The nature of convection as revealed by soaring birds and dragonflies

Birds and dragonflies can be used as indicators of the extent and intensity of vertical currents, particularly of the nature of convection below cloud base during its period of growth. The bubble theory of Scorer and Ludlam is used to interpret the observations of Hankin in India. The possibility of deriving information from the flight of swallows in Britain is also discussed. In hot sunshine and no wind the air appears to stream steadily upwards from fixed places for the first few metres. The birds indicate that convection develops upwards, the heavier ones which soar at greater heights taking to the air last. The upcurrents are stronger and wider at greater height and drift with the wind; they react more slowly at greater height to changes of sunshine on the ground. When convection is intense the upcurrents appear more sharp‐edged. Puffs of wind are associated with the descent of air from above the superadiabatic layer to replace a part of that layer that is ascending as a newly‐created bubble. When the air is stirred mechanically by wind, bubble formation in the lowest layers is reduced. It is suggested that air movements on the scale of ordinary convection are an element in the climate as important as any other in the life of many birds.