its gravity.

What's In It?

Rather boringly, most of the atmosphere is disappointingly inert nitrogen¹. 78% of it, in fact. The next commonest gas is oxygen, which you can use to set fire to things at least. This takes up 21% of the total, so there really isn't much left by this point. Nonetheless there are some important constituents around, despite their scarcity.

Other constituents, each accounting for a few parts per million, are other noble gases (like helium, neon and krypton), methane and nitrous oxide. Some of these gases have chemical structures which cause them to trap outgoing radiation. The main 'greenhouse gases' are: water vapour, carbon dioxide and ozone, with methane, nitrous oxide, carbon monoxide and chlorofluorocarbons (CFCs) also contributing. These gases are very important since they ensure the Earth does not radiate away all its heat energy - without them the average temperature on the planet would be around -18 degrees Celsius.

Finally we have the non-gaseous bits. These are mainly aerosols (solid particles) and water droplets in clouds. These are only a minuscule proportion of the total mass, but they are very important. Aerosols provide 'condensation nuclei' onto which water vapour can condense to form clouds and hence rain.

Of course, the make-up of the atmosphere varies slightly according to local conditions. For example, volcanic eruptions spew dust and gases out into the air, as does human pollution.

Incidentally, the total mass of the atmosphere can be calculated from the global average surface pressure (around 985 mb⁴) and the acceleration of objects due to gravity (around 9.8 m/s per second). It works out at 5.1x10¹⁸kg, or 51 with 17 zeros after it, or 5.1 million million million kilograms. Since pressure is nothing more than mass per unit of areas, that's around 10 tonnes for every square metre of surface. Think about that next time you look up into the sky. Luckily, the air inside you is at the same pressure. If you had no air in you the air outside would try to rush in to equalise the pressure, and you'd be squashed.

Vertical Structure

The atmosphere is divided into several sub-spheres, each with specific characteristics.

Troposphere - this is where most of the 'weather' happens, in the bottom 10 kilometres. Here, temperature decreases with height, at a rate (called a lapse rate) of around 6.5 degrees C per kilometre. The Earth's gravity holds most of the air down at this height, so even though it is a tiny slice of the atmosphere, it holds something like 80% of its mass. Stuck right at the top is a region called the tropopause, where temperature does not change with height for time. The tropopause can be seen when tall clouds form an 'anvil' shape as upwelling air spreads out as it encounters warmer air above.

Stratosphere - approximately located between 10 and 50 km. Here, the temperature begins to increase with height (which is why it is very rare to get clouds in the stratosphere - it is not cool enough for what water vapour there is at this height to condense. This region is the richest in ozone, and it is the absorption of ultraviolet (UV) radiation from the Sun that causes the unexpected increase in temperature. The ozone layer acts like a layer of factor 30 sun-block around the Earth, without which complex life would be unable to survive. The stratosphere is bounded at its top by the stratopause.

Mesosphere - temperature starts decreasing with height again here. It's top boundary is called - you guessed it - the mesopause. It is a very poorly understood part of the atmosphere so there is little more to say than that it is located between 50 and 80 km altitude, and that this is is about as high as aircraft can fly.

Thermosphere - a rather strange place, this. Temperature starts increasing with height again. The processes are either rather complex or rather poorly understood, but the warming involves the absorption of solar radiation by nitrogen and oxygen, and the stripping of electrons from atoms. This region contains a sub-region called the ionosphere, which is where auroras occur. The ionosphere itself contains a sub-region called the Heaviside layer⁵, extending from 90 to 150 km. It's rather important because it reflects medium-frequency radio waves, which is why radio signals can be sent round the curved surface of the Earth rather than just heading in a straight line into space. The thermosphere typically extends up to 400 km. However, this is a nebulous realm, which may or may not count as space depending on the definition used⁶.

Above this is the exosphere, which can extend up to 1,000 km in some cases. It is little more than a region of free-moving particles where there are virtually no collisions between particles. Some relatively light molecules, like hydrogen, can escape the force of gravity and 'leak' out of the atmosphere here. Earth has been losing such molecules to space every since it was formed.

There are other terms for atmospheric layers. One system is based on composition. It contains the homosphere, where molecules are not layered according to their mass; and the heterosphere, where the lighter molecules form a layer above the heavier ones. The homosphere extends from ground level to the mesosphere. Molecules do not stratify here because they are being constantly mixed by turbulence. The only two constituents that are not well mixed in the homosphere are water vapour and ozone. Once above the homosphere there is less mixing and so the molecules form layers according to their mass.

Atmospheric layers can also be defined by their electrical properties. Below 100 km is what is termed the neutral atmosphere. Shockingly (pun intended), the atmosphere is neutral (has no electric charge) in this region. The ionosphere, which has already been mentioned, contains ionised particles, which have been given an electric charge by UV radiation from the Sun. The magnetosphere is a subsection of the ionosphere (anywhere up to 64,000 km). Here, high energy particles from the Sun (the 'solar wind') are trapped by the Earth's magnetic field. It's a good thing they are, because they could do all sorts of nasty things to us if they got down to ground level.

