The previous record holder was Sept. 13, 1922, in El Azizia, Libya, with the temperature of 136.4 degrees F. But according to the World Meteorological Organization (WMO), an agency of the United Nations (which also comes up with the annual list of hurricane names), El Azizia is the heat champion no longer. Following an in-depth investigation into the accuracy of the measurement, WMO awarded the record to Death Valley, Calif., which had been in second place with 134 F recorded on July 10, 1913.
Just how do you investigate century-old temperature readings? The investigation by an international panel of experts began in 2010, sparked by a post on the meteorological blog Weather Underground. According to the WMO committee, members of the Italian military who took the Libyan measurement in 1922 (present-day Libya was controlled by Italy at that time) used an outdated thermometer called a Bellani-Six. It was a household instrument used as a last-minute replacement for the broken official thermometer. The Bellani-Six used alcohol as well as mercury; it had to be reset each day with a magnet. It was abandoned as a "serious meteorological instrument" in the mid-1800s.
No one knows who made this 136.4 degrees Fahrenheit reading. But whoever took the measurement was probably inexperienced, Randy Cerveny, WMO?s Rapporteur of Weather and Climate Extremes, tells PM. For one thing, temperatures for the month of September 1922 are recorded in the wrong columns of the logbook, indicating that a new person unfamiliar with the system had taken the job. Plus, with such a spike in daily maximum temps, you?d expect the daily minimum to be proportionally higher, since there are only so many hours in the day for the air to cool down again. But the daily low temps at El Azizia stayed close to average that September. The Italian officer may have misread the high temp but read the low correctly. The WMO panel estimates the actual temperature that day at about 51 degrees Celsius, or 123.8 F.
How can they be so sure about one particular day more than 90 years ago? Cerveny says the team used data from the 20th Century Reanalysis Project, published in 2011, which fused reliable sea-surface-temperature data with European, Mediterranean, and some African surface-pressure data. By doing so, scientists reconstructed daily air temperatures for every six hours from 1871 to the present at intervals of 2 degrees latitude and longitude.
"We didn?t have the technology to do this kind of investigation until very recently," Cerveny tells PM. This re-analysis told them something was amiss?this Libyan location would have been scorching hot, but not world-record hot. The Reanalysis data indicates an average temp of 87.8 F for 13 September 1922 at El Azizia, making a high of 136.4 F virtually impossible.
As for Death Valley, U.S. government surveyors travelling by camel reported the earliest weather observations there in the 1860s, according to a comprehensive report on the history of the valley in the Journal of the American Meteorological Society. But the period of official record doesn?t begin until 1911, when trained observers at a permanent weather station at Greenland Ranch, now called Furnace Creek, took daily maximum and minimum temperatures with two separate thermometers, each with a slide inside the tube. This was the standard for official measures at the time.
"The practices and instrument used by the Death Valley observer were substantially better than the new inexperienced person at El Azizia and the replacement thermometer that he was forced to use," Cerveny says. In fact, the Death Valley thermometers were only graduated up to 135 F. A note accompanying the record reading suggests that the actual temperature was even higher than 134 F, "because other ordinary thermometers at the ranch showed a much higher temperature."
Since those days, however, the valley has never again reached 130, Death Valley National Park wilderness coordinator Charlie Callagan says. But now, the valley has a world record to celebrate, even if it has been nearly a century in coming. "I expect Death Valley will be printing out new T-shirts now," Cerveny says.
The change in ranks comes just after the warmest July on record in the contiguous United States, according to NOAA. But even with its extreme droughts, 2012 came nowhere near the heat Death Valley saw 99 years ago.
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