current rate of progress, 2017 will turn out to be the second
hottest year since 1880, when the recording of global temperatures
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.
far this year world records have been broken in both high
temperature and low humidity.
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.
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.
days are more bearable if the nights are significantly cooler.
Indeed that differential is often what determines the existence of a
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
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.
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.
readers in the Middle East, when the temperature is in the middle
40s during the summer, the typical 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'.
humidity, 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
air can carry more water vapour than cold air so using relative
rather than absolute humidity allows a comparable figure at any
verified by Maximiliano Herrera.
thanks to Jeff Masters et al at Weather Underground.