The 'pauses', or layers of constant temperature, stop air from different layers mixing effectively. So the troposphere is especially well-mixed, but there is little exchange between the troposphere and the stratosphere.

Wind and Pressure

Without wind, things would be rather dull. And it would be rather too hot or cold in most parts of the world. Winds transfer heat energy from where there's a lot of it (at the equator) to where there isn't (polewards). On a basic level, they flow from where there is high air pressure to where there is low air pressure. Relatively low regions are called depressions, and high pressure centres, anticyclones.

The Coriolis Force

But now we hit a snag. Because you've just been told air flows from high pressure to low. But the wind blows anticlockwise around depressions (low pressure) and clockwise around anticyclones (high pressure). Why does it not go straight? It's all down to the Coriolis force.

Now, the Coriolis force isn't actually a force. Rather, it's a perceived force⁷. It doesn't actually exist, but for us on Earth it appears to. For the sake of simplicity, imagine the Earth is a flat disk. The disk is spinning around like a roundabout. If we roll a ball, fire a missile or whatever across this disk, it will head straight across. Why shouldn't it? There's no other force acting on it, right?

Aha, but there is (kind of). Consider this from the viewpoint of someone else, say, the target of the missile (he can see a long way). He will see the missile fire, but he won't worry. This is because the missile has been aimed directly at him. It actually needs to be aimed at where he will be, because in the time it takes for the missile to travel the required distance the rotating disk will have carried him away from the line of fire. Our target has had a lucky escape. He is not aware of the disk rotating. From his perspective, it looks like a force has bent or deflected the hypothetical missile away from him. This effect is only really discernible when large distances are involved, so you needn't worry about it when throwing a ball, for instance.

This imaginary deflecting force in the Coriolis force, and it 'bends' winds. Now, isobars are lines drawn on weather charts connected areas of equal pressure. You would expect the winds to flow across isobars, perpendicular to them, from high to low pressure. But when you take the Coriolis force into account, the winds are deflected so they travel roughly parallel to the isobars.

Wind Fields

Although there is a lot of variation, it is possible to put together a 'background wind field' which represents the most persistent throughout the year. Often, when summarising basic wind fields, meteorologists talk about an 'aqua-planet' - an ocean-covered Earth with the sun directly overhead at the equator. This simplified view is simply because in reality things get far too complex, with mountains and the tilt of the Earth's axis.

The aqua-planet's wind behaviours are very similar to those of Earth, or at least similar enough for most purposes. The Sun's heat causes moist air to rise at the equator, which gradually cools as it climbs, forming a belt of clouds called the Inter-tropical Convergence Zone (ITCZ). After spreading out away from the equator, this cooler air sinks back to the surface, where it moves back towards the equator, to the fill the space (the area of low pressure) which it previously vacated. This continuous loop of moving air is called a convection cell, and there are several such cells in the atmosphere: the equatorial one is called the Hadley Cell. George Hadley was an English lawyer during the early 1700s, but fancied himself a bit of a meteorologist. He did a lot of work on the winds blowing in towards the equator to replace the air rising at the ITCZ (called the trade winds⁸).

At the poleward end of the Hadley cell there is an area of high pressure, formed by the sinking air, at around 30 degrees latitude. Further poleward again (45 degrees latitude) are the westerly wind belts. These are much more pronounced in the southern hemisphere, because there is a huge belt of open sea north of Antarctica. With no pesky high land to get in the way these winds blow very powerfully in a great circle in the southern seas. This is the origin of the phrase 'roaring forties', as these winds blow around 40 degrees south. They get stronger further south, giving rise to new terms such as the 'furious fifties'.

Smaller Scale Air Movements

There are many local winds of variable nature which depend on specific conditions. They can be katabatic (flowing downslope) or anabatic (flowing upslope). Katabatic winds are more common, and can be divided into cold and warm types. Cold katabatic winds (such as the Mistral flowing from the Alps out into the Mediterranean) occur when air cools over a glacier or a mountain. Cold air is denser than warmer air, so it flows downhill. The warm type (such as the Chinook in western North America and the Foehn of central Europe) is caused by moist air rising over mountains. As it gains height it cools, the water vapour contained in it condenses, and rain or snow falls. By the time it descends on the other side of the high ground it has lost much of its water content. This wind then warms up as it gets further down, giving areas on the lee side of the mountains warm, dry, blustery conditions.

Apart from convection, which has already been mentioned, there are other small-scale motions in the atmosphere. Are they important? Well, perhaps not consistently vital, but they have their moments. Mountains and the like can produce some interesting waves and eddies in the air which in turn produces rippling cloud forms. There is also general turbulence: air is never still. Wave-like clouds can form when eddies break up cloud tops into shapes very reminiscent of waves breaking on a beach. Turbulence occurs all the time, at all sorts of scales. Lewis F Richardson (1881-1953) was an English meteorologist with rhyming tendencies:

Big whirls have smaller whirls that feed on their velocity, and little whirls have lesser whirls, and so on to viscosity...in the molecular sense.

