Climate and clouds
List of content
General
Clouds and atmospheric water vapour
Cloud albedo
Cloud formation
Cloud types
Cloud clover effects on climate
Cloud cover change observed
Total cloud cover versus global surface air temperature
Cloud data
Cloud cover change observed for different cloud types
Low cloud cover versus global surface air temperature
Tropical cloud cover and global air temperature
Clouds, evaporation and climate
General
A cloud is a visible mass of condensed droplets or frozen crystals floating in the atmosphere above the surface of the Earth or another planetary body. On Earth the condensing substance is typically water vapour (see diagram below), which forms small droplets or ice crystals, typically 0.01 mm in diameter. When surrounded by billions of other droplets or crystals they become visible as clouds. Clouds form when the dewpoint of water is reached in the presence of condensation nuclei in the troposphere, the lowermost 10-15 km of the atmosphere. Dense deep clouds exhibit a high reflectance (70% to 95%) throughout the visible range of wavelengths: they thus appear white when seen from above. Cloud droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with depth into the cloud. Depending on their thickness and the density of water droplets, clouds may therefore appear grey or dark when seen from below.
Cloud condensation nuclei or CCNs are small solid or liquid particles (typically 0.0002 mm) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. When no CCNs are present, water vapour can be supercooled below 0°C (32 °F) before droplets spontaneously form (this is the basis of the cloud chamber for detecting subatomic particles).
There are many different types of atmospheric particulates that can act as CCN. The particles may be composed of dust or clay, soot or black carbon from grassland or forest fires, sea salt from ocean wave spray, soot from fires, sulphate from volcanic activity, phytoplankton, or by nuclei formed by cosmic radiation. The ability of these different types of particles to form cloud droplets varies according to their size and also their exact composition.
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