The motion in and around isolated thermals

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Abstract
Laboratory measurements show that the motion of an isolated ‘thermal’ is like that of a vortex ring. The centre is rising at slightly more than twice the rate of the thermal cap and on the edges the buoyant fluid is descending, relative to a stationary observer. As the thermal rises, mixing with outside air takes place, approximately 60 per cent at the front and 40 per cent at the rear, and the thermal traverses a cone of half angle about 15°. Heavy particles, with a terminal velocity of less than 0·4 Wc, where Wc is the rate of rise of the cap, will not escape from the thermal; whilst those with a terminal velocity exceeding somewhat less than 1·6 Wc cannot be retained in it. It is argued that the isolated thermal is the ultimate form of all convective elements in the atmosphere and that this ultimate form is generally reached between 200 and 2,000 metres from the ground.

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