The location of the Southwest and the topographical extremes across this area strongly influence its weather. The Southwest experiences nearly every variety of extreme weather; heat and cold waves, droughts, floods, blizzards, and even tornados are all considerations for residents of the Southwestern states.

Figure 8.8: Lake Bonneville's maximal extent during the Pleistocene. Inset: Graph of the lake's changing level.

Although much of the Southwest falls within the category of an arid zone, using a single label to describe the Southwest's climate would belie its diversity. The main features that influence the area’s climate are latitude, regional topography, and a low atmospheric moisture content that leads to quick evaporation. For example, parts of the Colorado Rockies experience cool annual temperatures and over 8 meters (25 feet) of snowfall every year, while the dry deserts in southwestern Arizona receive only about 8 centimeters (3 inches) of precipitation a year and can experience as much as a 15ºC (60ºF) degree temperature difference between night and day.

Average temperatures found in the Southwest tend to decrease northward, which is largely the influence of latitude and elevation. Lower latitudes receive more heat from the sun over the course of a year: for each degree increase in latitude, there is approximately a 1ºC (2°F) decrease in temperature. Higher elevations (such as those found in the Rockies and on the Colorado Plateau) are also cooler, with approximately a 1.5ºC (3°F) decrease in mean annual temperature for each 300-meter (1000-foot) increase in elevation.

The warmest temperatures in the Southwest are found in Arizona and New Mexico, while the coolest are found in Utah and Colorado (Figure 8.9). The Southwest's overall average high temperature of 19.2°C (66.6°F) and average low of 2.8°C (37.0°F) are indicative of a varied climate, one much less uniform than that found in many other parts of the United States. By comparison, the average high and low temperatures for the entire United States are 17°C (63°F) and 5°C (41°F), respectively.

Another factor besides latitude and elevation that influences temperature in the Southwest is its arid climate. The lack of moisture in the air allows heat trapped in the earth during daylight hours to rapidly radiate away, leading to cool evenings. Thus, each Southwestern state experiences both extreme highs and lows. In New Mexico, for example, the average difference between the daily high and low temperatures ranges from 14° to 19ºC (25° to 35°F). Record high temperatures for the Southwest range from 53°C (128°F) in Arizona to 47°C (117ºF) in Utah, while record low temperatures range from −56ºC (−69°F) in Utah to −40ºC (−40ºF) in Arizona.

The average amount of precipitation for the United States is 85.6 centimeters (33.7 inches). In the Southwest, average precipitation ranges from only 34 centimeters (13.4 inches) in Utah to 39.9 centimeters (15.7 inches) in Colorado, which is indicative of the area's general aridity (Figure 8.10). Elevation does, however, play a key role in precipitation received throughout the Southwest. In New Mexico, for example, average annual precipitation ranges from less than 25 centimeters (10 inches) within the Great Plains and Basin and Range regions to more than 50 centimeters (20 inches) at the higher elevations to the northwest. Arizona's highest elevations receive an average of 65 to 76 centimeters (25 to 30 inches), with lower areas in the state’s southwestern portion averaging less than 8 centimeters (3 inches). In Utah, areas below 1200 meters (4000 feet) receive less than 25 centimeters (10 inches) per year, while higher elevations in the Wasatch Mountains receive more than 100 centimeters (40 inches).

Across New Mexico, Arizona, and Utah, summer rains originate from moisture brought into the area from the Gulf of Mexico. Warm, moist air from the south occasionally but infrequently moves into Colorado during the summer. During the winter, moisture travels from the west, as storms from the Pacific Ocean move east. Pacific storms lose most of their moisture as they pass over the Rocky Mountains, so much of the Southwest's winter precipitation falls as snow within the area’s mountainous regions.

The Köppen Climate Map

Wladimir Köppen developed a commonly used system of climate categorization based on the kinds of vegetation areas sustain. He defined 12 climate types, many of which are familiar: rainforest, monsoon, tropical savanna, humid subtropical, humid continental, oceanic, Mediterranean, steppe, subarctic, tundra, polar ice cap, and desert. Updated by Rudolf Geiger, it has been refined to five groups, each with two to four subgroups.

