Halfway through the year and we have already set new heat records

At current rate of progress, 2017 will turn out to be the second hottest year since 1880, when the recording of global temperatures started.

Last year was boosted a little by El Nino, which would make it even more remarkable were 2017 even to be ranked second because El Nino has gone for now.

So far this year world records have been broken in both high temperature and low humidity.

The heatwave in May that covered northern India, central Pakistan and eastern Iran created a new world record. Turbat in Pakistan recorded 53.5C on May 28, the new highest May temperature in the 137-year continuous list. It was also the highest temperature recorded for any month in Pakistan.

Then, on June 20 in Death Valley, California, the thermometer read 52.5C, making it the highest temperature measured in the Western Hemisphere so early in the year.

Hot days are more bearable if the nights are significantly cooler. Indeed, that differential is often what determines the existence of a deadly “heatwave”. Temperatures this high are rarely accompanied by cool enough nights and on June 17, Khasab in Oman set another world record: 44.2C became the highest night minimum temperature on record.

From a human point of view, such hot weather is only survivable if the humidity of the air is low enough for the body to cool by evaporation of sweat. Luckily for us, this always the case on this planet, in normal circumstances. Sometimes the humidity is extraordinarily low, as it was this month in Iran and the desert US.

Safi-Abad Dezful in Iran measured less than 0.4 percent relative humidity on June 27 with a temperature of 46.5C. This effectively ties with Needles, California for the lowest known relative humidity reading on earth. The California reading was made in May 2014 but on June 20 this year Needles was as dry as 0.8 percent.

For readers in the Middle East, when the temperature is in the middle 40s during the summer, the typical relative humidity (RH) is around about 10 percent. That is why it is possible to be outside even though that heat is above body temperature. Perspiration works to keep us cool enough in those conditions. This is often called “dry heat”.

RH is the measure of how much water vapour is in the air compared with how much would be needed to saturate that air and form fog.

Hot air can carry more water vapour than cold air, so using relative rather than absolute humidity allows a comparable figure at any temperature.

Figures verified by Maximiliano Herrera.

With thanks to Jeff Masters et al at Weather Underground.