Climate of the Southwest
By Melanie Lenart | The University of Arizona | September 12, 2008
The climate of the Southwest takes its shape from global, regional, and local forces. Although many factors influence the climate of a particular year or season, predictable patterns hold across the years and decades to define the region’s climate.
Figure 1. The Hadley cell circulation illustrates how rising air in the superheated tropics descends in the subtropics. This creates high-pressure zones in subtropical regions, including the U.S. Southwest.
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Credit: Barbara Summey, NASA Goddard VisAnalysis Lab
- The overall aridity relates to a global circulation pattern known as Hadley circulation
- Relatively high temperatures with dramatic daily swings define this geographic region
- Mountains and other differences in elevation affect local climate patterns
Much of the U.S. Southwest lies in the subtropical zone, where warm, dry air is flowing back down to Earth following its rain-inducing rise in the tropics.
Descending air in the subtropics is caused by Hadley Circulation (Figure 1). Where the descending branch of Hadley circulation comes down, dry, descending air creates a zone of atmospheric high pressure that makes it difficult for clouds to form.
This regional high has some benefits. Southwestern cities such as Phoenix and Las Vegas boast of having more than 300 days of sunshine a year. The warm, sunny days attract retirees and other “snowbirds” escaping freezing northern climes.
All those sunny days have a downside, though. Evaporation rates often soar in the absence of clouds and rainfall. Phoenix sunshine, for instance, could evaporate roughly 10 times more water from the landscape than the eight inches of rainfall it receives in an average year.
High evaporation rates coupled with low precipitation rates create the region’s arid to semi-arid climate and its characteristic vegetation. The signature saguaro and other cacti grow in the lower elevations, while forests are restricted to the mountains. Grasslands can grow at various elevations, favoring sites with precipitation levels generally falling between the amounts that sustain cactus and those that support forests.
Regional temperatures run higher compared to northern climes because the subtropics receive more direct sunlight, and there is little water available to temper its power.
Our arid regional climate tends to switch more rapidly from cool to warm than the humid northern climes that experience four distinct seasons. Climate researchers in the southwest often think in terms of the “cool season,” roughly October through March, and the “warm season,” the other half of the year. The “rainy season” comes during summer in areas affected by the regional monsoon.
Precipitation and a related factor, humidity, also have important influences on temperature at both the local and regional scale.
When humidity levels are low, most commonly during winter and spring, there are greater swings from daytime highs to nighttime lows (Figure 2). When humidity levels are relatively high, such as during the summer monsoon, temperatures often fluctuate less dramatically from day to night.
Figure 2. The average annual diurnal temperature variation. Daily temperature swings from early morning lows to afternoon highs are greater in the Southwest than along the Pacific coastline and the Eastern U.S. because of the region's lack of atmospheric moisture and clouds that moderate diurnal temperature patterns.
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The presence of water in the environment causes a daytime cooling effect for two main reasons. Clouds, formed from water vapor, can block some sunlight from reaching the surface to heat it. Water also moderates temperature through evaporative cooling.
Swamp coolers take advantage of this process to cool air by removing some heat as they circulate water for evaporation. Similarly, southwestern restaurants often employ the process of evaporation to cool outdoor seating areas by spraying a fine mist around the perimeter.
When there’s not much water available for evaporation, more of the sun’s energy can go toward boosting temperatures. That’s partly why Phoenix temperatures can regularly climb above 110 degrees Fahrenheit while the mercury rarely surpasses 100 degrees in more humid climates even closer to the equator.
Because water vapor is a greenhouse gas, its presence in the atmosphere tends to hold in heat that might otherwise escape. In desert climates, this influence is most noticeable on summer nights, when cloudy skies can keep nighttime temperatures higher than they are on clear nights.
Mountains, and differences in elevation in general, affect climate at the local scale. The effect is complex, but follows certain patterns on average.
- Temperature drops with height in the atmosphere, at a rate that averages out globally to about a 3½-degree-F drop for every 1,000-foot rise in elevation. This “lapse rate” applies from sea level up to the tops of mountains.
- Precipitation tends to increase with mountain height. Meanwhile, evaporation rates tend to drop with declining temperature. Geographic or climate features also wield important influences.
- The sunlight reaching south-facing mountainsides is more direct, while that reaching north-facing mountainsides is more oblique. Mountainsides facing north, east, and west also receive varying degrees of shading from the mountain itself.
Figure 3. Precipitation patterns (lower left), biomes (upper right) and topography (upper left) in Arizona and the Southwest region are interrelated and, as a consequence, have similar spatial patterns. Source for elevation map: National Geophysical Data Center. Source for average annual precipitation, 1971-2000: PRISM Group, Oregon State University (2006).
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Credit: Jeremy Weiss, The University of Arizona
The type of vegetation favoring a site depends largely on climate, namely precipitation, evaporation rates, and temperature (Figure 3). Thus, different types of vegetation thrive at different elevations (Figure 3). Climatic factors interact with other factors, such as soil type and fire frequency, in complex ways that create a mosaic of different vegetation types on the landscape.
Slope orientation also comes into play. For instance, pine forests may thrive at lower elevations on the shadier north-facing slope than they do on sunny south-facing slopes of a mountain range. Mountains also capture more rainfall on the side facing prevailing winds, leaving the downwind side with a deficit. This creates a “rainshadow effect,” leaving some areas downwind of mountains drier than other parts of the nearby landscape.
Global circulation patterns and the local landscape affect climate patterns that persist over many years. Within this context, a variety of other factors affect climate variability in a particular year or season. The average across the years defines the climate—but so does the seasonal and annual variability. As in most subtropical regions, precipitation variability runs high in the U.S. Southwest. The next sections will discuss the seasonal precipitation patterns and their year-to-year variability.
- Sheppard, P.R., et al. 1999. The Climate of the Southwest. CLIMAS Report #CLI-99. Institute for the Study of Planet Earth, University of Arizona, Tucson.