(A) Instead of moving along a straight line, the jet stream flows in the wavelike fashion; the waves propagate eastward (in the Northern Hemisphere) at speeds considerably slower than the wind speed itself.

(B) Jetstream, narrow, swift currents or tubes of air found at heights ranging from 7 to 8 miles (11.3-12.9 km) above the surface of the earth.

(C) They are caused by great temperature differences between adjacent air masses.

(D) Since the progress of an airplane is aided or impeded depending on whether tailwinds or headwinds are encountered, in the Northern Hemisphere the jet stream is sought by eastbound aircraft, in order to gain the speed and save fuel, and avoided by westbound aircraft.