Mark J. Barrett/Alamy
At any one time, how much of Earth is covered by cloud? Does this vary?
Vittoria Dessi, Zuoz, Switzerland
Cloud cover locally can easily be measured by observation from a single location on the ground. It is quantified in oktas, or the number of eighths of the sky covered by visible cloud of any type.
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However, on a larger scale, the distribution of cloud across Earth can be measured better from space, by beaming pulses of either infrared or visible light waves down and analysing the reflection of pulses coming from the upper surface of a cloud layer.
Earth’s cloud coverage has been measured by several NASA satellites, including the Ice, Cloud, and land Elevation Satellite (ICEsat), which has provided the most accurate figure to date. ICEsat’s measurements show that, on average, 70 per cent of the world is covered by cloud at any time.
This figure can vary by up to 30 per cent from day to day depending on weather conditions. For example, at higher temperatures there would be more evaporation and an increase in the moisture in the air that can condense into clouds. Annually, cloud cover varies by only a few per cent on average across the year, since the hydrological cycle is a closed system.
Brian Brown, Dronfield, Derbyshire, UK
NASA’s International Satellite Cloud Climatology Project (ISCCP) has been gathering information on global cloud coverage at 3-hourly intervals for 38 years.
As part of the project, NASA has collected data from several geostationary and polar orbiting satellites. This shows the average percentage global cloud cover at any time is about 65 per cent.
The average amount of cloud cover is slightly different in the northern and southern hemispheres, possibly because there is more land in the northern hemisphere. There are also seasonal changes in cloud cover and some more long-term changes. For example, over 23 years from 1983, the average percentage cover of low cloud fell from about 12 to 10 per cent. The reason for this isn’t clear.
The ISCCP data on total cloud cover includes clouds at all levels. But the percentage coverage of low cloud is much less, at about 11 per cent. Low cloud coverage is also much more variable than total cloud cover.
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