Weather Almanac for July 2009

DEATH VALLEY: THE HEAT IS ON

Climatologically, the hottest temperatures in the United States occur during the month of July. And within this large and meteorologically diverse nation, the acknowledged hot spot is found in Death Valley California. Indeed, the hottest temperature ever recorded in the US and North America occurred at Death Valley’s Greenland Ranch (now the site for the Furnace Creek Inn) when the mercury sizzled at 134^oF (56.7^oC) on 10 July 1913. The temperature that day set the record for hottest temperature ever recorded on the planet. This reading was not surpassed until the thermometer peaked at 136^oF (45^oC) at Azizia, Libya on 13 September 1922. It still ranks third in global heat records.

Death Valley National Park as compiled from Enhanced Thematic Mapper Plus
sensor on the Landsat 7 satellite on June 11 and July 20, 2000.
Photograph courtesy NASA

The legends of the West include this dry valley, and while gunslingers lead the list of the infamous killers of the American Southwest, the Death Valley terrain feature names echo the trials and tribulations of those who tried to cross the region or mine its resources. They include: Funeral Mountains, Coffin Peak, Hell's Gate, Starvation Canyon and Dead Man Pass. J. Ross Brown, who authored the first mineral resources report on the region for the American government, commented on the summer heat: “...[P]erhaps fastidious people might object to the temperature in summer . . . . I have even heard complaints that the thermometer failed to show the true heat because the mercury dried up. Everything dries; wagons dry, men dry, chickens dry; there is no juice left in anything living or dead, by the close of summer.”

The region reportedly got its name when a group now known as the Lost Pioneers of 1849 struggled out of the valley after attempting to follow a shortcut off the Old Spanish Trail out of Salt Lake City. The group of about 20 wagons became lost and wandered through the desert for two months unable to find a pass out of the valleys. Forced to abandon their wagons and belongings, they attempt to walk out of the desolate landscape from a place today known as “Burned Wagons Camp” located near the sand dunes of Death Valley. There, they killed their oxen and burned their wagons to dry the ox meat for jerky. The party then headed toward Towne Pass before turning south over Emigrant Pass to Wildrose Canyon. As they left the valley, legend has it that one of the women in the group turned and said “Goodbye Death Valley” and thus the valley got its name. Only one in the group, an elderly man, actually died in the Valley. The group was, in fact, fortunate, they had travelled across the valley in December rather than in the searing heat of summer. The ordeal actually became worse as the party crossed the Mohave Desert Plateau, saved from dying of thirst by puddles of water and ice. The Lost ‘49ers were eventually rescued by Spanish cowboys near present day Newhall, California

Among the memories of my youth associated with the area are the television program Death Valley Days, hosted at one time by future US President Ronald Reagan and the song “Mile-Long Train” (though the account of the Lost 49ers is not correct) by country star Jimmy Dean. Say the name, and images of hot, dry miles ahead of a lost traveller appear in my mind.

Location, Location, Location

Death Valley, covering about 3000 square miles (7800 sq km), is a unique place and that uniqueness contributes to the stifling heat experienced at this legendary location. DeathValley sits in southeastern California (Inyo County) near the Nevada border in the Great Basin and the Mohave Desert, southeast of the Sierra Nevada mountains The valley runs north-south between the Amargosa_Range on the east and the Panamint Range on the west; the Sylvania Mountains and the Owlshead Mountains form its northern and southern boundaries, respectively. Currently Death Valley National Park with headquarters at Furnace Creek Ranch encompasses a large part of the valley.

High and low points in Death Valley National Park, Diagram Courtesy Wikipedia

The Sierra Nevada range contributes to the valley’s brush and scrub desert found by stealing the last bits of Pacific moisture headed eastward into the Great Basin. In the rain shadow of this range, only a couple inches of rain falls annually, mainly during the winter.

Another factor contributes to its uniqueness of the valley: its elevation. Within Death Valley in the Badwater Basin, lies the lowest spot in North America, 282 ft (85.5. m) below sea level. (For trivia buffs, this point ironically is 76 miles (123 km) east of Mount Whitney, the highest point in the contiguous US.)

