A cloud is a visible mass of water droplets, ice crystals, or a mixture of both suspended in the atmosphere. Clouds form when water vapour cools and condenses around tiny particles of dust, salt, smoke, or other material. A typical cloud droplet may measure only 0.01 mm across, yet a large cloud can contain hundreds of tonnes of water and stretch for more than 100 km.

Clouds occur at very different heights. Low clouds generally form below 2,000 m, while middle-level clouds are commonly found between 2,000 and 7,000 m. High clouds may develop at altitudes of 5,000 to 13,000 m, where temperatures can fall below -40°C. Some storm clouds extend through several of these levels, rising from around 500 m to more than 12,000 m above the ground.

The modern system of cloud classification developed from the work of English meteorologist Luke Howard. In 1802, Howard proposed 3 principal categories: cirrus, cumulus and stratus. His terminology was expanded over time, and today the World Meteorological Organization recognises 10 main cloud genera. These include Cirrus, Cirrocumulus, Cirrostratus, Altocumulus, Altostratus, Nimbostratus, Stratocumulus, Stratus, Cumulus and Cumulonimbus.

Cumulus clouds are among the most familiar. Small examples may be only 1 or 2 km wide, appearing as bright, rounded forms with relatively flat bases. Under unstable atmospheric conditions, however, they can develop into enormous cumulonimbus clouds. A mature storm cloud may reach 10-15 km in height and contain more than 500,000 tonnes of water. Inside it, powerful currents of air can travel upwards at speeds exceeding 100 km/h.

Clouds play an important role in Earth’s climate. Approximately 67% of the planet is covered by cloud at any given time, although the amount varies considerably between regions. Clouds reflect incoming solar radiation back into space while also absorbing and re-emitting heat from Earth’s surface. Their overall effect depends on factors including altitude, thickness, composition and time of day.

Temperature is particularly important in determining cloud structure. At 0°C, water can begin to freeze, although tiny droplets may remain liquid well below this point. At temperatures between 0°C and -40°C, clouds can contain a mixture of liquid droplets and ice. Below approximately -40°C, most cloud particles consist entirely of ice crystals.

Clouds are also central to the water cycle. Around 505,000 km3 of water falls as precipitation across Earth each year. Most returns directly to the oceans, while the remainder falls over land before eventually flowing through rivers, groundwater and ice back towards the sea.

Despite these enormous quantities, individual clouds are temporary. Some survive for less than 10 minutes; others form part of weather systems lasting several days and travelling thousands of kilometres. A cloud observed at 09:00 may have changed completely by 09:30, while a larger weather system can remain recognisable for 24, 48 or even 72 hours.

Clouds are constantly changing. Their shapes are influenced by temperature, humidity, atmospheric pressure, wind and landscape. Mountains can force air upwards by 1,000 m or more, producing clouds along ridges and peaks, while coastlines and cities create local conditions that affect their formation.

From a distance, a cloud can appear almost solid. Up close, it has no fixed surface at all. It is instead a changing concentration of microscopic particles: something measured in millimetres and kilometres, at temperatures from 20°C to -40°C and beyond, yet defined as much by movement as by size.