Planets: A Very Short Introduction (Very Short Introductions) by David A. Rothery

Author:David A. Rothery [Rothery, David A.]
Language: eng
Format: mobi
Publisher: Oxford University Press
Published: 2010-11-24T16:00:00+00:00

Clouds

Clouds are highly reflective, so the cloudier an atmosphere, the more solar energy is reflected directly back into space. However, a cloudy sky increases the ability of an atmosphere to trap heat from the sunlight that does reach the ground, so the effect of clouds on global temperature is complex. The unbroken clouds of Venus have not saved its surface from being thoroughly cooked by the greenhouse effect.

Clouds form when the temperature and pressure make it favourable for some constituent of the atmosphere to condense as liquid droplets or ice particles. In the case of the terrestrial planets, the relevant constituent is usually water. Although water is only a small fraction of Venus’s atmosphere, there is enough to form a continuous layer of cloud at the top of its troposphere, between about 45 and 65 kilometres above the surface. There, water vapour condenses as droplets about 2 micrometres across. These remain suspended, being too small to fall, and are described as aerosol droplets. Atmospheric sulfur dioxide dissolves in them, so they turn into sulfuric acid. However, if anyone tries to tell you that it ‘rains sulfuric acid on Venus’, they are wrong. Wherever the droplets are drawn down below about 45 kilometres by atmospheric circulation, the heat causes them to evaporate again, and they never have the chance to become raindrops large enough to fall groundwards.

Above about 6 kilometres, Earth’s clouds consist mostly of tiny ice particles, and below that altitude they are mostly droplets of water. Rainclouds are not really grey, they just look that way because they are thick enough to blot out so much light. On Mars, clouds are comparatively rare. In most of its troposphere, they are made of water-ice, whereas around 80 kilometres, near the troposphere/mesosphere boundary, clouds of carbon dioxide particles have been observed.

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