In a broad sense, the Southwest’s climate is mostly dry and hot, with much of the region characterized as arid (represented by "B" in the Köppen system). Such conditions are common throughout the Great Plains, Colorado Plateau, and Basin and Range. Cold continental conditions (represented by "D") dominate the higher altitudes, especially within the Rocky Mountains. Scattered pockets of drier, Mediterranean temperatures (represented by "C") can also be found.

Figure 8.9: Mean annual temperature for the Southwestern states.

Figure 8.10: Mean annual precipitation for the Southwestern states.

A strong temperature difference at different heights creates instability—the warmer the air near the surface is relative to the air above it, the more potential energy it has to move up. The Great Plains receive warm, moist air moving north from the Gulf of Mexico, and cold, dry air moving in from the Rocky Mountains and the northern US. Where these air masses meet, vigorous mixing causes thunderstorms. Because warm air can hold more moisture than cool air can, convective mixing with cool air forces moisture to condense out of warm air as vapor (clouds) and precipitation. This movement of air in different directions is also the reason for the high incidence of powerful tornados that occur along "Tornado Alley" in the Great Plains (Figure 8.11).

Figure 8.11: Frequency of tornados in the continental US. "Tornado Alley" is an area of the central US known for its violent tornados.

Colorado has a generally cool and continental climate, with low humidity. The climate of the eastern plains is fairly uniform, with hot, windy summers and prevalent thunderstorms. The state's highest temperatures occur in the northeastern plains, where they can exceed 46ºC (115ºF). Moving westward, Colorado's foothills and mountainous areas experience an overall cooler climate and higher levels of precipitation. Here, the state’s varied topography leads to wide changes in climactic conditions that occur across short distances. For example, the difference in annual mean temperature between Pikes Peak (4302 meters [14,114 feet]) and Las Animas (1188 meters [3898 feet]), only 145 kilometers (90 miles) to the southeast, is equivalent to that between Iceland and southern Florida! Precipitation also varies widely—Cumbres in the San Juan Mountains receives nearly 7.6 meters (300 inches) of snowfall annually, while Manassa, less than 50 kilometers (30 miles) away in the San Luis Valley, receives only about 63 centimeters (25 inches) of snow a year.

Arizona's climate is influenced by three main topographical areas: the high Colorado Plateau (about 1520–2130 meters [5000–7000 feet] in elevation), the rugged mountains to the west (2740–3660 meters [9000–12,000 feet] high), and the low southwestern mountains with desert valleys (as low as 30 meters [100 feet] above sea level). While the state is generally arid, its high western mountains experience more precipitation each year than the desert southwest and the high northeastern plateau do. The desert also experiences higher temperature extremes, especially between day and night, with a daily change of as much as 15°C (60°F) during the driest parts of the year.

In New Mexico, climate is characterized by arid, semiarid, or continental conditions, with light precipitation, low humidity, and abundant sunshine. As in Arizona, the desert experiences a high temperature range daily; the state's mountainous areas, however, have climate characteristics that more closely follow those found in the Colorado Rockies. Summer rains fall almost entirely during brief but intense thunderstorms on the Great Plains, although the occasional hurricane in the Gulf of Mexico may push heavier precipitation inland. Winter is the driest season in New Mexico, because precipitation from eastward-travelling Pacific storms is left behind in the western mountains of Arizona and Utah.

Utah’s distance from both the Pacific Ocean and the Gulf of Mexico prevents heavy precipitation, and much of the state is typically sunny year-round, with light to moderate winds, although changes in atmospheric pressure during the late fall and winter can lead to an accumulation of haze. Light precipitation travels eastward over the Sierra Nevada and Cascade mountains after dropping heavy snowfall in areas of high elevation. Because high mountains to the west and north act as a barrier to cold Arctic air masses, most areas of Utah rarely experience temperatures below freezing or prolonged periods of extreme cold.