These are 'cotton-wool' clouds occurring in a clear sky in the daytime, each fluffy cloud sitting on top of an invisible thermal. Such cumulus is formed at the convection condensation level (Figure 8.1), generally between 500 m (over the sea) and 4 km (over dry land). A fair-weather cumulus cloud appears more substantial than altocumulus or altostratus, because it is lower and therefore warmer, so that saturated air contains more water (Table 8.3).
Fair-weather cumulus clouds usually arise in calm conditions, and then are temporarily anchored to the warm spots generating their thermals. The number and size of the clouds increase in the course of two or three hours, until further solar heating of the ground is prevented by the clouds themselves—another case of negative feedback. Individual clouds generally last 10-30 minutes, persisting only if continually nourished by strong daytime thermals. They disappear by subsidence and evaporation, by mixing with the surrounding dry air, or by spreading out to form stratocumulus.
Sometimes the thermal needed to create a fair-weather cumulus cloud is generated by a surface fire. A large fire in a timber yard in Melbourne in 1976 resulted in a cumulus with cloud base at 1.6 km and cloud top at 3.5 km. Heat from a power-station can work the same way.
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