This section of the valley held a huge lake nearly 100 miles (160 km) long and 600 feet (183 m) deep during the Pleistocene ice age, known to geologist s as Glacial Lake Manly. Today, part of this ancient lakebed will on rare occasion be the site of Lake Badwater. Usually the former Lake Manly is a dry, salt-coated playa, but between 1 July 2004 and 30 June 2005, the valley accumulated around 6.44 inches (163.5 mm) of rainfall, about three times the 1971-2000 Climate Normals average of 2.33 inches (59.2 mm) at the official climate station at Furnace Creek and nearly four times the annual average between 1911 and 1960 of 1.6 inches (41.0 mm) as measured at the Greenland Ranch Station. The resulting runoff filled the Glacial Lake Manly depression with a wide, but shallow lake: Lake Badwater. As the accompanying Landsat 5 photographs below show, in February 2005 a greenish lake covered much of the basin. Two years later, however, it was gone, leaving a coating of recrystallized salt behind.

Lake Badwater, Death Valley on 9 February 2005	Lake Badwater, Death Valley on 15 February 2007
Landsat 5 Satellite Photos Courtesy NASA

The Climate and Weather of Death Valley

From 1911 to the present, the main climate station has essentially been located at Furnace Creek (named in the climate records as Death Valley), though the exact present location has only been in use since 26 April 1960. Prior to that time the instruments were located at Greenland Ranch, a site very close to the current one. From 1 July 1934 to 31 December 1961, the official park climate station was at the Cow Creek Park Service Headquarters just north of the current climate station location. Normals quoted herein will be for the 1971-2000 period at Furnace Creek unless otherwise specified. Generally, extreme events quoted in this report occurred prior to 2003 which was the end of the period studied by Steven Roof and Charlie Callagan (The Climate of Death Valley, California, Bulletin AMS, 2003). A few instances between then and 2008 have been included, but I did no comprehensive study of the data to check for all extremes.

Death Valley is renown for its heat and aridity, ranking as one of the combined hottest and driest places in the world on an annual basis, although it is not the hottest and driest US location for every month of the year. The Normals mean annual temperature is 76.0^oF (24.4^oC) with a mean maximum temperature of 90.2^oF (32.3^oC). at the Furnace Creek climate station.

By comparison, the Normals mean annual temperatures at Miami Beach, Florida is 75.9^oF (24.4^oC) and Key West, Florida is 77.8^oF (25.4^oC), but their mean annual maxima are 80.3^oF (26.8^oC) and 82.9^oF (28.3^oC), respectively.

The winters are mild with average temperature of around 54^oF (12.2^oC), the average maximum temperature in December—January is 51.7^oF (10.9^oC). In contrast, the mean summer temperature averages at 98^oF (36.7^oC) with July averaging 100.6^oF (38.1^oC). Daily maximum summer temperatures average 112^oF (44.4^oC).

Death Valley Monthly Temperature and Precipitation Normals
Table Courtesy Western Regional Climate Center

For precipitation, the Normals annual precipitation total is 2.33 inches (59.2 mm) at Furnace Creek, making it the nation’s driest spot. The 1911-2002 average was 1.9 inches per year (4.8 cm/yr). (Less likely falls in other areas of the valley. Some climate reports show total rainfall at nearby Cow Creek averaging less than 2 inches (50.8 mm) annually.)

In recent years, the region has received more precipitation than earlier years within the climatic record. For the first 65 years in the climate record, the annual precipitation total averaged about 1.6 inches (40.6 mm). The thirty years of the most recent Normals has seen an increase in average precipitation to 2.33 inches (59.2 mm) at Furnace Creek.

Of the Normals annual total, 1.37 inches (34.8 mm) falls between 1 December and 31 March. This winter rainfall originates in storms coming off the Pacific Ocean onto the California coast. In summer, such storms are blocked, and rainfall originates from the Arizona, or Southwest, Monsoon mostly falling from thunderstorms. Much of the area’s rainfall comes during intense single events lasting about a day. The total precipitation falling on the wettest day of the year generally contributes about a third of the annual accumulation. Only 18.1 days per year on average see more than a trace of precipitation fall and only 6.4 days accumulating 0.10 inches (2.5 mm) or above.

When intense winter storms hit the region, several inches of rain may fall over the course of few days, heightening the risk of flash flooding in the surrounding canyons. Thunderstorm rains can be hit-and-miss across the valley, drenching one location while other areas stay dry. One example of such an event occurred in September 2000 when 1.5 inches (38 mm) of rain fell at the Cow Creek Administration Office while none fell at Furnace Creek climate station, 3 miles (5 km) to the south.

