Clouds are found everywhere throughout the Earth’s atmosphere. On average 67% of the Earth’s surface is covered by cloud at any given time, with this rising to 72% over the oceans and decreasing to 55% over land 1.
Clouds are the source of all the rain and snow we experience on Earth
Clouds form when the local relative humidity in the atmosphere exceeds 100%. The air is then saturated with water in the vapour (gas) form, meaning it cannot hold any more water vapour. If the relative humidity exceeds 100% droplets begin to grow through a process called condensation. Even when the relative humidity is greater than 100%, condensation still requires small specks of dust or dirt in the atmosphere, referred to as cloud condensation nuclei (CCN), to allow droplets to form.
The lower atmosphere (below ~5km altitude) generally contains enough aerosol that droplets readily form once the relative humidity exceeds 100%. Depending on the local air temperature, clouds can be made up of liquid droplets, ice crystals or a mixture of both. In very cold and clean atmospheres (with very low aerosol amounts), even when the temperature is below 0°C, clouds can still contain liquid droplets, so-called supercooled liquid. This is because the lack of aerosols prevents the formation of ice crystals and liquid droplets form more readily.
Most clouds are made up of a large number of small droplets or ice crystals. These clouds generally do not produce any rain or snow (referred to as precipitation in climate science). If the conditions are such that the relative humidity of an air parcel stays at or above 100% for a long time, the process of condensation can lead to some droplets (or ice crystals in a cold cloud) growing to become bigger than other droplets in the same cloud. The effect of gravity then leads to these bigger droplets falling through the air at speeds greater than the smaller (lighter) droplets. In doing so, the larger droplets collide with and “collect up” numerous smaller droplets as they fall, rapidly increasing their size.
Sometimes raindrops can become sufficiently large that they break apart while falling, forming new raindrops. This process of condensation followed by the collection of small droplets by larger ones is the main route by which rain droplets form. Once these large droplets reach the bottom of the cloud they fall out as rain or snow, ultimately providing water to the Earth’s surface.
A second process through which rain and snow form occurs in clouds that contain both liquid droplets and ice crystals (so-call mixed phase clouds). As a result of the thermodynamics of water, at temperatures less than 0°C in mixed phase clouds liquid droplets rapidly evaporate and the resulting water vapour freezes onto any ice crystals present. This leads to a rapid growth of ice crystals to sizes sufficiently large that they fall through the cloud due to gravity, collecting up other ice particles as they fall and producing snow 2.
Most of the water in the atmosphere that leads to cloud formation and precipitation ultimately comes from the ocean surface (through evaporation). Due to the way the land and ocean are heated by the sun and the resulting atmospheric circulations, more rain generally falls on land than over the oceans. This leads to a net transport of water from the ocean to the land. This transport of water is what supports life on Earth. The cycle is completed by rivers transporting this excess water back from the land to the ocean.
Clouds play a key role in shaping the Earth’s energy budget
Clouds interact with both radiation from the sun (solar radiation) and radiation emitted from the Earth’s surface (terrestrial radiation). Clouds reflect around 23% of the solar radiation that reaches the Earth’s atmosphere and absorb a further 3% 3. This reflection cools the Earth the Earth . Clouds also absorb longer wavelength terrestrial radiation, reemitting this both upwards and downwards, warming the both the atmosphere and surface below.
Clouds high up in the atmosphere contain relatively small amounts of water and are typically made up of ice crystals. These clouds are fairly transparent to the sun’s radiation (i.e. most of it passes through the cloud) whereas they are very effective at absorbing the longer wavelength radiation emitted by the Earth’s surface. Hence, high clouds tend to have a warming effect.
Low clouds generally contain more water and are mainly made up of liquid droplets. These clouds are highly reflective to the sun’s radiation. They are very efficient at absorbing terrestrial radiation from the Earth’s surface. Once absorbed, this terrestrial radiation is re-emitted upwards and downwards, with the intensity determined by the local cloud temperature. For low level clouds, the cloud top temperature does not differ greatly from the temperature of the Earth’s surface, hence the amount of radiation re-emitted upwards by low level clouds is similar to that emitted by the Earth’s surface. As a result, low level clouds do not have a big impact on the terrestrial radiation budget, but they do have a big impact on the sun’s radiation, reflecting away a lot of it. Low clouds therefore strongly cool the climate. On balance the cooling effect of clouds, through reflection of the sun’s radiation, is greater than their warming effect, so clouds act to cool the Earth in today’s climate 3.
What could happen in the future? The cloud feedback: The number one source of uncertainty in estimates of future climate change
How clouds will respond to future warming due to an increase in carbon dioxide (CO2) is the single largest source of uncertainty in estimates of future climate change. This is known as the cloud feedback problem 4 . Only a very small change in global cloud amounts can act to either negate or double the direct warming from a doubling of CO2. Even without a change in total cloud amount, if clouds respond to the initial warming by changing what they are made up of (e.g. containing more liquid and less ice), their vertical location or even their seasonal and/or geographical location this can lead to significant modifications to the initial warming due to an increase in CO2.
Accurately predicting the myriad of potential cloud responses to an initial warming is a major challenge in climate science. We generally expect that the warming from increasing CO2 (which increases the absorption of radiation emitted by the Earth’s surface in the atmosphere) will lead to an atmosphere with a similar relative humidity as today. From this one might expect cloud amounts to not change too much. How clouds change in different parts of the globe is important to consider though, as the sun’s radiation varies in intensity depending on latitude and season, as does the amount of radiation emitted by the Earth’s surface.
 King et al. 2013
 Storelvmo and Tan 2015
 Trenberth et al. 2009
 Stephens 2005