page_title: Climate of the United States
section_title: Southwest
hierarchical_section_title: Climate of the United States / Regional overview / Southwest
caption_attribution_description: Saguaro National Park
context_page_description: The climate of the United States varies due to changes in latitude, and a range of geographic features, including mountains and deserts. Generally, on the mainland, the climate of the U.S. becomes warmer the further south one travels, and drier the further west, until one reaches the West Coast.
  West of 100°W, much of the U.S. has a cold semi-arid climate in the interior upper western states, to warm to hot desert and semi-arid climates in the southwestern U.S. East of 100°W, the climate is humid continental in northern areas, transitioning into a humid temperate climate from the Southern Plains and lower Midwest east to the Middle Atlantic states.
  A humid subtropical climate is found along and south of a mostly east-west line from the Virginia/Maryland capes, westward to approximately northern Oklahoma, north of the greater Oklahoma City area. Along the Atlantic seaboard, the humid subtropical climate zone extends southward into central Florida. A Mediterranean climate prevails along most of the California coast, while southern Florida has a tropical climate, the warmest region on the US mainland. Hawaii and the U.S. territories also have tropical climates.
context_section_description: The Southwest has a hot desert climate, at lower elevations. Cities like Phoenix, Las Vegas, Yuma, and Palm Springs have average highs over 100 °F (38 °C) during the summer months and lows in the 70s or even 80s. In winter, daily temperatures in the southwest are cooler with highs in the 50s and 60s F, and lows in the 40s F.
  In Phoenix, Las Vegas and similar Southwestern desert areas, on average June is the driest month, after Pacific-originating winter storms have concluded and before the Southwestern summer "monsoon" begins. The Southwest and the Great Basin are affected by said monsoon from the Gulf of California from July–September. This results in some increase in humidity and cloud cover, bringing higher nighttime low temperatures and localized thunderstorms to the region, which can result in flash flooding. Further eastward in the desert Southwest (Tucson, AZ eastward toward El Paso, TX), winter-season precipitation decreases, while the summer monsoon increasingly provides a summer precipitation maximum. For example, El Paso and Albuquerque, NM have a pronounced July to September precipitation maximum. Still, drought has been frequent in the region, often lasting for periods of years or longer. Forest fires across the Western United States (especially the southwest) occur many years, and can be severe to extreme in especially hot, dry summer seasons.
  Northern Arizona and New Mexico, central and northern Nevada and most of Utah (outside higher mountain areas) have a temperate semi-desert to desert climate, but with colder and snowier winters than in Phoenix and similar areas, and less-hot summers (as at Salt Lake City, Utah). Summer high temperatures often reach the 90s, but low temperatures drop into the low 60s and even 50s. As in other temperate desert climates, the dry air results in large differences (sometimes over 40 degrees) between daytime high and nighttime low temperatures. Precipitation, though scarce, often falls year-round, influenced both by summer thunderstorms brought by the Southwestern monsoon (primarily in southern areas), and by winter-season storms from the Pacific Ocean.
  The coast of California has a Mediterranean climate. Daily high temperatures range from 70 to 80 °F (21 to 27 °C) in the summer to 50 to 65 °F (10 to 16 °C) in winter, with low temperatures from the 60 °F (16 °C)s in summer to the mid 40s F in winter. Like most Mediterranean climates, much of coastal California has a wet winter and dry summer. Early summers can often bring cool, overcast weather (fog and low stratus clouds) to coastal California. As such, the warmest summer weather is delayed until August, even September in many areas of the California coast; on average, September is the warmest month in San Francisco, CA. Upwelling of cold Pacific waters also contributes to the frequent cool spring and early summer weather in coastal California. In California's inland river valleys (Bakersfield, Sacramento areas), the wet-winter, dry-summer pattern remains, but winters are cooler and more prone to occasional frost or freeze, while summers are much hotter, with blazing sunshine and daytime high temperatures not uncommonly in the 90s °F to over 100 °F (38 °C).
caption: Sonoran Desert at Saguaro National Park
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