The lee, short for leeward, is the backside, opposite the windward side of an object. The term lee-side and “rotor zone” may be used interchangeably. Both terms refer to areas behind an object which is obstructing the wind flow. As the speed of the wind and roughness of the obstruction increases, so does the amount and intensity of the turbulence. If the wind speed is relatively low and/or the terrain is smooth or rounded, the level of turbulence will be relatively low. Conversely, if the winds are high and/or the terrain is rough, such as a sharp ridge or mountain, the level of turbulence in the lee will be high. You may hear pilots talk about flying in the lee, but you should understand the serious risk involved before attempting it yourself. Thermals in the lee side are protected from the wind and have a chance to build, but when they do release, they are pushing up into an air mass that is moving horizontally, which may create severe turbulence and shear. Also, if you fly into this area looking for lee-side thermals, you are flying into an area of mechanical turbulence.
Rotor is the reason it’s imperative for you to know the direction of the winds around you, as well as the wind at the surface. Knowing the direction and speed of the forecasted winds will keep you from launching into lee-side conditions, or flying into rotor at some point during your flight. When flying in the mountains, be especially cautious, around ridges and valleys, the opportunities for encountering rotor are greatly increased. It is easy to get wind forecasts from weather sources and/or by watching the drift of the clouds, so there is no excuse for being unaware.