The windward side is the side of the mountain that has the lower temperatures. This is due to the much higher precipitation that the windward side experiences, and that it has much more cloudy days, while the leeward side is much drier and has much more sunny days.
The mountains are a natural barrier for the air masses, and the air masses tend to accumulate on the windward side. As they accumulate, the windward side becomes cloudy, and very often it receives all of the rainfall, and that makes it cooler than the opposite leeward side.