The aridity of Death Valley results in large part from its location east of four major mountain ranges — the Sierra Nevada Range, the White/Inyo Mountains, the Argus Range, and the Panamint Range — all of which diminish the moisture moving inland from the Pacific Ocean. By the time the maritime air mass reaches Death Valley, very little moisture is left to fall as rain. (These ranges successively deplete winter storms of their moisture by precipitating it out on the ranges’ western flanks as the air ascends over the ridge lines, leaving Death Valley in their rainshadow.) Often what moisture does fall as precipitation from the overlying clouds evaporates on its way down through the dry, hot surface layer.

Sand Dunes north of Stovepipe Wells
Photo Courtesy US National Weather Service Office, Las Vegas

Snow has fallen in Death Valley. The weather observer at Furnace Creek measured 0.5 inches (1.3 cm) of snow on the ground on 10 January 1922. There is a likelihood that snow also fell later that month around the 29th-30th. Local newspapers reported snow in the surrounding communities such as Stovepipe Wells, 16 miles (25 km) west of Furnace Creek, and the low temperatures for the period were 36° and 34°F (2.2° and 1.1°C), respectively. Since then, the official record is a trace of snow on 5 January 1974.

As one would imagine, the humidity levels in Death Valley are generally low, and during the summer, the relative humidity can remain below 30% for weeks at a time. This coupled with the high air and surface temperatures and windy conditions rapidly evaporates any standing fresh water. From standard evaporation pans, the typical summer daily evaporation is determined to be 0.75 inches/day (1.9 cm/day) from May through August with maximum rates at 1.50 inches/day (3.8 cm/day) during the summer. Over the long term (1961-2002), the annual total potential evaporation is 143 inches per year (363 cm/yr). This rate of moisture loss greatly exceeds the average annual precipitation of 1.9–2.2 inches (4.8–5.6 cm) per annum, making the region arid. Standing water and damp mud, however, can exist on the surface on the salt flats in Death Valley because evaporation is hindered by high salt content of the water.

Wind also plays a significant role in the dryness of Death Valley since it is a major component in evaporation. While there have been no long-term direct measurements of wind speed, the climate record does include daily wind movement data. This measure determines the wind movement per day by counting the total distance of wind moving past the anemometer during the measurement period. (Each rotation of the anemometer corresponds to a given “distance” of wind movement, and the meter’s counter clicks off the distance of “wind travel” in much the same way our car odometer counts miles travelled.) Average daily wind movement at Death Valley is lowest during the winter and peaks in early spring. From March to May the average daily wind movement is 250 to 300 miles (200-480 km) per day. Dividing that number by 24 hours gives a rough estimate of average daily wind speed: 10-4 to 12.5 mph.

It is the heat, however, for which Death Valley is best known. In the study by Steven Roof and Charlie Callagan, the authors suggest six likely factors for the Valley’s extreme heat:

• Solar heating of dark, sparsely vegetated surfaces by radiation through clear, dry air. Since the vegetation cover is sparse, little solar radiation is used for evapotranspiration and instead heats the ground and surface air;
• Large-scale air mass subsidence into the low-elevation valley warms the air adiabatically as it descends into the valley;
• The heated air becomes trapped near the surface by inversions forming in the subsiding air mass. This inhibits heat convection upward from the surface;
• The deep north–south orientation of the valley keeps air trapped when west to east winds blow over rather than pushing the hot air out;
• The surrounding desert preheats air moving into the valley where it can warm further.
• The foehn (Chinook) effect heats air moving across the mountains. First the air cools adiabatically as it ascends the western flank of the mountain ridges, Then the moisture within condenses to form precipitation. In this process, the latent heat of condensation warms the air. Some of the moisture precipitates out around the ridge line leaving behind a warmer air mass which then heats adiabatically as it descends from the ridge line to the valley surface.

Meteorologists know that the hottest days usually occur in Death Valley when a high pressure ridge centers over western Nevada. The ridge thus blocks cooler maritime air from pushing east while a thermal low located in southern California directs hot air from the deserts of southern Arizona and Mexico into the valley.

