Earth's spin and the latitudinal gradient in solar energy input both contribute to prevailing wind patterns at Earth's surface. These surface winds—the northeast trades, the southeast trades, the westerlies, and the easterlies—blow primarily in certain directions at certain latitudes around the globe. For example, the westerlies are prevailing winds that blow from west to east at latitudes between 30 and 60 degrees north or south of the equator. Most of North America receives weather from the westerlies.
Regional climates on Earth are influenced by prevailing surface winds, the spatial arrangement of water and land, and by land topography. In the accompanying animation, we examine the causes of a rain shadow, which is a region of low precipitation on the leeward (wind-protected) side of a mountain range.
Suppose now that the prevailing air movement, or wind, is from the ocean onto the land. For example, this will be true on the west side of continents in middle latitudes between about 30 and 60 degrees. Let's imagine a given volume of air, which we will call a packet of air, and follow its movement. As the air moves onshore, this packet of air encounters a mountain range and rises up the windward slope of the mountains. As it rises, there is less atmosphere above it to press downward, the air pressure decreases, and the packet of air expands in volume.
As the packet of air expands, it cools, and more and more water molecules in it coalesce to form the tiny water droplets or ice crystals of a cloud. In other words, more and more of the water vapor undergoes a phase change back from gaseous to liquid phase or from gaseous to solid phase. If the water droplets or ice crystals become massive enough, they fall to Earth as rain or snow. The windward side of the mountain range is therefore relatively moist and the vegetation is relatively lush.
The packet of air continues over the crest of the mountain range and descends on the leeward side. As it does it is compressed by the increasing mass of the atmosphere above it, and it warms. There is now far less water vapor in the packet of air than before, and this water remains in gaseous phase. Thus the leeward side of the mountain range and the lands to the leeward of the range are relatively dry, and vegetation is relatively sparse in this so-called rain shadow.
In North America, the Great Basin of Nevada and Utah is a classic rain shadow. This rain shadow occurs because the prevailing surface winds are westerlies that blow from the Pacific Ocean onto the California coast. As the air moves eastward it rises over the towering Sierra Nevada, which runs north and south along the eastern edge of California. Much of the water vapor in the rising air condenses and falls as precipitation on the western side of this mountain range. Far less rain and snow fall to the east of the crest of the Sierra Nevada. As a consequence, the lands to the east are part of the Great Basin Desert.