The outer parts of most of the planets in the Solar System are made mainly of gases; this gaseous outer part of a planet is called its atmosphere. For us, the Atmosphere is all important – it provides the oxygen we breath, it controls the weather and climate that we experience, and it provides water to the land surface, in the form of rain and snow, on which all land plants and animals depend.

When we look at images of the planets in the Solar System, sometimes the disk we see is actually the atmosphere. This is true of Venus, and the “gas giant” or Jovian planets Jupiter, Saturn, Uranus, and Neptune. The atmosphere of Mars is quite transparent, so we see the rocky surface of the planet. Mercury, like Earth’s Moon, has almost no atmosphere, so images show the rocky surface too. The Earth is special: views of the Earth from space show white, reflective clouds, but with enough clear areas that the surfaces of the land and oceans are also visible. Those cloud patterns reveal a constantly changing Atmosphere. As inhabitants of the Earth we feel those changes as weather.

Earth’s Atmosphere extends above the land and ocean surface and fades into space between ~100 and 1000 km higher, depending on the criteria used. The Atmosphere supports and protects life, stores moisture and solar energy, and moves materials around in the Hydrosphere and on the surface of the Geosphere.  Earth’s Atmosphere, especially the lowest part, called the troposphere, constantly changes. Short-term changes, from seconds to weeks, result in weather, studied by meteorologists.  Longer-term characteristics of the Atmosphere, from a few years to billions of years, are described as climate, studied by climatologists (or climate scientists).  Studies of climate must use averaged weather conditions. 30 years is a common time period for calculating climate averages.

Gases: temperature and pressure

The Earth’s Atmosphere is largely composed gases:- chemical substances in which the molecules are in constant motion, colliding with one another, and against the walls of any solid objects in the Atmosphere. The Atmosphere is a mixture of gases called air. The most abundant gas in the Atmosphere is nitrogen, but of course oxygen is the most important to animal life, including humans.

Unlike atoms and molecules in liquids and solids, the molecules of gases are not connected to one another by any kind of atomic bond. The molecules in a gas are in constant motion, and the average kinetic energy (energy of movement) of the molecules in a gas is measured by its temperature. In everyday life, temperature is measured above and below an arbitrary zero point (0°C in the Celsius scale). However, scientific temperatures are measured in kelvin (K). The zero on the kelvin scale is the point where molecules have no kinetic energy, known as absolute zero. Zero kelvin is about -273°C.

The temperature of a gas is not the same thing as its heat. Heat measures the total kinetic energy of the molecules in a mass of gas. In scientific terms this is expressed by saying that temperature is an intensive variable whereas heat energy is an extensive variable. What this means that if measure the amount of heat in an amount of gas, and then double the amount of gas, we have double the amount of energy. However, the temperature does not change when we double the quantity. In the upper Atmosphere the air is very thin, so there are not many molecules, but the average kinetic energy of each one is high. There, the temperature may be very high but the amount of heat energy is quite low.

The motion of gas molecules causes impacts against any solid object, and that results in a force. The pressure is the force that acts on a unit area (e.g. a square metre) of any solid or liquid surface that a gas is in contact with.

The pressure of a gas is closely related to its volume: if you squeeze a gas into half its volume, without changing the temperature, the pressure doubles (a relationship known as Boyle’s law). Conversely, if you reduce the pressure (for example by raising a mass of gas from sea level to a position high in the Atmosphere) it expands, and the density (mass per unit volume) decreases.

In the Atmosphere, the pressure and density are lowest at the edges of space, at the top of the Atmosphere over 100 km above the surface of the Geosphere. The weight of this gas presses on parts of the atmosphere below. Each lower layer of the Atmosphere is compressed by the weight of the air above it, with the result that the pressure and the density of the Atmosphere increase downward.

The average pressure of the Atmosphere at sea level is about 101325 Pa or 101 kPa (kilopascals). This pressure is sometimes referred to as 1 atmosphere (atm).

As you might expect, there is a relationship between the temperature and the pressure of a gas, because both are caused by the motion of the molecules. If a gas is heated inside a sealed container, the pressure rises in proportion to the absolute temperature (in kelvin). Eventually this may cause the container to explode. (Don’t try this at home!) On the other hand if a mass of gas is heated in the Atmosphere, not in a container, it is free to expand and its density falls. This has several important consequences. First, hot air will tend to rise in the Atmosphere, and cold air will flow in to take its place. Second, where solar heating is most intense, near the Equator, the pressure falls because the density of the overlying Atmosphere is less.

