Mountains, valleys and local topography affect the movement of air, precipitation and temperature. This results in areas that are wetter, drier or warmer than surrounding flatlands.
Mountains are natural barriers to the movement of wind. They are colder than surrounding flatlands because the temperature decreases with elevation. As a result, there may be a tropical climate at the bottom of the mountain and snow on top. Mountains are wetter on the windward side because of this temperature differential. Winds carrying moist air rise when they reach the mountain and cool as they rise higher. Cold air cannot hold as much water as warm air, and precipitation is the usual result. This is why it rains so much in Seattle. However, the leeward side of the mountain tends to be drier because the wind loses all of its moisture on the windward side, and the air compresses and warms as it works its way down the mountain.
Valleys tend to be warmer than surrounding flatlands. Death Valley, California, for example, is the hottest and driest place in the United States. It is dry because dry air descends from the Sierra Nevada mountains and into the valley. As it descends down into the valley, it warms and settles there. Local topography near large bodies of water affect the temperature of air currents causing them to absorb more water than usual.