Convective Cloud

This is the most common kind of cloud inland and near the equator, and results from thermal convection which is either shallow or deep. Shallow convection results from daytime heating of the ground's surface and produces non-precipitating 'fair-weather cumulus' clouds (Section 8.6). Cloud base is at the connective condensation level (CCL), shown in Figure 8.1. Cloud forms at this height when the surface is at the convection temperature Tc, represented by K in Figure 8.1 (Note 8.B).

'Deep convection' reaches through most of the troposphere and results in thunderstorms (Chapter 9). It is initiated by several factors, including local surface heating. But the trigger might be daytime 'anabatic flow' up mountains, or uplift caused by sea breezes near the coast (Chapter 14), both of which lead to most thunderstorms in the afternoon. As an example, the sky is normally clear at night on Mt Wilhelm (which reaches 3,480 m in Papua New Guinea), but clouds begin to form at about 2,000 m around 8 a.m., and then they grow until there are intermittent showers from 11 a.m. until sunset. However, thunderstorms are more common during the night over the waters of tropical archipelagos like Indonesia, because of uplift started by low-level convergence of nocturnal land breezes (Chapter 14) from nearby islands.

Whatever cloud is formed, an immense amount of latent energy is released as the water vapour condenses. The condensation that provides a modest shower of 6 mm over an area of 10 km2 releases 150 million megajoules, which is as much as is produced in a whole day by Australia's 2,000-megawatt Snowy Mountain hydro-electric scheme, and about equal to the energy in the two atom-bomb explosions at Hiroshima and Nagasaki together.

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