The second point above is particularly important since Death Valley is lower in elevation than most other desert areas in the United States In this process, the air sinking into the valley from aloft is heated adiabatically by compression, and that heat is retained in the air (here adiabatically means “without transfer of heat out of the air mass in question”). Since air heats adiabatically at a rate of about 1 Celsius degree per 100 m, air descending to the floor of Death Valley, about 80 m (262 ft) below sea level, would heat an additional 0.8 C degrees (1.4 F deg) more than a similar air mass descending to a sea-level desert surface. To put that into perspective, nearby Las Vegas has an elevation of about 2160 ft (659 m). If the temperature of the air over Las Vegas was 100^oF (37.8^oC), the same air descending to the floor of Death Valley would heat to 112^oF (44.4^oC)

Death Valley Sand Dunes	Zabriskie Point, Death Valley
Photos Courtesy Wikimedia Commons

The average annual temperature for the 1971-2000 Climate Normals is 76.0^oF (24.4^oC). The warmest month July averages 100.6^oF (38.1^oC) with average high temperature of 114.9^oF (46.1^oC) and average low temperature of 86.3^oF (30.2^oC). The coldest month on average is December at 52.3^oF (11.3^oC) with average high temperature of 65.1^oF (18.4^oC) and average low temperature of 37.5^oF (3.1^oC).

The average number of days with maximum temperature above 100^oF (37.8^oC) in a year is 138. Nearly all days have a maximum temperature above 50^oF (10^oC): an average of 363.9 days per year. June, July, August and September, all have average maximum temperatures which exceed 100^oF (37.8^oC). It is an unusual day in July and August when the peak temperature does not reach 100^oF (37.8^oC). Only December and January have monthly record high temperatures below 90^oF (32.2^oC): 88^oF (31.1^oC) and 89^oF (31.7^oC), respectively.

While heat is the rule in the summer, winters in Death Valley can be much cooler. The December and January average daily temperatures at Furnace Creek are slightly warmer than those registered in San Francisco and cooler than Los Angeles. Daily minimum temperatures drop below 60^oF (15.6^oC) on about half the days in the year and sub-freezing temperatures (32^oF/0^oC) can occur between 1 October and 1 May. The coldest weather generally hits from late December to early January. Only 11.7 days per year see temperatures that drop to freezing or below.

The difference between a day’s minimum and maximum temperatures, the daily temperature range, generally averages around 30 F degrees (17 C deg), a few degrees greater than 30 F deg (17 C deg) in summer, a few degrees less in winter. The largest temperature ranges experienced at Furnace Creek usually fall within the 45 to 50 F deg (25-28 C deg) range. A few days, mostly in the autumn, experience temperature ranges greater than 55 F deg (31 C deg).

Temperatures cool as one rises above the low Death Valley floor by an average of 3-5 F deg (1.7-2.8 C deg) per 1000 ft (305 m) rise in elevation.

View from AguereberryPoint, Death Valley
Photo Courtesy Wikimedia Commons

For The Record Books

The Death Valley heat extremes began filling the record books in 1913, two years after the first permanent weather station was established the Greenland Ranch. On five consecutive days in July, the thermometer reached 129^oF (53.9 ^oC) or above, peaking at 134^oF (56.7 ^oC) on 10 July. That measurement held the global heat record for a decade until it was surpassed by the 136.4^oF (58^oC) recorded at El Azizia, Libya, in the Sahara Desert in September 1922.

(Note that while both of these temperature extremes are officially recognized as the hottest recorded temperatures for the world and for North America, there are some who dispute the validity of the readings on the grounds of the observational method and lack of thermometer calibration after the record reading.)

Since that record year, no temperature measurement has exceeded 129^oF (53.9^oC) in Death Valley. However several readings of 129^oF (53.9^oC) have occurred: 18 July 1960, 17 July 1998, 19 July 2005 and 6 July 2007. There have also been six readings of 128^oF (53.3^oC). Only one year 1960 has failed to register a reading of 120^oF (48.9^oC) or higher. (In 2009, the 120^oF (48.9^oC) mark was reached on 27 June and exceeded the next day.)

Death Valley, however, is not the only location in the United States to register high temperatures of 120^oF (48.9^oC) or above. Ten different states have recorded such highs including the Dakotas. In fact, Death Valley does not hold the national record for highest temperature in the month of June. That honor goes to Volcano in Amador County, California, reached 129^oF (53.9^oC) in June 1902. Death Valley ties for second, a Fahrenheit degree back, with Lake Havasu City, Arizona.