If gas rises in the Atmosphere it may gain or lose energy in the process. If air rises rapidly enough, it may neither gain nor lose heat, in which case the rise is referred to as adiabatic. As we shall see, adiabatically rising air doesn’t usually maintain its temperature, but to understand why, we need to introduce another intensive variable, pressure.

The behaviour of rising or falling air depends on whether heat is added or not. If no heat is added or removed, the movement is described as adiabatic. If no heat is added or removed, you might think that the temperature would stay the same, but actually this is not the case. This is easily demonstrated by pumping up a bicycle tire. As you work the pump, compressing the air, it gets hot. The energy that you expend on squeezing the air into a smaller space is converted to heat, and the temperature rises. As air rises in the Atmosphere, the opposite happens. The air expands, and this means that work is done against the pressure of the surrounding air, so some energy is lost in the form of mechanical work done. As a result, rising air cools. The rate of cooling is known as the adiabatic lapse rate. For dry air the lapse rate is 9.8°C per kilometre, or about 1°C per hundred metres. This is one of the reasons the air is typically cooler on the top of a mountain than it is in an adjacent valley. The lapse rate for damp air may be different, but before considering the behaviour of damp air, we need to look at temperature and pressure in the Atmosphere.

Vertical changes in the Atmosphere

The temperature in the Atmosphere changes in quite a complicated way, as shown in the diagram below. To understand how this works, we need to consider where the energy is coming from.

The outer edge of the Earth’s Atmosphere is quite hard to define, as it fades away into the emptiness of space. One suggestion is known as the Kármán Line, and is set at 100 km. For legal purposes, it’s the upper limit of national airspace. Above 100 km the Atmosphere is very thin. Although there’s not much energy in it, the molecules are so dispersed that each one has quite a large amount of kinetic energy so the temperature is high.

As the air becomes more concentrated downwards, the amount of heat energy per molecule declines so the temperature falls to a minimum around 80 km above the Earth’s surface, known as the mesopause. The portion of the Atmosphere above this minimum is known as the thermosphere. In both regions, solar radiation removes electrons from atoms and molecules, producing charged ions: the mesosphere and thermosphere are parts of the larger ionosphere.  Surges of charged particles from the sun (solar wind) may change the ionization state of atoms and molecules, particularly in the thermosphere, producing visible light – the aurora borealis (northern lights) and aurora australis (southern lights).

Below 50 km, oxygen and ozone in the Atmosphere become concentrated enough to start trapping large amounts of incoming radiation, especially in the ultraviolet range. This trapped energy heats the air, so there is a temperature maximum at this height, known as the stratopause. The temperature at the stratopause, 50 km up, may be similar to that at the Earth’s surface, and this heats the mesosphere from below, accounting for the downward rise in temperature. The stratopause marks the base of the mesosphere and the top of the stratosphere. The temperature in the stratosphere increases upward, because it is warmed by ultraviolet absorption in its upper parts.

Continuing downward, the next big absorber of solar radiation is the land or ocean surface, which is intensely warmed by solar energy. Therefore, the bottom part of the Atmosphere, known as the troposphere, is heated from below. Temperature decreases upward to a boundary about 10–15 km above the land or ocean surface, known as the tropopause, marking the transition from stratosphere above to troposphere below. The height of the tropopause, and the thickness of the troposphere, is actually quite variable. Near the equator, where the ground is intensely heated, the troposphere may extend up to about 17 km. In polar latitudes, with less solar heading from below, the troposphere is typically less than 10 km thick. Almost all weather happens in the troposphere.

Composition of the Earth’s atmosphere

Mixed gases: partial pressure

The atmosphere is a mixture of different gases, as we shall see below. In a mixture of gases, it’s possible to break down the total pressure into partial pressures exerted by the different gases. Dalton’s law of partial pressures states that the total pressure of the mixed gas is the sum of the partial pressures of each gas in the mixture. The partial pressure of each gas depends on the proportion of its molecules in the mixture. For example, the partial pressure of oxygen in Earth’s atmosphere at sea level is about 21% of the total pressure. This indicates that 21% of the molecules are oxygen.

Nitrogen

Nitrogen is the most abundant gas in Earth’s Atmosphere. It occurs as N₂ molecules that are relatively unreactive. Most of the time we are unaware of this largest component of our Atmosphere. However, nitrogen is a critical part of all living things, and therefore the Biosphere must capture nitrogen from the Atmosphere. This can occur in two ways. Nitrogen can be oxidized by lightning in the Atmosphere, which combines it with oxygen to make nitrate (NO₃^–) ions. Alternatively, it can be reduced, by biological processes that occur, for example, in the roots of certain plants, to make ammonium (NH₄^–) ions. Both processes make nitrogen more accessible to living organisms. These processes are important parts of the nitrogen cycle, on which all life depends.

