Clouds are visible or otherwise detectable collections of tiny liquid water droplets, ice crystals, or mixtures of both suspended within the atmosphere. They form when moist air reaches conditions in which water vapour becomes supersaturated relative to available condensation or ice forming surfaces, allowing water to condense or deposit into particles large enough to remain suspended for significant periods.
Cloud particles are much larger than individual water molecules but generally remain small enough that their terminal fall speeds are low and atmospheric turbulence can keep them aloft. Clouds occur at many altitudes and temperatures and vary greatly in structure, optical appearance, composition, lifetime, and physical processes. They are fundamental components of weather and climate because they participate in precipitation formation, reflect incoming solar radiation, absorb and emit terrestrial infrared radiation, transport latent heat, and redistribute water through the atmosphere.
Cloud formation usually requires cooling or moistening of air until saturation is reached. Rising air often cools because pressure decreases with altitude and the expanding air performs work, reducing its temperature. If the air cools sufficiently, relative humidity approaches saturation and water vapour can condense onto aerosol particles that act as cloud condensation nuclei. Ice formation can occur through several pathways, including deposition of water vapour onto ice nuclei, freezing of supercooled droplets, and interactions between ice and liquid particles.
Cloud formation can therefore depend on aerosols, temperature, humidity, vertical motion, atmospheric stability, and microphysical processes. Saturation alone does not guarantee immediate cloud formation because nucleation can have kinetic barriers and depends on the availability and properties of suitable particles. Clouds consequently represent a complex interaction between atmospheric thermodynamics and microscopic particle physics.
Cloud droplets and ice crystals undergo continual microphysical evolution. Condensation increases droplet size when water vapour deposits onto liquid particles, while evaporation can shrink droplets when surrounding air becomes subsaturated. Collision and coalescence allow droplets to merge, particularly in warm clouds, while ice particles can grow through deposition, aggregation, riming, and interactions with supercooled droplets.
The Bergeron Wegener process can promote precipitation formation in mixed phase clouds because the equilibrium vapour pressure over ice is lower than that over supercooled liquid water under relevant conditions, allowing ice crystals to grow at the expense of liquid droplets. Once particles become sufficiently large, gravity causes them to fall as precipitation or virga if they evaporate before reaching the surface. These processes determine cloud lifetime, precipitation efficiency, particle size distributions, and radiative properties.
Clouds are classified according to their appearance, vertical extent, and altitude. Common morphological groupings include cirrus, cumulus, stratus, and nimbus related forms, while standard cloud nomenclature combines these elements to describe recognised genera and varieties. High clouds are generally composed largely of ice because temperatures are low, while lower clouds may consist predominantly of liquid droplets or mixed phase particles depending on temperature.
Towering clouds can extend through several atmospheric levels and may contain strong updrafts, downdrafts, ice, liquid water, lightning, and heavy precipitation. Layered clouds can cover large areas with relatively uniform bases, while convective clouds develop vertically in unstable environments. Cloud classification provides a standard descriptive language, but visual appearance alone does not reveal all microphysical or dynamical properties. Remote sensing instruments and in situ measurements are therefore used to determine cloud phase, thickness, temperature, particle characteristics, and vertical structure.
Clouds have competing effects on Earth's radiation balance. Cloud tops generally reflect a significant fraction of incoming solar radiation back to space, producing a cooling influence, while clouds also absorb and emit infrared radiation and can reduce the rate at which heat escapes from the lower atmosphere to space. The net effect depends on cloud altitude, optical thickness, particle properties, coverage, surface reflectivity, and atmospheric conditions.
Low thick clouds often have strong shortwave reflection, whereas high thin ice clouds can allow substantial solar radiation through while still affecting outgoing infrared radiation. Cloud feedbacks are consequently among the important complexities in understanding climate sensitivity. Cloud processes operate on scales much smaller than many climate model grid cells, requiring parameterisation or increasingly detailed modelling approaches.
Clouds also influence atmospheric dynamics because condensation releases latent heat. When rising moist air condenses, latent heat is released into the surrounding air, reducing the rate at which the parcel cools with altitude and potentially increasing buoyancy. This process can strengthen convection and contribute to the development of deep clouds and thunderstorms. Cloud formation also redistributes water between vapour, liquid, and ice phases and can alter atmospheric stability.
Precipitation removes water and releases latent heat at different locations, coupling cloud processes to larger circulation systems. Clouds can therefore both respond to atmospheric motion and modify that motion, creating feedbacks between microphysical processes and large scale weather. The interaction is especially important in tropical convection, frontal systems, and severe thunderstorms.
Cloud observation uses ground based observations, weather radar, aircraft measurements, satellites, lidar, radiometers, and other instruments. Visible satellite imagery can reveal cloud distribution and structure, while infrared observations provide information about cloud top temperature and allow observation at night. Microwave measurements can provide information about cloud water and precipitation under appropriate conditions. Radar can detect hydrometeors within clouds and precipitation systems, while lidar is particularly useful for thin clouds and aerosols.
Aircraft instruments can directly measure droplet and ice particle properties, temperature, humidity, and turbulence. Because each instrument responds to different physical properties, combining observations is often necessary to obtain a reliable picture of cloud structure. Remote sensing retrievals also depend on assumptions and algorithms that introduce uncertainty, especially when clouds contain mixed phases or complex overlapping layers.
Common misconceptions include the idea that clouds are made only of water vapour or that a visible cloud necessarily produces precipitation. Water vapour is invisible, whereas clouds consist of condensed or deposited particles that scatter or interact strongly with light. Many clouds contain droplets or ice particles that remain too small to fall efficiently to the surface. Clouds can also evaporate or dissipate when air becomes warmer, drier, or dynamically subsident.
Their scientific importance extends far beyond appearance because clouds connect water cycling, atmospheric chemistry, radiation, convection, precipitation, and climate. Understanding clouds remains difficult because their microscopic processes interact with atmospheric dynamics over scales ranging from individual particles to planetary circulation. This scale interaction is one reason cloud representation remains a major challenge in numerical weather prediction and climate modelling.
