Field Observations of Thermals and Thermal Streets, and the Theory of Cross-Country Soaring Flight

The classical theory of cross-country soaring flight postulates climbs in discrete thermals, separated by glides through stationary air, and prescribes an speed for the inter-thermal glides, based on the performance of the glider or bird, and the rate of climb in thermals. However, it is known that glider pilots usually fly slower than the theoretical optimum speed between thermals, and often achieve faster cross-country speeds than the maximum predicted by the theory. The reasons for this are analysed in relation to a survey of real thermals over the Serengeti National Park, Tanzania. About 1500 km was flown in straight lines, in a Schleicher ASK-14 motor glider, measuring the airmass lift (vertical velocity of the air) at 15-s intervals. It is shown how the bird or pilot can increase the mean rate of climb in the sample of thermals that are actually used, and so increase the cross-country speed, by selecting an inter-thermal speed below the supposed optimum. If the air between thermals is not stationary, the time lost in circling can be further reduced by variations of speed during the inter-thermal glides. Eventually, if thermals are organised into lines or streets, circling can be eliminated altogether, and linear soaring becomes possible. Linear soaring is also sometimes possible along ridges and escarpments. Soaring in discrete thermals may be seen as one end of a spectrum of soaring techniques, with linear soaring at the other end, giving much higher cross-country speeds. In straight flight, soaring and flapping are end members of another spectrum, connected by various intermediate techniques. A bird migrating by flapping flight in a straight line can reduce its power requirements by flying through thermals or other forms of rising air.