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March-April 2015

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The Weather and Climate of Arizona

Picture Arizona in your mind's eye… There are truly amazing physical contrasts (the one-mile-deep Grand Canyon, volcanic peaks like Mount Humphreys, and seemingly endless deserts). Arizona also hosts a similar array of weather contrasts, from heat to cold and dryness to floods, as well as dust storms, forest fires, unparalleled lightning displays, and observable human impacts on local climate.

Consider a typical April climb down from the Grand Canyon's South Rim (7,000 feet) to an overlook of the Colorado River (3,800 feet). Here, one encounters subfreezing temperatures on the Rim at night, yet by noon it will be nearly 100°F at the overlook. In a sense, the Grand Canyon compresses the whole state's range of climates and altitudes (Figure 1) into a few miles horizontally and less than a mile vertically. At the bottom, the climate is similar to Phoenix and Tucson—warm in winter and very hot in summer. Yet the North Rim can be below freezing on many or most mornings of the year.

Figure 1.  Arizona elevation map with cities and other significant features.

Figure 1. Arizona elevation map with cities and other significant features.

In the book Over the Edge: Death in Grand Canyon, by Myers and Ghiglieri, the authors describe weather-related deaths from heat stroke (hyperthermia), cold (hypothermia), floods, river floods, and lightning. These are the main features of Arizona's climate—temperature extremes, long periods of high temperatures in the major cities at lower altitudes, altitude dependence, lightning, and occasional flooding. One can add dust storms (haboobs), snowstorms (especially at higher elevations), droughts, and forest fires to the mix.

Altitude Dependence

Elevations across Arizona range from near sea level in the southwestern deserts and along the lower Colorado River Valley to over 12,000 feet in the San Francisco Peaks near Flagstaff (Figure 1). This fluctuation in elevations generates extremes in temperature and precipitation, both annually and monthly. For example, on average, the town of Alpine in the Apache National Forest at 8,020 feet reaches 90°F only one summer out of seven, averages 65 inches of snowfall, and has 10 nights of freezing temperatures or below in June. But in the lowest deserts of southwestern Arizona, near the Mexican border (less than 100 feet elevation), some locations exceed 100°F on 30 days in June, and have not recorded rain for decades on some of the days in the middle of June.

Elevation also contributes to a range of annual precipitation totals, from less than three inches to over 40 inches. The Mogollon Rim is a cliff-like feature that rises from less than 5,000 feet near Payson to over 8,000 feet near Alpine, and extends over 200 miles, dividing the lower deserts in the southwest from the Colorado Plateau in the northeast. The orientation of the Rim (northwest to southeast) produces a steep gradient in orographic precipitation when southwesterly winds are forced up the slope. During a March 2013 science-based high school field trip through Arizona, students from a Massachusetts high school were treated to thunderstorms near Phoenix and 15 inches of snow as the car caravan climbed Highway I-17 en route to Flagstaff. The transition from rain to snow-covered highways through the climb up the Rim was dramatic and almost instantaneous.

Moisture Sources

Higher humidity arrives at low levels from the Gulf of California to the southwest, over the ridges of the Mexican and New Mexican mountains to the southeast and east (summer), and over the top of the Californian ranges to the west (winter). The distance from the Gulf of California (also called the Sea of Cortez) to Tucson is only about 100 miles. Because the Gulf waters warm to as much as 90°F in summer, moisture from this source can be a potent source of thunderstorm development when lifted over higher terrain. In fact, many areas of southwestern Arizona receive half or more of their annual rainfall from moisture arriving from the Sea of Cortez. However, sometimes dew points rise to over 75°F at Yuma (low elevation), but without a lifting mechanism from large mountains, outflow boundaries, or other self-propagating features during the summer monsoon season, no storms form. That is when temperatures exceed 100°F and make for dangerous heat stress conditions. Nevertheless, this is a prime agricultural region wherever surface water is made available through irrigation.

Rain, Floods, and Even Snow

Arizona is relatively dry, but devastating flash floods and heavy snowstorms occur somewhere every year. The danger of flash flooding is exemplified by the August 1997 Antelope Canyon (near Page) flash flood that killed 11 hikers. This dramatic narrow sandstone canyon (averaging only five feet wide but up to 100 feet deep in places) was flooded when a thunderstorm five miles away dropped less than two inches of rain. As the runoff was funneled into the canyon, water levels rose up to 50 feet in minutes. In August 2008, up to eight inches of rain fell in two days in the tributaries to the Grand Canyon near the Havasupai reservation village of Supai; a small earthen dam there broke and stranded 426 hikers, boaters, and other visitors. Helicopter rescues were required.

