Cloud is created almost immediately an atmosphere is cooled to its dewpoint temperature (Note 8.D). The droplets form by condensation on motes in the air, called cloud condensation nuclei (CCN). Without these, the air's relative humidity would need to be 110 per cent or more before droplets could form by the spontaneous collisions of water-vapour molecules (Section 4.3). But there are generally ample CCN in each cubic metre, even in clean air.
They are classified according to size—Aitken nuclei are less than 0.2 pm in diameter, so-called large nuclei are 0.2-2 pm, and giant nuclei are larger still. A cubic metre of maritime air reaching Cape Grim in northern Tasmania in summer typically contains 300 million particles larger than 0.01 pm, 100 million over 0.1 pm, and just 2 million giant nuclei. Such air contains many fewer CCN than continental air, so the droplets in marine clouds are fewer and larger.
The relatively clean air from off the ocean contains nuclei consisting mostly of sulphate crystals formed from dimethyl sulphide, a gas evolved by phytoplankton, which are minute organisms in the sea. (Extra sulphide is produced in a warmer ocean, resulting in more, smaller cloud droplets. This would increase cloud albedo and thereby perhaps reduce the initial warming—another possible negative feedback process, see Note 7.A). CCN above cities are mostly ammonium-sulphate particles, produced by sulphur dioxide and ammonia from air pollution reacting within cloud droplets. Other CCN are formed biologically.
Large nuclei are the most effective in creating cloud droplets (Note 8.D). Droplets form exclusively on the large nuclei if cooling of the humid air is gradual, so that there are bigger but relatively few droplets. Sea-salt particles are notably suitable. Even small nuclei are adequate if moist air is cooled rapidly, as in the strong updraughts of convective clouds (Table 8.1). This produces a dense white cloud of numerous, tiny droplets, with consequent poor visibility.
Cloud droplets are typically around 10 pm in diameter, so tiny that they float in the air, the fall speed being less than 1 mm/s. The space between them is likely to be about 200 times their diameter, so they amount to much less than the water vapour within the cloud's volume (Note 8.E). Nevertheless, the huge volume of a large cloud means that it may contain thousands of tonnes of liquid water.
The droplets are no more than a millionth of the mass of typical raindrops and the change from one to the other is not straightforward. The formation of raindrops usually requires particles called ice nuclei, which are quite different from cloud-condensation nuclei. Such raindrop nuclei are not always adequately available, unlike CCN. As a result, even slightly supersaturated air always produces clouds, but most clouds do not yield rain (Chapter 9).
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