A thermal is a column of warm-air bubbles, expanding as they rise, each with a central updraught and peripheral subsidence. Thermals commonly occur on warm, calm afternoons above relatively hot patches of the ground (such as a dry field of low albedo amongst irrigated fields, or a slope facing the Sun), forming huge bubbles of warm air, lighter than the air around. They eventually lift off, entrain more air and join other bubbles. Within a couple of hundred metres they form a continuous column, the diameter of which is then around a quarter of its height (Figure 7.7). The thermal extends upwards to the top of the PBL at 1,000 m or beyond, usually slanting in the wind and moving slowly along if the ground is uniform. Their upwards growth is usually limited by the stable layer at the top of the PBL but thermals in a hot desert may extend to 4 km, and may reach the Lifting Condensation Level, so that a cumulus cloud forms. The distance to the next thermal is about twice its height.
Locusts may be drawn together into the base of a thermal and lifted up. Then they drift down over a great distance, extending the damage they cause.
Figure 7.7 Typical conditions associated with a thermal. The dimensions are only indicative; the vertical dimension is exaggerated.
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