Question: You’re flying in a light single-engine airplane and about to fly over a mountain range. The winds aloft are blowing perpendicular to the mountain range. Which side of the mountain range is more hazardous: the upwind side or the leeward side?
Answer: The leeward side presents the greater hazard due to the possibility of downdrafts that may exceed the climb capability of the aircraft. (Source: Aeronautical Information Manual 7-5-6)