Precipitation

Interesting fact for you: the globally averaged precipitation rate is 1 metre per year or around 0.275 cm per day. It can be a lot more than this in areas such as the ITCZ. All that air rising in the Hadley Cell cools down and the water vapour contained in it condenses and a lot of it falls as rain. Much of this rain waters the rainforests of the world. Regions either side of the equator have just two seasons: dry and wet. This is because the ITCZ shifts north and south through the year due to the tilt of the Earth. There can also be variations from year to year. For example, Canton Island at 3 degrees south, 170 degrees west, has been known to have no rainfall some years to more than 30 cm per month (for months on end) in others. All this is down to subtle changes in the sea surface temperature in the Pacific, which is affected by the El Niño phenomenon.

The deserts of the world are mainly located around 30 degrees latitude, where sinking air from the Hadley cell warms up. This air has lost most of its moisture as rain over the ITCZ, then moved polewards a little before sinking. As it sinks it warms up, so what water vapour is left is even less likely to condense, meaning very little rainfall. Which is why deserts exist, broadly speaking.

The Atmosphere: A History

Mark I

Originally the Earth's atmosphere was mostly composed of the two most abundant elements in the Universe: helium and hydrogen. It is generally thought that this 'first atmosphere' dissipated into space after acquiring enough heat energy from the molten rock which covered most of the planet. Since the elements in it were relatively light it did not take much to dissipate them.

Mark II

The 'second atmosphere' came about when things had cooled down a little, and the molten rock hardened to form the crust - the rocky bit we live on. Like today, there was still molten rock underneath the crust; but the additional heat of the youthful planet mean volcanic eruptions were commoner and more intense. These eruptions released a lot of water vapour, some carbon dioxide and a little nitrogen.

The second atmosphere was very dense, containing around 100 times more gas than our current atmosphere. This meant there was a great deal of water vapour, which over time formed clouds. Clouds generally end up as rain, and rain generally ends up forming puddles. Some of these puddles got very big and became lakes and oceans. As more water settled on the surface, the more carbon dioxide was removed from the atmosphere and dissolved in it. So water vapour and carbon dioxide content decreased and inert nitrogen came to the fore.

Remember this was all taking place over millions of years. The second atmosphere is thought to have come into being around 4.4 billion years ago⁹. All this water was lying around under the Sun, and ever so slowly, the energy from the Sun's rays split water vapour¹⁰ into hydrogen and oxygen - a process called photodissociation. The light hydrogen likely escaped into space, while the oxygen hung around, although still at much lower levels than we are used to today.

Mark III

This began to change when a type of bacteria called cyanobacteria became the first organism to begin producing oxygen. Later, two to three billion years ago, primitive plants began to evolve. Plants (and cyanobacteria) rely on the process of photosynthesis to provide them with sugars - 'food', if you will. In this process, plants combine carbon dioxide and water, in the presence of sunlight, and produce oxygen. Several hundred million years later, after rather a lot of photosynthesising, the atmospheric concentration of oxygen is thought to have reached its present levels.

Over time, ultraviolet light from the Sun caused oxygen molecules to rejig, and ozone began to form, which is nothing more than three oxygen atoms bonded together. The ozone was created in the stratosphere and formed the layer which, as has been said, is vital to our survival on Earth. In the new oxygen-rich atmosphere new organisms developed and, after a few millennia, hey presto: you have humans.

Human Effects

The atmosphere is not a stable entity. It is constantly changing, as shown by the three different versions we have had so far. As well as natural change, humans can also effect changes. CFCs from fridges and aerosol cans cause a depletion of the ozone layer, which resulted in the hole discovered over Antarctica in 1985. CFCs were phased out as a result. Particulate matter from industrial chimneys provide a greater number of 'nuclei' onto which water vapour can condense, so, on a local scale at least, humans can influence rainfall patterns ('cloud seeding' involves using planes to add particulates to clouds to induce rain). Industrial processes also give off gases like sulphur dioxide, which can combine with water in raindrops to acidify them further (rain is naturally slightly acidic). Acid rain can damage forests and kill fish by acidifying lakes, but also damage buildings. Pollution from combustion engines and other sources can cause respiratory problems, though this effect has lessened with the introduction of clean air legislation. Finally, human industrial activity gives off gases like carbon dioxide, methane and fluorocarbons, which increase the magnitude of the greenhouse effect and contribute to global warming.

Why is the Sky Blue?

Ah, the perennial question, always worth answering. This is down to something called Rayleigh scattering. Now, the colour of light is affected by its wavelength. As light from the sun enters the atmosphere the long-wave radiation passes straight through. But short-wave radiation (blue light has a short wavelength) is absorbed by particular gas molecules that have suitable chemical structure. This light is then scattered, or re-radiated in different directions. This scattering means that, wherever you look, you will always see some blue light. This also explains why the sky is paler nearer the horizon. The light has had to pass through more air to reach you, so more has been scattered in other directions, and less blue light reaches you. This effect is also responsible for the Sun being yellow. The atmosphere filters out the short wavelength blues and violets, leaving colours which combine to form yellow. Outside the atmosphere, the Sun appears white.