Nevada lies in the Great Basin region of the western United States. A high plateau broken by long mountain ranges and desert lowlands dominates the state. Las Vegas sits at the southern limit of the plateau, in land that gradually slopes downward toward the Colorado River, which flows to the east and south.
Hot deserts surround the city of Las Vegas. While continental air masses move generally from west to east, by the time low-pressure weather systems have crossed California, their precipitation has fallen in the greater elevations of the Sierra Nevada Mountains. Mountain ranges in northern Nevada, at the limit of the Great Basin, block winter weather systems that move down from Canada and bring snow to locations in the Rocky Mountains that lie at the same latitude as Las Vegas.
The Great Basin contains few streams or permanent bodies of water. Most of the short streams have no outlet, and disappear completely in the long, hot summer season. As a result, the natural evaporation rate into the atmosphere is low, and clouds have little chance to form.
As a result of these factors, Las Vegas usually enjoys sunny weather, which takes place an average of 300 days of year, and low annual precipitation that seldom reaches more than twelve inches in a year. As there is little heavy industry in Las Vegas, pollution is not a major factor in the region's climate. Automobile smog, however, often collects over the city during the crowded winter tourist season.