Oxygen

Oxygen represents about 21% of Earth’s Atmosphere at the present day, and is the second most abundant component. Oxygen is essential to all animal life and many plants, which obtain their energy by reacting oxygen with carbon compounds, a process known as respiration. The element oxygen actually occurs as two different gases in the Atmosphere.

Most oxygen exists as molecular oxygen (O₂), consisting of two oxygen atoms joined by a double bond, but a small proportion exists as ozone (O₃) in which three oxygen atoms are joined. Ozone forms by the action of ultraviolet radiation on oxygen in the Atmosphere, but the ozone that forms is even more effective at absorbing ultraviolet radiation, particularly at the shorter wavelengths that are most harmful to human and other animal life at the Earth’s surface. Ozone is most abundant in the stratosphere between 15 and 50 km above the Earth’s surface, though at maximum it typically only accounts for about 10 parts per million in the Atmosphere.

Water vapour

Water vapour^[1] is the most variable component of Earth’s Atmosphere, but in most parts of the Atmosphere it’s the third most abundant gas, after oxygen and nitrogen. The average partial pressure of water is 2 to 3% of atmospheric pressure. However, very warm air may hold over 4% of water vapour, while in very cold air, the partial pressure of water vapour may be as low as 0.01%. Because water vapour pressure is so variable, it’s common to express the composition of the rest of the Atmosphere as “dry” percentages, after water has been removed.

For any given temperature, there is a saturation vapour pressure that represents the maximum amount of water vapour that can exist in the air if it’s in equilibrium with liquid water or (below freezing) with solid ice. Under most circumstances, if the partial pressure of water rises much above this saturation point, liquid water or solid ice will start to condense.

Water vapour is an important greenhouse gas, but because its saturation behaviour leads to frequent return of water from the Atmosphere to the Hydrosphere by precipitation, it is a less serious concern than carbon dioxide (below) and methane.

Argon

The next most abundant component, argon, is an inert gas: – one with eight electrons in the outer shell of its atoms, with the result that it participates in almost no chemical reactions. Argon accounts for about 1% of the gas in the Atmosphere, but because of its inert character, it does not participate in significant interactions with the other spheres.

Carbon dioxide

After argon, the next most abundant component is carbon dioxide or CO₂, an important greenhouse gas. The abundance of carbon dioxide over most of the past 10,000 years (based mainly on measurements of air bubbles in glacier ice of known age) was about 280 parts per million, or 0.028 %. However, in the last 200 years, the amount of CO₂ has been rising steadily to a present-day (2023) value of nearly 420 ppm.

Carbon dioxide has the ability to dissolve in water, producing an acidic compound called carbonic acid by the reaction:

CO₂ + H₂O = H₂CO₃

Carbonic acid undergoes further reactions once it’s dissolved in water, which we look at in more detail in the chapters on the hydrosphere.

Daily, weekly and monthly measurements of atmospheric CO₂ are available from an observatory at Mauna Loa in Hawaii.

Other gases

Three of the next four gases by abundance are inert gases, which play passive roles in the Atmosphere similar to that of Argon. They are

• Neon 18 ppm (parts per million)
• Helium 5 ppm
• Krypton 1.1 ppm

Just ahead of Krypton in abundance is methane CH₄ at 1.8 ppm. Methane is produced by natural processes in the Biosphere, but atmospheric methane is also increasingly contributed by the accidental release of natural gas, of which it is the main constituent. Methane is a potent greenhouse gas, second in importance to carbon dioxide. Its contribution to warming of the Atmosphere is less than that of carbon dioxide because its absolute contribution is low, and because it undergoes slow natural oxidation by reacting with oxygen, giving it a lower residence time.

There are of course many other gases present in the Atmosphere in trace amounts, some of which we will meet in other sections as we look at human impacts on the Earth system.

Aerosols

In addition to gases, the Atmosphere contains very small particles of solids and liquids. If they are small enough, these particles may remain suspended in the Atmosphere for long periods of time, as they are continually buffeted by the fast-moving gas molecules. These small solid particles and liquid drops are known as aerosols. They include:

• Water droplets
• Ice particles
• Solid particles of rock and other dust
• Small particles of carbon derived from forest fires
• Particles of salt derived from the oceans
• Etc.

Water in the Atmosphere

Phases of water

Water is an unusual substance. It can exist at the Earth’s surface as three different phases. (Phases are materials that are separated from each other by distinct boundaries.) The three phases of water are water vapour, liquid water, and solid ice. In the Atmosphere, water vapour is part of the gas phase. Liquid water and ice exist in the Atmosphere as separate phases as aerosols, small particles that are suspended within the air.