But when it comes to the hottest temperature in the contiguous United States on a daily basis, Death Valley stands out. Steven Dutch of the University of Wisconsin, Green Bay assessed the frequency various locations reported the highest temperature in the nation using the USA Today online database that stretches back to 1995. Dutch found Death Valley, accounted for 1022 high temperature readings or almost 19% of the total.

Summer 1917 saw 52 days when temperatures reached 120^oF (48.9^oC) or above, with 43 of them consecutive. The longest string of days with a maximum temperature of 100^oF (37.8^oC) or above was 154 days in the summer of 2001. The summer of 1996 was another scorcher with 40 days over 120^oF, and 105 days over 110^oF (43.3^oC). July 2005 had 7 consecutive days (July 14-20) with high temperatures equal to or above 125^oF (51.7^oC).

The highest minimum temperatures ever recorded in Death Valley occurred prior to 1937: 110^oF (43.3^oC) in 1918, 1922, 1923, and 1924. Since then the hottest minimum temperature occurred on the night of 23-24 July 2003 when the temperature only fell to 104^oF (40^oC). Death Valley is also the only location in the US (and perhaps one of only two in the world) where minimum daily temperatures have been recorded at 100^oF (37.8^oC) or higher. Minimum daily temperatures of 100^oF (37.8^oC) or higher have occurred in ten of the years since 1937. In all but two years (1911 and 1912) on record, the highest annual minimum temperature has been 93^oF (33.9^oC) or above.

The highest ground temperature ever recorded was 201^oF (93.9^oC) on 15 July 1972. The air temperature at standard thermometer height that day peaked at 128^oF (53.3 ^oC).

The hottest month ever was July 1917 averaging 107.2^oF (41.8^oC); the second hottest, and the hottest of modern record, was July 2005 averaging 106.5^oF (41.4 ^oC).

The coldest month on record was December 1990 at 44.9^oF (7.2 ^oC). The coldest recorded daily temperature was 15^oF (-9.4^oC) on 18 January 1913 (the same year as the record high). During the winter 1928/29, 72 consecutive days recorded temperatures at or below freezing. The lowest summer temperature since 1961 is 54^oF (12.2^oC) on 6 June 1996.

The wettest calendar year in Death Valley climate history was 1941 which saw 4.63 inches (118 mm) accumulate. The most precipitation over a 12-month period fell between 1 October 1977 and 30 September 1978: 6.40 inches (162 mm). Second was the period July 1977 to June 1978: 5.09 inches (129 mm). The wettest month was January 1995 when 2.59 inches (66 mm) accumulated. The wettest day was 15 April 1988 when 1.47 inches of rain fell. The only other day on record to exceed an inch of rain was 26 September 1997.

Death Valley precipitation records include calendar years with no precipitation: 1929 and 1953. A string of 40 months from 1 March 1931 to 30 June 1934, recorded a total of only 0.64 inches (16 mm).

Last Words

Despite the intense heat of Death Valley, the national park hosts over a million visitors each year for camping, sightseeing, biking, hiking and star-gazing. Recently, visits have been increasing during the summer months as folks come to experience the unique summer conditions. Death Valley National Park is one of the largest protected desert wilderness areas in America with a many unique flora and fauna.

Telescope and Wildrose Peaks as seen from Emigrant Canyon Rd, Death Valley
Photo Courtesy Wikimedia Commons

If you are interested in exploring Death Valley, the US Park Service has an online guide to the park available by clicking here.

Learn More From These Relevant Books
Chosen by The Weather Doctor

• Heidorn, Keith C.:And Now...The Weather, 2005, Fifth House, ISBN 1894856651
• Burt, Christopher C.: Extreme Weather: A Guide and Record Book, 2007, W. W. Norton & Company, ISBN 039333015X, paperback.
• Mogil, H. Michael: Extreme Weather: Understanding the Science of Hurricanes, Tornadoes, Floods, Heat Waves, Snow Storms, Global Warming and Other Atmospheric Disturbances, 2007, Black Dog & Leventhal Publishers, New York, ISBN 1579127436, Hardcover.

Written by
Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
July 1, 2009

The Weather Doctor's Weather Almanac: Death Valley: The Heat Is On
©2009, Keith C. Heidorn, PhD. All Rights Reserved.
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