Christopher C. Burt ·

Above: Death Valley experiences temperatures among the hottest on Earth almost every summer. July 2017 was its warmest single month (average monthly temperature) since records began in 1911. Image credit: Brocken Inaglory/Wikimedia Commons.

Sustaining extreme heat for an entire month is a more impressive feat than doing so for just one day. This past July, Furnace Creek station at Death Valley, California, measured an average monthly temperature of 107.4°F—the hottest single month ever reliably measured anywhere on Earth.

Average monthly temperatures are typically calculated by adding the highs and lows for each day of the month, then dividing by 2 and dividing by the number of days in the month. A more precise method is to include every hour’s temperature reading and to divide accordingly. By using this technique, which is applied by the Global Historical Climatology Network (GHCN), the Death Valley average for July 2017 was 107.24°F. (Note that conversions from degrees Fahrenheit to Celsius, and vice versa, may have a small effect on the results from Death Valley.)

Various media sources, including USA Today and the Washington Post, reported the Death Valley reading as being the second-hottest monthly average ever observed on the planet, behind the 107.44°F reported at the King Khaled Military City site in northern Saudi Arabia during August 2014. New evidence clearly shows the value from King Khaled to have been in error. Thus the Death Valley figure from this past July is, in fact, the warmest single month (average monthly temperature) reliably measured to date in the world.

The problem with observations from the King Khaled site

Brian Brettschneider (International Arctic Research Center, University of Alaska Fairbanks) discovered this week that during each day of August 2014—and, in fact for many other months and years around this time—the King Khaled Military City site was recording temperature data only once per hour, and only between the hours of 9 am and 9 pm local time.

Below is the site’s raw temperature file, hour by hour, for the first two days of August 2014 (the same diurnal measurement pattern continued all month).

Figure 1. Hourly temperature data for King Khaled Military City, Saudi Arabia, for August 1-2, 2014. Image credit: Brian Brettschneider, data from GHCN.

Note that each day’s initial report begins at 9 am, and then the final daily report is at 2100Z (9 pm). Thus, the daily average temperature (from which the monthly average temperature was derived) consists only of daytime readings. Obviously, this means that the cooler nighttime temperatures were not incorporated into the data set, and hence the GHCN came up with the erroneous figure of an average monthly temperature for the site in August 2014 of 107.44°F.

Comparing the King Khaled site to surrounding sites from the GHCN database, we can see how this issue of daytime-only readings erroneously boosted the site’s average monthly temperature in August 2014 relative to nearby sites in Saudi Arabia and the northern Gulf region.

Figure 2. Map of temperature averages for August 2014 across the Persian Gulf region. The King Khaled Military City report of 107.4°F is inconsistent with other neighboring reports. Image credit: © Brian Brettschneider; data from GHCN.

Below is a table of all the sites in the world that have measured a monthly average temperature of at least 105°F. Death Valley’s average from July 2017 holds the #2 spot, right behind the questionable King Khaled report.

Figure 3. All sites in the world that have reported months with an average temperature of at least 105°F. Nearly all of the top readings come from Death Valley or nearby locations. Note that only six of the dozens of examples above (including the discredited King Khaled average of 107.44°F) were reported in August. Desert climates typically experience their hottest readings around and for several weeks after the summer solstice (or during late June and July in the Northern Hemisphere), so the hottest single months on Earth would more likely be in July as opposed to August. Image credit: Brian Brettschneider, data from GHCN.

From the information above, it is reasonable to assume that this past July’s figure from Death Valley is indeed a new global record for hottest average monthly temperature. In fact, Death Valley now holds the top nine spots on the list!

Caveat: Using the GHCN for extreme weather event research

There is a lesson to be learned here: using datasets from the GHCN for specific extreme temperature events—daily, monthly, or otherwise—should be approached with caution. The GHCN database is composed of literally millions of figures, and overall it is the best, most accurate source of temperature data available (the data is correct 99.9% of the time). The GHCN is used routinely and successfully for examining such macro-climatic issues as temperature trends and climate change. That being said, the accuracy of this database should not be taken for granted in verifying specific one-off extreme events without proper vetting. We should never assume that any one report or series of reports in the GHCN database is necessarily spot-on.

Kudos: Thanks to Brian Brettschneider (@climatologist49) for the information concerning temperature data from King Khaled Military City.