ROLE OF CONVECTION When the surface of the ground is heated by sunshine the lowest layer of air may be heated in turn. If a large temperature lapse-rate is established, the atmosphere becomes unstable, because the less dense ground-layer of air tends to rise and carry its load of microbes upwards, being replaced by cooler air from above. The pattern of this overturn is not yet clear. A regular `cellular' pattern of ascending and descending air has been suggested, but more recently the ascending air has been pictured as taking the form of `chimneys' or of `bubbles'.
Glider pilots are familiar with the properties of warm ascending currents of air or `thermals', as described by Yates (1953). In still air a glider sinks at about 90 cm. per sec. (about 20 times the terminal velocity of a pine pollen grain). On dull days thermals do not develop. They reach their maximum upward velocity of 3 metres per sec., or up to 25 metres per sec. in cloud, at midday in summer. Yates indicates that, depending on the type of soil, on wind strength, and on sun height, a sizeable thermal is released from an area of P25 sq. km. every 5 to 15 minutes in summer. At a height of 30o metres, thermals may be 30o metres in diameter, though they are probably often smaller when lower, whereas at still greater heights a diameter nearer 2 km. was reported by Ludlam & Scorer (1953). Their vertical movement may cease at a temperature inversion, or may continue to from Io,000 to 50,000 ft. The temperature in a thermal appears to average e-2°C. higher than the surrounding air through which it is ascending.
The theory of thermals is still a matter of controversy, but there seems no doubt that air rises from some surfaces more often than from others. Green vegetation and wet soils may be relatively cool, while a ripe cereal crop, buildings, roads, or bare rock, may heat up rapidly in the sun and become the source of rising warm air. Thermals can also arise at a cold front, and glider pilots regard hilly country as the best source of thermals.
The pattern by which cool air sinks to replace the ascending warm air is also a matter of debate. Downward draughts reported in the neigh bourhood of thermal upcurrents appear to be comparatively feeble. Probably the downward movement occurs over a much wider area than 28 the thermal, as a slow sinking of the atmosphere. The sinking speed may be comparable with that of a fungus spore (Hirst, 1959), but the local rising velocity may commonly be Ioo or more times this velocity. Some bird species soar in large thermals, as do dragonflies in smaller ones near the ground. Other birds haunt thermals to prey on the insects carried upwards (Scorer, 1954).