As pressure and temperature change in the Atmosphere, phase changes occur. Ice melts to produce liquid water. Water evaporates to produce water vapour. Ice may sublimate to produce water vapour. Each of these phase changes requires input of energy to break the bonds between the water molecules. This energy is called latent energy, or latent heat, meaning energy that is added or removed without changing the temperature. For example, it takes about the same amount of energy to warm ice from -16 °C to zero degrees Celsius as it does to bring ice at 0°C to water at 0°C.

Changes in the opposite direction release latent energy. Water freezes to produce solid ice. Water vapour condenses to produce either liquid water or solid ice. Each of these phase changes requires the removal of latent energy as bonds between water molecules are made. In thermodynamics, latent energy is explained as the energy that must be supplied to go from a more ordered, or lower entropy state (like ice), to a more disordered or higher-entropy state like water vapour.

Equilibrium between phases

The figure shows a body of liquid water (maybe a lake) in contact with a gas (maybe the Atmosphere). The two have the same temperature, so the average kinetic energy of the molecules is the same, but the energy of individual molecules varies a great deal. Every so often, a molecule of water has enough energy to free itself from the surface of the liquid and enter the gas as water vapour. The partial pressure of water vapour in the gas will gradually increase.

However, the molecules in the gas also have a range of kinetic energies, and sometimes the energy of a molecule of water vapour will be sufficiently low that, when it hits the surface of the liquid water, it will return to the liquid phase. A state of equilibrium will be reached when the rate transfer of molecules is the same in each direction. We say that the air is saturated with water vapour. If the water-vapour content of the Atmosphere increases beyond this level, it will be counteracted by increased condensation (provided there is a water surface available).

Saturation vapour pressure

The partial pressure of water vapour that can exist in equilibrium with liquid water varies with temperature, and is measured by the saturation vapour pressure. At 20°C the saturation vapour pressure of water is about 2.3 kPa or 0.023 atm, meaning that water makes up a little more than 2% of the molecules in saturated air at this temperature. At 40°C this value rises to 7.3%. At freezing point it is only 0.6%. Below freezing, the saturation vapour pressure is usually expressed relative to ice; it continues to fall with falling temperature. By minus 20°C, air is saturated when it contains only 0.1% of water vapour molecules.

Relative humidity

Often, air in the Atmosphere is not saturated with water vapour. For example, in continental interiors far from the sea the air is often quite dry. If the water vapour pressure is only half as large as the saturation vapour pressure, we say that the relative humidity is 0.5 or 50%.

So

Relative humidity = (Water vapour pressure) / (Saturation vapour pressure)

Dew point

As temperature falls the saturation vapour pressure of air also falls. Therefore, if a body of unsaturated air is cooled, it eventually gets to a temperature where the relative humidity hits 100% – the air has become saturated as a result of falling temperature. We say the air has reached its dew point – the temperature that is cold enough for a deposit of water, known as dew, to form on exposed surfaces.

Condensation of water and ice

What happens when there is no surface available for condensation, for example high in the Atmosphere? The formation of a first, very small droplet of water is difficult, because the molecules in small drops with curved surfaces are more loosely bound than those in a large body of water, so their vapour pressure is a little higher than the saturation pressure. As the temperature falls, therefore, the air may become oversaturated. Extra energy must be taken out to achieve the first condensation of a very small water drop. That energy is known as the nucleation energy. As a result, condensation may not start until the temperature has fallen a little below the dew point.

Once nucleation starts, water droplets grow rapidly to about 20 μm in diameter. The aerosol typically scatters the light, forming clouds. If there’s more than one drop per cubic millimetre of Atmosphere, frequent collisions between droplets form larger drops that are no longer suspended, and those raindrops fall as rain.

If the temperature is below freezing. The nucleation of ice crystals is particularly difficult. From zero Celsius down to about -9°C, the high nucleation energy of ice inhibits ice formation. So supercooled water droplets form instead, producing freezing mist (which may deposit ice on any surface that it touches). Once the temperature becomes low enough for the nucleation of ice, the supercooled water droplets release vapour which then condenses on the ice crystals, allowing them to grow. This is known as the Bergeron process. If the ice crystals become large enough that they are no longer suspended in the Atmosphere, then snow will fall.

Adiabatic lapse rate of condensing air

In an earlier section. We saw that as warm air rises, it tends to cool adiabatically by about 10°C/km. However, if condensation starts, it releases latent energy as heat which offsets the adiabatically cooling. For air that is actively condensing the rate of cooling is reduced to about 6°C/km. This is known as the moist adiabatic lapse rate.