These are a few paragraphs taken from a blog by Bill Reid, an AMS member. His photograph of lightning near the Hollywood sign I use as my avatar.
An Investigation of Death Valley’s 134°F World Temperature Record
By: Christopher C. Burt , 7:06 PM GMT on October 24, 2016
An Investigation of Death Valley’s 134°F (56.7°C) World Air Temperature Record
In 2012 the WMO (World Meteorological Organization) disallowed what had long been considered the hottest air temperature ever measured on Earth: a 58.0°C (136.4°F) reading measured at El Azizia, Libya on September 13, 1922. As a result of this record being struck from the books, the temperature of 134°F (56.7°C) recorded at Greenland Ranch at Furnace Creek in Death Valley, California on July 10, 1913 became, by default, the new world’s hottest air temperature yet measured. In this guest blog we will investigate the credibility of that measurement. This blog is courtesy of William T. Reid, a geographer and climatologist who has been studying the desert climate of California and, in particular, the Death Valley temperature record for some 30 years. Mr. Reid and I worked together to come to a commensurate conclusion regarding the validity of this significant planetary weather record: It is possible to demonstrate that a temperature of 134°F in Death Valley on July 10, 1913, was essentially not possible from a meteorological perspective, using an officially sanctioned USWB shelter and thermometer and following proper procedures observationally. Thus, the best explanation for the record high report(s) in July 1913 is observer error.
Death Valley is the lowest, driest and, during the summer months, hottest location in the United States. At its lowest spot, Badwater Basin, the altitude stands at 282 feet below sea level. Some 15 miles north of Badwater and about 100 feet higher is the Furnace Creek oasis and resort. A weather station was established here under the aegis of The USWB (U.S. Weather Bureau) in June 1911, and named ‘Greenland Ranch’. The ranch was developed by the William Tell Coleman Borax Company in 1883, and was named such for the alfalfa fields planted for the mule teams used to transport borax and for feed for other ranch animals consumed by the miners, ranch hands and visitors. In 1934, the oasis area became more commonly known as ‘Furnace Creek Ranch’ and Death Valley became a National Monument. The U.S. Park Service established a weather station in 1934 at their Cow Creek headquarters, three miles north of Furnace Creek. Both Cow Creek and Greenland Ranch stations closed in 1961 when the Park Service set up a new cooperative weather station, again under the aegis of the USWB, at the Death Valley National Park Visitors Center where it remains to this day. This new station, named ‘Death Valley’, is approximately 1000 feet (300 meters) north of the former Greenland Ranch.
The focus of this investigation is the unprecedented temperature record of July 7-14, 1913 when the USWB COOP observer at Greenland Ranch, Oscar Denton, measured a string of abnormally hot days. Maximum temperatures for July 7-14, 1913, were: 127°, 128°, 129°, 134°, 129°, 130°, 131°, and 127°F respectively. Minimums were near-to-slightly above average for this 8-day period, ranging from 85°F to 93°F. Each daily maximum temperature from July 7 to July 14, 1913 equaled or exceeded all other maximum temperature values at Greenland Ranch for the entire 50-year period of record (1911-1961), aside from a questionable maximum of 129°F in July 1960.
The Instrumentation and its Exposure in 1913
The location for the instrument shelter (a standard Stevenson screen), first installed by the U.S. Weather Bureau in 1911 at Greenland Ranch, was “carefully selected” according to George H. Willson (see Monthly Weather Review, June 1915). Mr. Willson was, at that time, the district forecaster and section director for the U.S Weather Bureau (USWB), Department of Agriculture, San Francisco station and was responsible for climate data in Death Valley during its early years. The shelter housed standard USWB maximum and minimum thermometers, and was well exposed to wind. Structures and trees at the ranch were of sufficient distance to disallow any very localized build-up of hot air around the station during bright sunshine. It was placed “over an alfalfa sod”, and “the location is such that the shelter is not exposed to the reflected heat from the desert.” Willson “italicized” the above quote for emphasis. Willson continued:
“Evaporation is excessive in this section and liberal irrigation is necessary to maintain plant life; hence, the cooling by evaporation from the surrounding damp ground and live vegetation is probably sufficient to lower the readings of the instruments several degrees. Undoubtedly the temperature down in the desert bottom of the valley is much higher than it is at Greenland Ranch.”
About 100 acres of land here was irrigated for the alfalfa crop and other fruits and vegetables. Willson made it clear that the new station was in a spot that was cooler, perhaps significantly cooler, than the rest of the basin bottom. This conservative siting (with regard to daytime temperature measurement) might not have been welcome by one of the earliest Greenland Ranch COOP observers, Oscar Denton (and may have influenced his observations). In a Historic Resource Study by Linda Greene (1981, see references) it is stated that by the late 19th century temperatures at the ranch “ranged from eight to ten degrees cooler than elsewhere in the valley due to the presence of water, shade trees, and grass in the area.”