In late July 2006, a flooding event in Sabino Canyon north of Tucson resulted from seven inches of rainfall in the upper canyon. A debris flow ensued, such that at one point during the event, the flow in the normally narrow canyon exceeded the flow of the Colorado River in the Grand Canyon! Since it occurred at night, there was no loss of life, as in Antelope Canyon, but it could have been a different situation during a typical daytime, when Sabino Canyon is filled with hikers and tram riders.

January 2010 storms set new one-, two-, three-, four-, five-, and six-day watershed average precipitation records across the Salt-Verde watershed.

Flash flood events often occur during the summer, in association with the summer part of the North American Monsoon. Starting around July 1, moisture arrives at middle levels from the southeast and at low levels from the southwest (Gulf of California). For two to three months, dewpoints rise into the 50, 60s, and sometimes 70s at lower elevations. Given the high temperatures and the significant mountainous topography of the region, this moisture content is more than adequate to set off diurnal cumulus activity. It usually starts over the highest mountains (Figures 1 and 3), especially the Mogollon Rim, White Mountains, and the southeast Arizona mountains, which are an extension of the Sierra Madre Occidental Mountains in nearby Mexico. On some afternoons, these thunderstorms move off the higher elevations into surrounding valleys, and sometimes they do not. Often the storms propagate via the formation of new thunderstorms at the leading edge of outflow boundaries. The arrival of such storms in Phoenix, often near sunset, results in evening haboobs (at left), downbursts (below), spotty heavy rain, and a wide array of associated traffic impacts. Mesoscale convective systems sometimes form over southern Arizona in the summer; many more form south of the border in Mexico.

Occasionally in late summer to autumn, eastern Pacific tropical cyclones send deep moisture northward and help cause major floods and flash flooding over regions of the state. In late September 1997, Hurricane Nora moved northward up the lower Colorado River and entered the Arizona/California border as a tropical storm; the remnant circulation aloft resulted in drenching rains over the western half of the state to set an unofficial 24-hour rainfall record of 11.97 inches at the Harquahala Mountain weather station. Yuma received 3.59 inches, exceeding its annual average rainfall at the time of 3.17 inches. On September 8, 2014, moisture from Hurricane Norbert settled over south-central Arizona, dropping over five inches of rain in 12 hours in the Phoenix metropolitan area, causing widespread flooding and effectively closing Interstates 10 and 17 through Phoenix. This was a one in 1000-year rain event, and the third such rainstorm to occur in the Phoenix area during a four-week period that year. On August 12, 2014, a tropical air mass in the Phoenix area contributed to a 3.37-inch 90-minute rush hour rainstorm at South Mountain. This created debris flows and street flooding that required swift-water rescues of stranded motorists. One week later, over five inches of rain fell in the New River area just north of Phoenix. Washes (intermittent, usually dry stream beds) and rivers flooded during this event.

Not to be outdone by summer, winter brings its share of wild precipitation, too. The winter “Storm of the Century” in 1967 was actually two back-to-back winter storms during December 12–20 that dropped 86 inches of snow on Flagstaff, 102.7 inches at Hawley Lake in the White Mountains, and 84 inches at Mount Lemmon Ski Area near Tucson. Eight people died of exposure in that storm. More recently, heavy snow and high winds forced the closing of 180 miles of I-40 through northern Arizona in March 2012, as snowplows couldn't keep pace with the ice and drifting snow. Flagstaff received 26.4 inches of snow from the storm.

Sunshine and Dryness

Sunshine, however, is the most notable feature of Arizona weather. The dominance of sunshine is partly attributable to the state being at the latitude of descent in the global general circulation. Most places in the more heavily populated lower-altitude regions of the state see no more than one or two days a year with continuous middle or low clouds. These days typically occur in winter, when a low-pressure system or trough moves in from the southwest or west. Days of overcast cirrus that somewhat dim the bright sunshine are not unusual in winter, since middle and low clouds are often absent due to the dry lower atmosphere. Measures of incoming solar radiation indicate that Arizona is in the center of the maximum sunshine region of the United States, and has more annual sunshine (85% of possible) than any other state except Nevada; Arizona also has sunshine values far in excess of Florida's 70%. The United States Energy Information Administration reports that Arizona is ranked second in the country in solar-based, utility-scale electricity generation.

Between October 18, 2005, and March 10, 2006, no rain fell in the Phoenix area, or most of the southern part of the state. This established a new record of 143 consecutive dry days. When the precipitation finally arrived, only 0.26 inch fell at Sky Harbor Airport, but well over an inch was reported in areas of the east valley, and snow fell on most of the surrounding mountains. This cold winter storm was a punctuation mark to one of the driest winters on record.

