Weather is the measure of short-term atmospheric conditions such as temperature, wind speed, and humidity. The Southwest is an active location for atmospheric events such as thunderstorms and tornados. It also experiences a variety of other weather hazards, including high temperatures and drought.

Storms and Tornados

Several types of severe storms present challenges to people living in the Southwest. Summer brings severe thunderstorms associated with cold fronts. Fall and spring can bring ice storms, while winter brings snow and, in some cases, blizzard conditions. In March 2016, for example, a major blizzard dumped 60 centimeters (2 feet) of snow on the Denver metropolitan area and Colorado’s Front Range, knocking out power, shutting down the Denver International Airport, and closing schools. A second event in April 2016—dubbed Winter Storm Vexo—inundated the Southwest with more heavy snowfall, from 1.3 meters (51 inches) in Pinecliffe, Colorado to 28 centimeters (11 inches) near Questa, New Mexico and 18 centimeters (7 inches) in Bellemont, Arizona.

Rainstorms occur where colder air from higher latitudes abruptly meets warmer air. Severe thunderstorms are a common occurrence for people living in the eastern Southwest because the conditions over the Great Plains are perfect for the development of severe weather. The region’s flat, open fields are warmed by the summer sun, which sits high in the sky during this time of year. This results in large temperature differences when cold air masses move across the country. At the boundary between warmer and cooler air, buoyant warm air rises, and then cools because air pressure decreases with increasing height in the atmosphere. As the air cools, it becomes saturated with water vapor, condensation occurs, and clouds begin to form. Because liquid water droplets in the clouds must be very small to remain suspended in the air, a significant amount of condensation causes small water droplets to come together, eventually becoming too large to remain suspended. Sufficient moisture and energy can lead to dramatic rainstorms. Because warm air has a lower pressure relative to cold air, and the movement of air from areas of high pressure to areas of low pressure generates wind, the significant difference in air pressure associated with these boundaries and rainstorms also generates strong winds. Hail is also a possible occurrence during storms as a result of moisture high in the atmosphere that condenses and forms rain droplets. If the wind is strong enough to keep the droplets suspended, and cold enough to freeze them, they may become hailstones. If the wind continues to persist and keeps the hail suspended in the clouds long enough, they can even grow as large as golf balls. Once they reach a mass that is too great for the wind to keep them suspended, they fall to the Earth, where they can do considerable harm upon impact. Anyone caught in a significant hailstorm can expect some bruises or stinging sensations. If the hail is large enough, property can be damaged; car windshields, sunroofs, and canopies are especially susceptible.

With freak and intense thunderstorms comes the added risk of lightning strikes. Friction in the atmosphere from a chaotic storm can produce a buildup of static electricity and an unbalanced electrical charge. When the imbalance is great enough, the accumulated energy will discharge itself in the form of a lightning bolt. This discharge can be heard as the sound of thunder. The intense beam of energy can scorch or kill any life that is unlucky enough to be at the point of contact. If a lightning strike occurs in an arid, vegetated area, the resulting fires may develop into full-blown forest fires.

Some severe thunderstorms, called supercells, have the potential to develop into tornados that can cause serious property damage and endanger lives. These storm events are associated with wind shear, which occurs when the wind’s speed or direction changes with increasing height in the atmosphere. Wind shear can happen when a cold front moves rapidly into an area with very warm air. There, the condensing water droplets mix with the cooler, drier air in the upper atmosphere to cause a downdraft. At the frontal boundary, warm, moist air rapidly rises as cooler, dry air descends; in the meantime, the pressure differences between the warm and cold air masses cause strong winds. Clouds with a visible horizontal rotation can form, appearing to roll like waves crashing on the shore of a beach. This horizontal motion can tilt, lifting the rotating cloud vertically, and the rolling cloud will form a tornado. Most tornados will last a few seconds to several minutes. During that time, many tornado-prone areas will use tornado sirens to alert residents of the danger. A smaller tornado might generate flying debris that can cause injury or damage to buildings, while larger tornados can cause buildings and houses to be completely broken apart. Tornados are classified by their ranking on the En-hanced Fujita scale, or EF scale. These classifications are estimates of wind speeds based on the type of damage that is observed following the storm.

“Tornado Alley” is the nickname for an area, extending from Texas to Minnesota, that experiences a high number of exceptionally strong tornados due to its flatter topography and high incidence of severe thunderstorms. The Great Plains of Colorado and New Mexico are part of Tornado Alley, leading to more tornados in this part of the Southwest (Figure 9.30). From 1991 to 2010, for example, an annual average of 53 and 11 tornados occurred in Colorado and New Mexico, respectively (Figure 9.31). To the west, fewer tornado strikes occur, with an annual average of five and three striking Arizona and Utah, respectively. The boundaries of Tornado Alley vary in application, depending on whether the frequency, intensity, or number of events per location are used to determine its borders.

Measuring Tornado Intensity

Tornado intensity is measured on the Fujita scale, or simply F-scale, based on the amount of damage that a tornado can cause. The scale ranges from F0 to F5. The scale was modified recently to more accurately reflect specific wind speeds; this newer scale is known as the “Enhanced Fujita scale” and is labeled EF0 to EF5.

Dust Storms

In arid climates, even under non-drought conditions, dust storms are a hazard. Dust storms occur when winds hold dust aloft, sometimes briefly over a local area, and sometimes over broad regions for days. They can be hazardous to health and, because they drastically reduce visibility, dangerous to motor vehicle and airline traffic.

Among the most spectacular dust storms are those known as haboobs (or monsoonal dust storms), which occur when strong thunderstorm downdrafts blow loose sediments up from the desert, sending dust up to over 1000 meters (3300 feet) into the sky. Large haboobs can be as much as 100 kilometers (62 miles) across, and travel at speeds of 50 to 100 kilometers per hour (about 30 to 60 miles per hour) for over an hour. These storms occur in the summer, across southernmost New Mexico and Arizona (Figure 9.32), as well as in California and Texas.

Figure 9.30: Annual tornado reports per 29,500 square kilometers (10,000 square miles) in the continental US, between 1950 and 1995.

Figure 9.31: A tornado touches down over the hills near Roswell, New Mexico.

Figure 9.32: A haboob blows into the village of Ahwatukee, part of Phoenix, Arizona, in August 2003.

In addition to the inhalation of silt and clay dust, other health hazards associated with dust storms include fungi, bacteria, pollutants, and heavy metals. These materials can irritate the lungs and trigger asthma attacks, allergic reactions, and other illnesses. One fungus, Coccidioides, causes "valley fever," which causes cold- and flu-like symptoms and sometimes rashes. Though most recover without treatment, it can have serious consequences and even lead to death for some people with weak immune systems.