The rain shadow effect refers to less precipitation falling on the lee side of a mountain range. A mountain range acts as a barrier to moisture moving past the mountain range (especially low level moisture). The windward side of the mountain range is often closer to the ocean. A couple of exceptions to this are when different wind directions change which side of the mountain range is the windward and leeward side and when the moisture typically blows into a mountain range after being transported far inland over a continent.

The prevailing wind is the most common wind direction during the course of the year. Global wind patterns produce a favored wind direction for a location. This prevailing wind direction can vary seasonally or only vary some over the course of a year. The prevailing wind direction in the vicinity of a mountain range will determine which side is typically the windward side and which is the lee side.

As moisture moves into a mountain range, it will be force lifted into the vertical. This vertical lifting can condense out a significant amount of moisture. Thus, the windward side of the mountain range is wet. As air flows over the mountain range it has cooled and lost much of its moisture. The air then can sink on the leeside which decreases the relative humidity of the air and warms the air. The leeside is typically drier, warmer and sunnier. This dry and sunny weather on the leeside of a mountain range is what is termed the rain shadow effect, which is shown on the diagram below.