Climatology of clouds

The tiny droplets and crystals which make up clouds give rise to precipitation – rain, hail and snow – thus bringing the wheel full circle. Here again, satellites have proved to be very useful for observing this part of the cycle in the atmosphere, and especially for estimating the evaporation of water at the ocean surface. This is done indirectly from measurements of humidity, surface temperatures and winds, in particular by combining several passive microwave channels and measurements by infrared radiometers on meteorological satellites such as the AVHRR on board NOAA satellites, and SSM/I sensors on US defence satellites. The map of evaporation obtained in this way underlines the role of the trade wind regions, located in the vicinity of the Tropics of Cancer and Capricorn, as a source of humidity for the atmosphere.

This water vapour builds up in the equatorial regions where rain clouds are very common.

The arrival of Earth observation satellites has radically changed our physical view of clouds. Satellites have revealed structures and other masses of nebulosity that were hitherto unknown. It was in 1983, very early on in the short history of climate observation by satellite that the International Satellite Cloud Climatology Project (ISCCP) was set up.

This consisted in archiving data from meteorological satellites, standardising them, and processing them in order to construct a climatology of clouds for the fi rst time. Terms such as cirrus flocus, cumulus humilis, congestus and cumulonimbus were no longer used: clouds were classifi ed as being high, low or medium level, thin or thick. Instead of talking about oktas (the standard units for measurements of cloud cover by human observers) the term 'percentage cloud cover' was used. The current observation system is operational, and cloudiness is measured systematically. Every last detail of clouds is observed – altitude, temperature, phase (water or ice) – mainly using passive instruments in visible and infrared wavelengths.

However, the size of the droplets and crystals that make up clouds are also probed with active instruments (lidars* and radars such as those on the Aqua-Train.