Temperature is a measure of how fast molecules in a substance are moving, known as their kinetic energy. This includes gas molecules in the air, such as oxygen and water vapour – the higher the air temperature, the more kinetic energy the molecules have. Clouds form when water vapour coalesces into droplets around aerosol particles in the atmosphere. A crucial element of this is the process of condensation, by which water in gas form (water vapour) becomes liquid. This occurs when water vapour molecules are moving slowly enough so that, if they collide with a solid surface such as an aerosol particle in the atmosphere, they tend to stick to it rather than bounce off. So lower air temperatures – meaning lower kinetic energy – result in a faster condensation rate than higher air temperatures.

Air molecules have more kinetic energy at high temperatures than at low temperatures. More energetic water vapour molecules are less likely to condense into droplets and form clouds, because the speed at which they’re moving inhibits the condensation process. This results in a lower rate of condensation, so the water vapour molecules remain airborne for longer, since they’re less likely to fall as rain. The reverse is true if air temperatures fall – water vapour molecules have less energy, making them more likely to condense into droplets, form clouds and rain out. The result is that warmer air can hold more water vapour and has the capacity to sustain higher humidity levels than cooler air.

Water vapour is a greenhouse gas, so its atmospheric concentration affects the strength of the greenhouse effect which controls the Earth’s surface air temperature. But air temperature also controls humidity, and if global temperatures rise or fall, global water vapour levels rise or fall in response. This results in a positive feedback – an initial temperature rise leads to an increase in water vapour, strengthening the greenhouse effect and causing a further temperature rise. This also works in reverse following a decrease in temperature – less water vapour weakens the greenhouse effect, causing a further temperature drop. Scientists calculate that, globally, water vapour feedback approximately doubles an initial temperature change, turning a rise of 1 °C into 2 °C of warming, or a drop of 1 °C into 2 °C of cooling.

In the lowest atmospheric layer – the troposphere – temperatures decrease with height. This is known as the lapse rate and is central to the workings of the greenhouse effect. If the atmosphere had no greenhouse gases, the infrared energy given off by the Earth’s surface would escape to space directly from ground level. Instead, the energy is absorbed by greenhouse gases and eventually escapes from a higher level in the troposphere. Because the air at high altitude is colder than at the surface, energy escaping at these heights carries less heat away from the planet than energy escaping at the surface. As a result, more heat is trapped in the Earth’s atmosphere, keeping it warmer than it would otherwise be. So the more pronounced the tropospheric lapse rate is, the more heat is trapped by the greenhouse effect.

Any change in air temperature leads to a change in levels of atmospheric water vapour. This in turn changes the amount of infrared energy absorbed in the atmosphere, because water vapour is a greenhouse gas. For example, an initial rise in temperature causes an increase in water vapour, leading to more infrared energy absorption and a further rise in temperature. However, in areas of high surface humidity, such as the tropics, most of the available infrared energy is already absorbed by the existing levels of water vapour. So the extra water vapour would have a more pronounced effect at higher altitudes, causing the upper troposphere to warm at a faster rate than the surface and so reducing the lapse rate. Conversely, an initial decrease in temperature would result in an increased lapse rate.

Owing to water vapour feedback, an initial change in air temperatures in the troposphere leads to a change in the lapse rate. Warming decreases the lapse rate while cooling increases it. Because of the central role of the lapse rate in the greenhouse effect, any change affects temperatures in turn. For example, a reduced lapse rate following an initial warming means that temperatures at high altitudes are no longer as cold relative to the surface as before. As a result, the heat-trapping ability of the greenhouse effect is lessened and the initial warming is reduced. This is a negative feedback and partly counteracts the positive feedback effect of water vapour. Scientists calculate that, globally, the combined effects of water vapour and lapse rate feedbacks turn a 1 °C warming or cooling into a 1.5 °C warming or cooling.

• Met Office: Climate feedbacks
• NASA: Water vapour and climate change
• Koshland Science Museum: Water vapour