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Severe turbulence can kill aircraft passengers. Now, in test flights over the Rocky Mountains, NASA engineers have successfully detected clear-air turbulence up to 10 seconds before an aircraft hits it. However, passengers can avoid serious injury by fastening their seatbelts. It is the only antidote for this sort of thing, says Rod Bogue, project manager at NASAs The centres new turbulence detector is based on lidar, or laser, radar. Laser pulses are sent ahead of the plane and these are then reflected back by particles in the air. The technique depends on the Doppler Effect. The wavelength of the light shifts according to the speed at which the particles are approaching. In calm air, the speed equals the planes airspeed. But as the particles swirl in rough air, their speed of approach increases or decreases rapidly. The rate of change in speed corresponds to the severity of the turbulence. Bogue says that he had a comfortable amount of time to fasten his seatbelt. The researchers are planning to improve the lidars range with a more powerful beam. The system could be installed on commercial aircraft in the next few years. A A not very rough storm. C A kind of visible storm. 37 In December 1997, a United Airlines flight hit unexpected rough air B throwing its passengers out of the cabin. D forcing the pilot to make an emergency landing. A the speed of the plane. C the number of particles in the air. 39 We can infer from the fifth paragraph that B researchers are not sure about the effectiveness of the lidar detector. D no more test flights are needed. A the lidar detector needs improvement. C passengers often forget to fasten their seatbelts. 【参考答案】36. B37. C38. D39. A40. A | 