Thermometer shelters were routinely placed above grass during the first part of the 20th century, even in extremely arid areas where grass was scarce. It was thought important to have consistent environments around and below the shelters to allow valid and meaningful temperature comparisons from station to station. It goes without saying but needs to be emphasized: a desert weather station above grass is representative of a desert area above grass, and it is not representative of a desert area above a typical, mostly barren, desert surface.
Meteorology of Summer Heat Waves in the Death Valley Region
At near 36°N latitude, Death Valley is ideally situated geographically for hot weather during the summer months. At this latitude, subsiding air and clear skies dominate (associated with the northern fringe of the planet’s Hadley cell), and the vast majority of summer days are clear or mostly clear in Death Valley. During the late spring and early summer the jet stream and the associated ‘westerlies’ migrate poleward. With this cooler mid-tropospheric flow shunted well to the north, warm air is allowed to build aloft over the Desert Southwest and the Death Valley region. From June to August monsoon-related moisture and thunderstorms are common in the Desert Southwest, sometimes impacting Death Valley. This weather regime is associated with lower maximum temperatures due to cloudiness, precipitation, and general evaporative effects such as storm outflow. However, Death Valley is far enough north and west to avoid most of the monsoon activity that migrates from the Sonoran Desert. Because of the prevailing subsidence, coupled with the high terrain on all sides of Death Valley, it is difficult for low-level moisture to reach the depression itself. Normally, any low-level moisture mixes out into the drier upper levels of the atmosphere during daytime convection. The high mountain ranges to the west, especially the Sierra Nevada and Panamint Range, effectively cast a cool-season (October-April) ‘rain-shadow’ over Death Valley, so Pacific storm systems usually sweep through with little more than wind and clouds. On average, the basin of Death Valley receives only 2.0”-2.5” inches of rain annually, with less than an inch on average due to monsoon and tropical-related activity from May through September.
Death Valley’s barren surface is typically bone dry in summer, and it warms easily under the high desert sun. Surface temperatures of 200°F have been measured, as was the case on July 15, 1972 when the ground surface temperature reached 201°F; at the same time the shelter air temperature stood at 128°F. Since little solar radiation is utilized for evaporation and transpiration, nearly all ‘incoming’ solar radiation (that is not reflected skyward) heats the ground. Intense daytime heating of the surface creates a relatively hot, near-ground layer of air characterized by very steep lapse rates (a ‘super-adiabatic’ layer). This creates a very unstable environment and a strong vertical exchange of air is required to mitigate and regulate such. By early afternoon any vestiges of overnight cooling have ‘mixed out’ entirely, and there is deep mixing throughout the lower half of the troposphere throughout the region. Above the near-ground ‘super-adiabatic’ layer, the environmental lapse rate is at (or very near) the dry adiabatic lapse rate of 5.4°F per 1000 feet. The unconditionally unstable, deep, mixed layer typically extends up to about the 600-millibar level (about 15,000 feet above the bottom of Death Valley) on nearly every summer afternoon.
A consequence of the deep, mixed layer is a virtual connection between its top and its bottom. Once established and maintained, the entire column warms (or cools) as an entity. Any significant increase in ambient temperature at shelter level (1.5 meters above the ground) would be, and must be, associated with a similar degree of warming of the entire air column. Thus, the temperature typically changes little at desert stations during the hottest hours of the afternoon. Arnold Court, an expert desert climatologist for the U.S. Army in the 1940s (see references), found that shelter temperatures remained within 2 or 3 degrees (F°) of the maximum temperature from about 1:30 p.m. to 5:30 p.m. in Death Valley on July afternoons. The physics of the deep, mixed layer does not allow for the development of area-wide ‘hot spots’ within a region, and it does not allow significant afternoon temperature ‘spikes’ to occur. What it DOES allow is a fairly predictable pattern of temperature in the desert both diurnally and spatially. It promotes a strong correlation, on a regional scale, between daily maximum temperature and elevation.
Llewelyn Williams, a geographer with U.S. Army Natick Laboratories, examined the conditions favoring high surface temperatures at Yuma, Arizona, and determined that the key to high ambient air temperatures is “warm air between 5,000 and 14,000 feet and a well-developed vertical exchange induced during afternoon convection” (1967). The study also found that “there exists an upper limit to what the combination of radiation and ground surface temperature can do in developing high ambient air temperatures”.
Continue reading: https://www.wundergr...perature-record