What is a rain shadow?
A rain shadow is a dry area on the leeward side of a mountainous area.
Wind and moist air is drawn by the prevailing winds towards the top of the mountains, where it condenses and precipitates before it crosses the top. The air without much moisture left, advances across the mountains creating a drier side called the rain shadow.
The condition exists because warm moist air rises by orographic lifting to the top of a mountain range. As atmospheric pressure decreases with increasing altitude the air expands and cools to its adiabatic dew point. This causes the moisture to condense and precipitate on the top and windward sides of the mountain. The dry air then descends on the leeward side of the mountain, which creates an arid region.
For example:- San Jose, California and adjacent cities are usually drier than the rest of San Fransisco Bay Area because of the rain shadow cast by the highest part of the Santa Cruz mountains.