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Which effect does the foehn have on humidity?

  • A.The relative humidity rises because the descending air is compressed and the vapour becomes concentrated
  • B.There is no change
  • C.The relative humidity remains at saturation throughout the descent, so cloud persists on the lee slope
  • D.The relative humidity falls sharply

Answer: D

Having lost much of its water as precipitation on the windward side, and warming at the dry adiabatic rate on descent, the air arrives with a much reduced relative humidity. Falls from 80 per cent to 20 per cent within an hour are recorded. The dryness is as characteristic of the foehn as the warmth and contributes to the wildfire risk associated with it. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

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