Orographic rainfall occurs due to the presence of mountains in the way. In fact, the rainfall is on the windward side of the mountains and is caused by the motion of a large-scale flow of moist air rising across the ridges caused by the mountains. This results in adiabatic cooling and condensation. Trade winds greatly influence the wetter climate found on the windward sides of mountains in some parts of the world. It is also the reason why the air on the leeward side of mountains remains dry and why the leeward sides come to be known as rain-shadow regions. The interiors of certain large mountainous zones are often dry. One of the major examples of this phenomenon is the Great Bain in North America. Orographic precipitation is a common phenomenon which occurs on oceanic islands. Hawaii is one such group of islands that has spells of orographic rainfall. Much of the rainfall received on the mountains on the Hawaiian Islands is on the windward side of the mountains, whereas the leeward sides remain dry.