Dew points and associated relative humidity values at the surface are often very low. Relative humidity readings as low as 2% are common in the southwest desert; such low values allow for moisture evaporation off human skin and can make the temperature feel cooler than the thermometer reads. However, once temperatures exceed about 95°F, it's still hot. In the shade, with a breeze, the heat is often tolerable, except during much of the day during summer months. In winter, the days are usually mild and dry at lower elevations. The diurnal temperature range is large, since the air is often very dry. It is not uncommon to observe diurnal ranges of 30 degrees or more. In late spring to early summer, the rims of the Grand Canyon can range from near freezing at night to near 80°F during the day.

As a result of the abundant sunshine and dry atmosphere much of the year, as well as the very large elevation range, Arizona often experiences extremely large statewide temperature ranges. Figure 2 shows maps of record-high and record-low readings across the state. Every station has exceeded 90°F, most have gone over 100°F, and many topped 110°F. Quite a few low-elevation stations have exceeded 120°F or even 125°F! In contrast, every location in Arizona has been below freezing, most have been colder than 20°F, and some high mountain locations have been colder than –30°F (Table 1)!

Figure 2.  Maps of extreme maximum (left) and minimum (right) temperatures across Arizona.

Figure 2. Maps of extreme maximum (left) and minimum (right) temperatures across Arizona.

Table 1. Arizona Extremes

Type of eventValueLocationDateAltitude
All-time high128°FLake Havasu CityJune 29, 1994735 feet
All-time low−40°FHawley LakeJanuary 7, 19718,180 feet
Maximum annual precipitation58.92 inchesHawley Lake19788,180 feet
Maximum 24-hour precipitation11.97 inchesHarquahala Mountains Workman CreekSeptember 25, 1997 September 5–6, 19705,682 feet 6,975 feet
Most annual days over 90°F189 days, Mohawk (Dateland)  450 feet
Least annual days<0.5 day, Alpine  8,020 feet
Type of eventValueLocationDateAltitude
All-time high128°FLake Havasu CityJune 29, 1994735 feet
All-time low−40°FHawley LakeJanuary 7, 19718,180 feet
Maximum annual precipitation58.92 inchesHawley Lake19788,180 feet
Maximum 24-hour precipitation11.97 inchesHarquahala Mountains Workman CreekSeptember 25, 1997 September 5–6, 19705,682 feet 6,975 feet
Most annual days over 90°F189 days, Mohawk (Dateland)  450 feet
Least annual days<0.5 day, Alpine  8,020 feet


Forest Fires

During the weeks prior to the arrival of the summer monsoon around July 1, and anytime extended droughts affect the state, forest fires are a major concern wherever there are trees and large shrubs. The fire threat nearly ceases after the monsoon arrives, although dry thunderstorms can often spark wildfires. According to the National Interagency Fire Center, between 2002 and 2013, Arizona had an annual average of 2,408 wildland fires, which burned an average of 345,330 acres each. Arizona has a large forest fire every few years somewhere in its vast forested area, which covers 27% of the state. In 2003, the Aspen fire erupted on Mount Lemmon near Tucson in mid-June before the monsoon moisture arrived (see photo, page 18). Most of the town of Summerhaven (325 structures) at the summit was destroyed, and the National Interagency Fire Center reports that only now have trees begun to fill in the dead timber areas. In June 2002, two separate fires began in the White Mountains of east-central Arizona, one started by a stranded motorist and the other started by an out-of-work firefighter. The two fires became one, the Rodeo-Chediski fire, which consumed 468,638 acres and 491 structures (including 465 homes) over 60 days. The worst year for wildfires in Arizona history was 2011, when three major fires (all human-caused) flared near Alpine, Sierra Vista, and the Chiricahua Mountains and a total of 1,036,935 acres of forest went up in smoke. The deadliest fire was the Yarnell Hill fire, a lightning-caused blaze near Prescott in 2013, during which 19 hotshot firefighters lost their lives when the wind suddenly shifted due to outflow from a nearby thunderstorm, sending the fire back over their position.

Urban Heat Island

The urban heat island effect of Phoenix is evident year round. The minimum temperature has steadily risen, as the Phoenix metro area has grown from a few thousand people in 1900 to over 4 million in 2010. In recent years, Phoenix Sky Harbor airport, located in the center of the metropolitan complex, has seen several summer nights with middle or high cloud cover, no wind, and temperatures that remained at 90°F or above. Before 1980, Phoenix had essentially no nights with low temperatures above 90°F. In 2003, there were 15 nights at or above 90°F, and a new high minimum temperature record of 96°F was set on July 15, 2003. In the past five years, Phoenix has averaged just under 12 nights per year at or above 90°F. Temperatures at Phoenix reach 115°F or more on several days during the late spring and summer, although the dew point is usually no higher than the 30s. As the humidity rises, the daytime temperatures drop, but the nighttime temperatures increase. When summer thunderstorms are absent, temperatures skyrocket.

