Clouds

Formation and sources

Clouds begin when air containing water vapor cools to its dew point and condensation occurs on aerosol particles such as dust, sea salt, or pollen.

Moisture that feeds clouds comes from evaporation at the surface and from transpiration by plants; together these processes are often referred to as evapotranspiration.

Air is lifted and cooled by a variety of mechanisms—convection from surface heating, frontal lifting when air masses collide, orographic lifting over terrain, and large-scale convergence in weather systems—and each mechanism tends to produce different cloud forms and vertical structures.

Radiative cooling of the surface at night can also lead to low cloud formation, while local moisture sources such as lakes or irrigated fields can seed cloud development nearby.

Classification and how clouds look

Meteorologists classify clouds primarily by their altitude and their basic form. Broad altitude groups are high, middle, and low clouds, plus a separate category for clouds with strong vertical development:

• High clouds: cirrus, cirrostratus, and cirrocumulus; these are mainly composed of ice crystals and appear thin or wispy.
• Middle clouds: altostratus and altocumulus; these sit between the high and low layers and often signal changing weather.
• Low clouds: stratus, stratocumulus, and nimbostratus; these are dominated by water droplets and commonly produce overcast skies and light to moderate precipitation.
• Clouds with vertical development: cumulus and cumulonimbus; these range from fair-weather puffiness to towering thunderstorm anvils that produce heavy rain, lightning, and strong winds.

Cloud names can be combined to describe mixed characteristics (for example, nimbostratus for thick, rain-bearing stratiform clouds), and appearance varies with illumination, background sky, and moisture content.

Cloud microphysics and precipitation processes

Within a cloud, tiny droplets and ice crystals interact constantly. Droplets grow by condensation and by colliding and coalescing with other droplets; in warm clouds, this collision-coalescence process can produce raindrops.

In mixed-phase clouds where ice and supercooled water coexist, the Bergeron process causes ice crystals to grow at the expense of water droplets, often leading to snow or ice that then melts into rain below warmer layers.

Aerosol concentration, droplet size distribution, and the presence of ice nuclei all influence whether a cloud produces drizzle, steady rain, snow, or no precipitation at all. Temperature profile, vertical motions, and cloud thickness determine the dominant microphysical pathway.

Life cycle and dynamics

Clouds have a life cycle: initiation, growth, maturity, and dissipation. Convective clouds depend on buoyant updrafts; if updrafts are strong and sustained, clouds build vertically and may develop precipitation and turbulence.

Entrainment, the mixing of surrounding drier air into a cloud, can limit growth and encourage evaporation and decay.

Frontal and stratiform cloud decks are more dependent on large-scale lift and moisture convergence and often persist longer over a region. Local factors, such as surface heating, topography, and humidity, shape how long clouds live and how they evolve.

Role in weather and climate

Clouds are central to both short-term weather and long-term climate.

By reflecting sunlight, clouds increase planetary albedo and exert a cooling influence; by absorbing and re-radiating infrared energy from the surface, they exert a warming influence.

Whether a particular cloud layer has a net warming or cooling effect depends on its altitude, thickness, particle phase, and time of day. Low, thick clouds tend to cool the surface by reflecting sunlight, while high, thin clouds tend to warm by trapping outgoing radiation.

Because cloud feedbacks amplify or dampen temperature changes, they are a major source of uncertainty in climate projections.

Observation and measurement

Clouds are observed at many scales and with many instruments.

Trained observers describe cloud type and coverage from the ground; ceilometers estimate cloud base height; radiosondes measure vertical profiles of temperature and humidity that indicate where clouds can form; weather radar detects hydrometeors and precipitation structure; and satellites provide broad coverage with visible, infrared, and microwave channels that reveal cloud top height, phase, and movement.

Lidar and advanced remote sensors can profile thin clouds and aerosol layers that are hard to see with conventional instruments.

Numerical weather models use these observations to initialize forecasts and to represent cloud processes, although parameterizing clouds at model grid scales remains challenging.

Practical impacts and applications

Clouds affect many human activities and industries.

In aviation they influence visibility, turbulence, and icing risk; for solar energy they modulate photovoltaic output and forecast uncertainty; in agriculture clouds alter evapotranspiration, shading, and the timing and amount of rainfall; for water resource management cloud-driven precipitation patterns determine reservoir inflows and flood risk.

Cloud observations are also critical for air quality assessments, outdoor event planning, and emergency response during severe storms.