The Southwest region of the United States is defined by heat, aridity, and dramatic variation. Spanning Arizona, New Mexico, and parts of Nevada, Utah, Colorado, and west Texas, the region averages less than 16 inches of precipitation per year across most of its lower elevations, with some desert areas receiving as little as 3 inches. But calling it simply “hot and dry” misses the complexity. Elevation swings of over 10,000 feet create climate zones ranging from scorching desert floors to cool, snow-capped peaks, sometimes less than 100 miles apart.
Temperatures Across the Region
Summer heat is the Southwest’s defining feature at lower elevations. Cities like Phoenix and Las Vegas regularly exceed 100°F from June through September, and Death Valley in eastern California holds the record for the highest reliably recorded air temperature on Earth at 134°F. Winter lows in these same desert valleys are mild, typically staying above freezing, which is why the region draws so many seasonal residents during the cooler months.
What surprises many people is how cold parts of the Southwest get. Temperature drops roughly 3°F for every 1,000-foot gain in elevation. That rule plays out dramatically: the difference in annual average temperature between Pikes Peak (14,114 feet) and Las Animas, Colorado (3,898 feet), only 90 miles to the southeast, is equivalent to the difference between Iceland and southern Florida. Flagstaff, Arizona, sits at nearly 7,000 feet and averages over 100 inches of snow per year. Santa Fe, New Mexico, at 7,200 feet, has winter lows that regularly dip into the teens.
Desert areas also experience extreme temperature swings within a single day. During the driest parts of the year, the difference between daytime highs and nighttime lows can reach 60°F. Low humidity and clear skies allow heat to radiate away rapidly after sunset, which is why desert nights can feel genuinely cold even after a blistering afternoon.
How Precipitation Works in the Southwest
Rainfall in the Southwest follows two distinct seasons, and understanding both is key to understanding the region’s climate. Winter storms move in from the Pacific, carried by westerly winds. These systems bring steady, widespread rain and mountain snow between November and March, and they’re the primary source of snowpack that feeds the Colorado River and other major water systems.
The second rainy season is the North American Monsoon, which transforms the landscape each summer. The monsoon develops over Mexico in June, then pushes into Arizona and New Mexico by July. A shift in wind patterns pulls moisture northward from the Gulf of Mexico and the Gulf of California, fueling afternoon and evening thunderstorms that can be intense but highly localized. One neighborhood might get drenched while another a few miles away stays bone dry. By early to mid-September, wind patterns revert to their westerly flow and the monsoon ends.
These monsoon storms deliver a huge share of the region’s total rainfall. Arizona and New Mexico receive more than 50% of their annual precipitation during the July through September window. Northwestern Mexico gets upwards of 75%. For many desert ecosystems, the monsoon is the single most important climate event of the year.
Four Deserts, Four Climates
The Southwest contains four distinct desert ecosystems, each with its own precipitation timing and temperature profile.
- Mojave Desert (southern Nevada, southeastern California): Dominated by winter precipitation, so most plant growth and biological activity happens during the cool season. Summers are brutally hot and almost completely dry.
- Sonoran Desert (southern Arizona, parts of southeastern California): The hottest of the four deserts, but it receives both winter and summer rain, giving it two growing seasons. This dual rainfall pattern supports the iconic saguaro cactus and a surprisingly diverse range of plant and animal life.
- Chihuahuan Desert (southern New Mexico, west Texas, northern Mexico): Dominated by summer monsoon rainfall, so biological activity peaks during hotter conditions. It sits at higher elevations than the Mojave or Sonoran, making winters colder.
- Great Basin Desert (Nevada, Utah, parts of Oregon and Idaho): A cold desert where precipitation falls primarily as winter snow. Summers are hot but shorter, and elevations are high enough that freezing temperatures occur across much of the year.
All four deserts receive less than about 16 inches of rain annually, but the timing of that rain shapes everything from which plants grow to when animals are most active.
Humidity and Dry Heat
The Southwest’s reputation for “dry heat” is well earned. Average afternoon relative humidity in Tucson runs around 25%, and inland desert areas are often even lower. For comparison, San Diego, on the California coast, averages 62% in the afternoon. This low humidity is what makes 105°F in Phoenix feel different from 95°F in Houston. Sweat evaporates quickly, giving your body’s cooling system a real advantage.
That said, humidity spikes noticeably during the monsoon. July and August afternoons in Tucson or Phoenix can feel muggy by local standards as moisture surges in from the south. Longtime residents describe it as the most uncomfortable stretch of the year, not because temperatures peak (they’re actually slightly lower than June) but because the combination of heat and moisture makes the air feel heavier.
Water Scarcity and Long-Term Trends
The Southwest’s climate creates a fundamental tension: it’s one of the fastest-growing regions in the country, and also one of the driest. The Colorado River, which supplies water to roughly 40 million people across seven states, has been in a sustained deficit for over two decades. Lake Mead, the largest reservoir in the United States, currently sits at about 34% of its full capacity. That’s a recovery from its 2022 low point, but still far below historical norms.
Extended drought is not unusual for the Southwest. Tree ring records show “megadroughts” lasting decades have occurred repeatedly over the past 1,200 years. What makes the current period different is that rising temperatures increase evaporation from reservoirs and reduce snowpack even in years with near-normal precipitation. The region’s water math has fundamentally changed: the same amount of rain and snow produces less usable water than it did 30 years ago.
For residents, this plays out in water restrictions, landscaping rules, and ongoing negotiations over how the Colorado River’s supply gets divided. Cities like Las Vegas, Phoenix, and Tucson have invested heavily in conservation and recycling infrastructure, but the gap between supply and demand remains the defining resource challenge of the region.