Climate
Arizona is an arid state. Only Nevada receives less precipitation. Mountain ranges to the west and a persistent high pressure pattern over south-central Arizona prevent most Pacific storms from reaching the state. As a result, Arizo na has dry air and scant precipitation, averaging less than 10 inches per year. Southern Arizona leads the nation in sunny days.
The sunshine, infrequent storms, spectacular sunsets, and clear starry nights attract people to Arizona, but such a climate does not produce much surface water. The arid climate also increases water demand, because most crops and urban landscapes mus t be irrigated.
The Sonoran Desert of central and southern Arizona is characterized not only by infrequent precipitation but also by extreme temperature swings averaging 35 degrees F between day and night. Precipitation and temperature are affected greatly by elevat ion, with "mountain islands" much cooler and wetter than surrounding lowlands. Annual precipitation varies from 4 inches near Yuma to over 30 inches on mountaintops. The plateau region north of the Mogollon Rim has a cooler but still arid climate.
Arizona's precipitation comes in two "wet" seasons. Summer storms can produce intense downpours, spectacular lightning and flash floods. More important from a water supply perspective are the gentler winter storms that blanket the mountains and high country with snow, feeding streams and rivers with snowmelt.
Issues Summary