In Iran it was 115 degrees Fahrenheit today (46 C). Add in humidity and the heat index was a stunning 165 F (74 C). But what they really should be concerned about is the wet bulb reading…

A Limit to Human Heat Endurance

Thirty five degrees Celsius. According to recent research, it’s the wet bulb temperature at which the human body is rendered physically unable to cool itself in the shade. At which evaporation not longer cools the skin. A temperature that results in hyperthermia, heat exhaustion and heat stroke — even when sitting still and out of direct sunlight over the course of about 1-3 hours. Basically, it’s the physical limits of human heat endurance.

The primary factors involved in determining wet bulb temperature are atmospheric temperature and humidity. The temperature of an air parcel cooled to saturation (100 percent humidity). Basically, it’s the coolest temperature human skin is able to achieve by sweating.

One of the reasons why high heat and high humidity seem so oppressive is the fact that it interferes with water evaporating from your skin keeping your body at its natural temperature (98.6 F). High heat + high humidity means less cooling at skin level, which can result in a pretty rapid over-heating. We’ve all experienced it, that sense of stifling on a hot, muggy day. And there’s a bone-deep reason why it feels so bad. Hit a too-high intensity and it’s a killer.

(An oppressive heat dome high pressure system settles in over the Persian Gulf. Image source: Ryan Maue.)

At 47 percent relative humidity and 115 degrees Fahrenheit, it felt like 165 degrees (F) today in Bandar Mahshahr, Iran. That’s a wet bulb temperature of 34.7 C. A temperature near the edge of human limits and the second highest heat index value ever recorded in any official or unofficial measure (the highest unofficial measure was 178 F). It’s the kind of heat that is, quite frankly, deadly.

Heat Dome Settles Over Persian Gulf, Sea Surface Temperatures Spike

Bandar Mahshahr sits at the Northern end of the Persian Gulf. A region of water that features some of the highest sea surface temperatures on Earth. Over the past week, an oppressive heat dome high pressure system began to settle over the region. Air temperatures around the Gulf hit well above 110 F in many locations. In Baghdad, they soared to 122 degrees F (50 C). Yesterday and today, the sea surface temperatures also sweltered — ranging as high as 34.6 C (94 degrees F).

Since ocean surface temperatures produce latent heat and determine the maximum moisture loading of the Earth atmosphere, maximum sea surface temperature is a good basic yardstick to determine if surface wet bulb temperatures are capable of hitting or exceeding the human survivability threshold at 35 C. And what we are seeing is that the near Persian Gulf region is steadily entering this dangerous range.

(Sweltering sea surface temperatures like those now visible in the Persian Gulf can support heatwaves that the human body did not evolve to endure. Image source: Earth Nullschool.)

As the heat dome continues to settle in over the next week, there is increasing risk to the people living in the Persian Gulf region. Some have access to cooled shelters, life saving ice and water. But many do not. At particular risk are the over 3 million Iraqis displaced by the violent conflict wracking that fractured state. Chronic electricity and water cuts throughout the region also lends to the overall vulnerability. It’s a current crisis. But it is one that occurs in an overall worsening context.

As the world’s oceans continue to be warmed by heat trapped through human greenhouse gas emissions, sea surface temperature thresholds will be driven inexorably higher. The potential moisture content in the near surface atmosphere will rise and so will temperatures. This will increasingly generate heatwaves which the human body simply does not have the physical capacity to endure. Overall, this is one of the reasons we see more mass casualty events as a result of heatwaves — like the events occurring this year in Pakistan and India. It’s a case of pushing the atmospheric heat and moisture loading beyond human survivability thresholds. And we’re steadily doing that now. Let’s hope that this week’s Persian Gulf heatwave doesn’t add another hothouse mass casualty event to the growing list.

Links:

Physical Limits to Adaptation

Middle East Broils Under Extreme Heatwave

Iran City Hits Suffocating Heat Index

Earth Nullschool

Pakistan Heatwave Mass Casualty Event

India Heatwave Mass Casualty Event

Hat Tip to Robert in New Orleans