From SKYbrary Wiki
The process by which a mass of air is lifted by a geographical feature such as a line of hills or a mountain range.
When a moving Air Mass meets a geographical feature such as a mountain, some of the air will have no option but to rise up over it. This process is known as Orographic lift. If the rising air is moist, clouds will form.
Windward slopes of a mountain range will therefore tend to have higher rainfall than the leeward side slopes of the mountain range.