Hottest “reliably measured” temperature expected at Death Valley

California’s Death Valley is expected to exceed the hottest “reliably measured” temperature this week, as an ongoing heat wave grips the southwestern United States.

Temperature gauges at Death Valley are expected to reach 55°C.

Temperature records have already been set in recent days as the northern summer last week led to the world’s hottest day on record on Monday. That record was then broken the following day. And broken again the day after that. The next day? You guessed it: record broken yet again.

In fact, after the world’s hottest day record was broken four days in a row, last week was confirmed by the World Meteorological Organization (WMO) as the hottest week on record.

Sweltering conditions continue over much of the northern hemisphere. Over the weekend, the EU’s Copernicus Sentinel satellites measured ground temperatures in Spain at 60°C, near Seville.

But if that’s the case, how come we are talking about Death Valley potentially breaking the record?

It comes down to how temperature is measured.

Essentially, air temperature – the temperature that is given on weather forecasts and reports – is measured in the shade between 1.2 and 2 metres above ground (either grass or dirt). This prevents several factors from increasing the temperature measured by thermometers.

Today these thermometers are electronic, replacing thermometers which used liquids such as mercury or alcohol for safety reasons.

The electric thermometer is placed in a white box which resembles an artificial beehive. The box provides shade which prevents the thermometer from heating up from radiation from the Sun. The box is painted white to reflect as much light from the Sun as possible.

Airflow through the box is provided by air holes. The box is elevated to prevent additional heat permeating from the ground which acts like a heat sponge. This is why the ground temperature in Spain reached 60°C, but the air temperature was “only” 41°C.

In fact, even higher ground temperatures have been recorded. We again go to Death Valley to find this record – a ground temperature of 93.9°C was measured at Furnace Creek in Death Valley on 15 July, 1972.

Death Valley is known for being the hottest place on Earth. It’s desert climate and sheltered location nearly a hundred metres below sea level mean its summer air temperatures often reach 50°C.

As I pointed out in a recent podcast, I visited Death Valley in 2014 when the temperature was a balmy 52°C. Being the Mediterranean goth that I am, I braved that heat in long-sleaves and pants. While sun smart, it was not heat smart.

According to the Guinness World Records, the hottest air temperature recorded is 56.7°C – again, at, you guessed it, Furnace Creek, Death Valley. That record, however, established on 10 July 1913 does raise questions of accuracy given the much more advanced means for measuring temperature in use today.

In fact, another claim for the hottest air temperature (57.8°C) measured on 13 September 1922 in El-Azizia, Libya was stripped of the title because of questions over accuracy.

A 2012 investigation by the WMO concluded that the El-Azizia temperature could be up to 7°C off in accuracy due to multiple factors including the fact that the surface over which the measurement was taken was asphalt-like.

WMO member and geography professor at Arizona State University, Randy Cerveny, said in 2012: “We accept that [1913] Death Valley temperature extreme record. Obviously if any new materials on it surface, we will be prepared to open an investigation, but at this time all available evidence points to its legitimacy.”

Hence, if Death Valley does creep over 55°C in the coming days, it will be the hottest reliably measured air temperature recorded on Earth.