On Friday, Bandar Mahshahr, Iran registered an air temperature of 115 degrees and a dew point of 90, an extraordinarily rare combination of heat and humidity. The resulting heat index – a measure of what the air feels like – hit 165 degrees, the second highest we have ever seen reported, although official records for heat index are not maintained.

What does a heat index that high even mean and what does it feel like?

To begin, a heat index of 165 is, at best, a crude estimate. The heat index, developed by R.G. Steadman in 1979 is actually only designed to compute values up to about 136 degrees – which can be attained when the air temperature is 110 degrees and the dew point is 80.

For heat and humidity conditions beyond that, the heat index is off-the-charts. Under such circumstances, you can plug the temperature and humidity level (relative humidity or dew point) into an equation to compute a heat index, but the result “is not valid” the National Weather Service says.

So when it’s so hot and humid you can’t really even compute a heat index anymore, what does it feel like? For example, what’s it like to live in Dhahran, Saudi Arabia, where on July 6, 2003 the temperature was 108 degrees with a dew point of 95 (producing a “not valid” heat index of 178 degrees) – an incomparable combination?

Capital Weather Gang reader, John Hagner, who lived in Dhahran for several years starting in 1992, shared his experience via email:

When the winds come off the Persian Gulf you just can’t imagine how awful it gets.
On the hottest and most humid days, you’d walk outside and it felt immediately like someone pressed a hot wet towel, like you sometimes get on airplanes, over your entire head. I wear glasses, and they’d immediately fog up. You sweat instantly. People just avoid being outside in any way they can. In the summers, my friends and I would become nocturnal as a way to beat the heat. Crime is basically non-existent, so my parents didn’t worry about us being out all night. I’d usually have breakfast with my dad and then sleep through the heat of the day, waking up when he got home from work. At night it was still stifling, but the edge was off.
Air conditioning is everywhere. You can trace the population explosion in the country directly to the advent of air conditioning – it allowed people to settle down and stop living the nomadic life that was common into the middle of the 20th century. We lived on a compound for employees of the Saudi national oil company, and they treated air conditioning repair like ambulances or fire trucks – they had crews on 24-hour call, and you could have them dispatched at a moment’s notice by calling the special air conditioning emergency hotline. In the summer, the air-conditioned school buses would stop outside every individual kid’s house, so they didn’t have to wait at a stop and could stay in the AC. Off the compound, air conditioning is still common, even for the poorest migrant workers there. Shopping was done in huge air-conditioned malls. The great open-air souks operate in the winter or very early in the morning on summer weekends

Another reader, John Freivalds, who spent two summers in Andimeshk, Iran, about 100 miles north of Bandar Mahshahr, described his experience, also via e-mail:

It was so hot that when we laid clothes out to dry you could finish hanging and the clothes would be dry.
Before air conditioning the locals lived two and three stories underground. The surface temperature one day was 188 and we cooked eggs.

Note that Andimeshk, being 100 miles inland from the Persian Gulf, is land-locked and thus much less humid than Bandar Mahshahr. The astronomical humidity levels experienced in Bandar Mahshahr the past two days would likely not permit clothes to dry as described in Freivalds accounts.

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