In meteorology, leeward and windward are technical names for the directional sides of a mountain. The windward side is that side which faces the prevailing wind (upwind), whereas the leeward, or "lee" side, is the side sheltered from the wind by the mountain's very elevation (downwind).
Windward and leeward aren't just arbitrary terms, they are important weather and climate factors. One is responsible for enhancing precipitation in the vicinity of mountain ranges, and the other, for withholding it.
Windward Mountain Slopes Give Air (and Precipitation) a Boost
Mountain ranges acts as barriers to the flow of air across the surface of the earth. When a parcel of warm air travels from a low valley region to the foothills of a mountain range, it is forced to rise along the slope of the mountain as it encounters higher terrain. As the air is lifted up the mountain slope, it cools as it rises (a process known as adiabatic cooling). This cooling often results in the formation of clouds, and eventually, precipitation which falls on the windward slope and at the summit. Known as orographic lifting, this event is one of three ways precipitation can form (the other two are frontal wedging and convection).
The Northwestern United States and the Front Range Foothills of Northern Colorado are two examples of regions that regularly see precipitation induced by orographic lift.
Leeward Mountain Slopes Encourage Warm, Dry Climates
Opposite from the windward side is the lee side -- the side sheltered from the prevailing wind. (Because prevailing winds in the mid-latitudes blow from the west, the lee side can generally be thought of as the eastern side of the mountain range. This is true most times -- but not always.)
In contrast to the windward side of a mountain which is moist, the leeward side typically has a dry, warm climate. This is because by the time air rises up the windward side and reaches the summit, it has already stripped of the majority of its moisture. As this already dry air descends down the lee, it warms and expands (a process known as adiabatic warming), which causes clouds to dissipate and further reduces the possibility of precipitation. This occurrence is known as the rain shadow effect. It is the reason why locations at the base of a mountain lee tend to be some of the driest places on Earth. The Mojave Desert and California's Death Valley are two such rain shadow deserts.
Downslope winds (winds that blow down the lee side of mountains) not only carry low relative humidity, they also rush down at extremely strong speeds and can bring temperatures as much as 50+ degrees Fahrenheit warmer than the surrounding air. Katabatic winds, foehns, and chinooks are all examples of such winds. The Santa Ana Winds in Southern California are a well-known katabatic wind infamous for the hot, dry weather they bring in autumn and for fanning regional wildfires.