TOEFL Reading: ETS-TOEFL阅读机经 - 307T3JZ766I1DGD5L$

main idea. A. Birds can soar by finding an updraft, which is created either by air rising from ground that is warmer than the surrounding ground or by wind blowing up a hill or over a large obstacle. B. Soaring is possible in updrafts in which the air's upward speed is greater than the bird's sinking speed. C. Birds flying over a flat meadow often need to flap their wings in order to stay at the same altitude because thermal chimneys and ring thermals typically do not form in such areas. D. Birds can travel in an energy-efficient way by soaring upward in thermals and then leaving the thermal and gliding to lower altitudes. E. Birds that need to fly over long distances generally do not use thermals because they limit the distance the birds can soar. F. Thermals that form as a result of ridges have predictable speeds that depend on the incline of the particular slope and the climate of the local area.