Given the impact of the growing urban heat island effect, cities such as Phoenix and Tucson are not expected to set many more records for low nighttime temperatures. For Phoenix, with the exception of December 23, 1990, when the nighttime low reached 26°F, all the low minimum records for December and January were set prior to 1979. However, abnormally cold weather continues to occur. On both February 3 and 4, 2011, Tucson recorded 18°F for record nighttime low temperatures. The number of freezing days for Phoenix has dropped from around five to less than one, though 2011 had four days at or below freezing. Tucson still averages about 10–12 freezing days per year.

Severe Weather

Tornadoes are scarce and usually weak, and damaging hail is infrequent. That said, a strong low pressure system with supercell thunderstorms moved across Arizona on October 5–6, 2010, and brought 1.5 inches of hail to the Phoenix area, as well as eight confirmed tornadoes to Flagstaff and north-central Arizona. The largest hailstone was three inches in diameter (ranked third for all-time largest in the state) in Scottsdale, and the largest tornado was an EF3 near Flagstaff. Heavy rains produced flash flooding in the Tucson area, where the National Weather Association was holding its annual meeting!

A particular type of dust storm that originates from a thunderstorm, the haboob, occurs as the monsoon begins. These dust storms happen in late June and early July in the lower deserts as thunderstorm outflows from the mountains push up dust from the valley floor. According to the National Climatic Data Center database, there have been 70 dust storms in Maricopa, Pinal, and Pima counties during the monsoon season between 2003 and 2013. Later in the monsoon season, dust storms are less prevalent, as rainfall has removed most of the loose dust and dirt, forming a surface crust. In 2010, an early monsoon thunderstorm formed near Tucson and generated a haboob that traveled from Tucson through Phoenix, past Wickenburg (over 150 miles), rising well over 5,000 feet in height (see photo, page 14). Dust raised in the spring due to strong low-level winds in otherwise clear skies can also prove to be dangerous for drivers.

Downbursts (see photo, page 15) are quite common during the summer monsoon. Most are dry microbursts that occur because the middle levels are moist but the low levels are dry. The resulting winds cause tree and building damage every summer in nearly every portion of the state below about 5,000 feet. Dust devils are nearly ubiquitous during the warmer months in the lower elevations of the state due to intense heating of the ground. Since the surface needs to have open land to make the circulation visible, they are often not as frequently visible in Phoenix and Tucson as in the desert areas outside of built-up areas.

Lightning is one of the constant features of the monsoon months. Arizona averages over 600,000 cloud-to-ground flashes per year, and its density in terms of flashes per square mile per year is typical of many states (Figure 3). Since most of the lightning is confined to early July–early September, daily lightning activity during that time rivals or exceeds many parts of the Unites States. But the most dramatic feature of the Arizona lightning is its spectacular appearance. Relatively dry air, high cloud bases, sun-to-cloud positioning at sunset (allowing for colorful sunsets), and dramatic peaks and desert vegetation make Arizona the lightning photography capital of the world. If there's an appealing lightning photo in a book or calendar, it's probably from Arizona, and often from the Tucson area. In fact, there are several lightning photographers in southern Arizona who specialize in the picturesque lightning around the region.

Figure 3.  Cloud-to-ground flash density over Arizona on a 2-mile grid from 2004 through 2013 from Vaisala's National Lightning Detection Network. Peak lightning occurrence is along the Mogollon Rim and other higher-elevation areas of the state.

Figure 3. Cloud-to-ground flash density over Arizona on a 2-mile grid from 2004 through 2013 from Vaisala's National Lightning Detection Network. Peak lightning occurrence is along the Mogollon Rim and other higher-elevation areas of the state.

Beautiful Weather

During the course of the year, there is nearly always sunshine in Arizona. Warm winter temperatures below about 3,000 feet attract a large number of visitors and have led to the tremendous growth of the population in the Phoenix and Tucson areas. In summer, nearly the same warm weather and low humidity can be encountered at elevations above 5,000 feet, and has contributed to growth in areas such as Sedona, Flagstaff, Prescott, and other higher-elevation cities. The constants are mild temperatures, low humidity, and sunshine on nearly every day of the year, depending on the altitude.

So for the weather enthusiast, even though many of the extreme events are rare, the variety of weather—from violent flashfloods to stark aridity, from the bitter cold of the mountains to the blazing heat of the deserts, from dusty microbursts to magnificent lightning—makes Arizona one of nature's best natural laboratories for weather. Not surprisingly, many of the truly great meteorologists and climatologists of the past and present, such as Jerome Namias, Carl-Gustaf Rossby, Horace Byers, and Robert Maddox, have made Arizona the focus of some of their landmark research. Arizona remains not only a place of great beauty but also a place of great weather.

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