A spate of new research warns of lethally hot, humid weather in many regions by the end of this century- weather in which it would be dangerous even to walk outside, or turn off the air conditioning.

These are very long range projections, with all the uncertainty that that implies; it’s hard enough to forecast weather a few days ahead. But they got me thinking about heat stress. We see it every summer in Florida. I always thought of it as something to be dealt with and endured, not as a danger.

This July and August, the Heat Index (“feels like” temperature) seemed to climb into the “Danger” zone every day and stay there for hours, its peak often exceeding the forecast. Could we be approaching deadly weather already? What about other, even muggier parts of the planet?

Researchers descibe this extreme weather in unfamiliar terms, usually involving the “wet bulb temperature” in degrees Celsius. What we hear in daily weather reports and forecasts is the Heat Index, in degrees Fahrenheit. How do these measures relate- or do they? Here’s the official NOAA chart of Heat Index values:

Official NOAA chart of the Heat Index: How the combination of heat and humidity feels to a person wearing a short sleeved shirt and slacks, walking at 3.1 mph in shade. Direct sunlight can make you feel much hotter, up to an additional 15 degrees Fahrenheit.

Calculating the wet bulb temperature for each cell of the chart (Note 1), you see the same pattern in both measures. They line up so well you could estimate one from the other with fair accuracy, even though one is a physical measurement and the other is based on experiments with human subjects reporting how hot they feel. Both are measuring heat stress.

“Danger” on this chart means “Heat cramps and heat exhaustion are likely; heat stroke is probably with continued activity.” Crossing into the “Extreme Danger” zone, “Heat stroke is immiment.”

Potentially lethal wet bulb temperatures fall in the lower right corner of the chart above, deep in the “Extreme Danger” region where no heat index values are shown. The “Danger” zone values we felt in July and August were a lower order of danger.

What is a “lethal” wet bulb temperature? The value usually cited equates to 95 degrees Fahrenheit, midway between your body’s internal temperature of 98.6F and average skin temperature, about 91.4F. In this range, it’s already so hot that your body can only cool itself by sweating (details here.) Wet bulb temperatures of 95F and higher cover about half of the “Extreme Danger” zone. The transition from “Danger” to “Extreme Danger” comes at a wet bulb temperature of 86-88F.

When wet bulb temperature reaches 95 F, you can’t even cool your skin to its normal temperature. Six hours of this, even resting in shade next to a fan, is fatal.

With a wet bulb temperature around 91F, you can just barely cool yourself by sweating. Why? Wet bulb temperature – “Tw” – is read from the wet bulb (surprise!) thermometer of an instrument called a psychrometer, during routine weather observations. The psychrometer cools its wet bulb by evaporating water into the surrounding air, just like your skin. Tw is the lowest temperature it can cool its wet bulb to under current conditions- and likewise the lowest temperature you could cool your skin to. Tw isn’t usually reported, but is used to compute other humidity measures.

Florida gets unusually high Tw’s, but some other regions are even muggier. The maps below show the highest daily maximum Tw from 1979 thtough 2015. The highest values occur over water surfaces- the Persian Gulf and the Red Sea. Lesser hot spots are found along the shores of the Indian Ocean, in the South China Sea and the Phillipine Sea, as well as the Gulf of California and the Gulf of Mexico. On land, the highest values are found in major river valleys of Pakistan, India and China; also the Mississippi valley and the Parana valley of Argentina (not shown).

From figure “Spatial distribution of highest daily maximum wet-bulb temperature, TWmax (°C), in modern record (1979–2015)”, in Science Advances article referenced below. Published under a creative commons license. This is the inset map from the figure, with the color scale. I’ve added degrees Fahrenheit below the scale.

Continental US from the same map, enlarged, with same scale. South America not shown.

The Persian Gulf has been extremely hot this summer. On Weather Underground’s map of sea surface temperatures (here), it has been showing up bright white, which on their color scale means temperatures over 94F- far higher than anywhere else on the map. Checkng the morning reports from a number of stations in high Tw regions, over several days, I found a few reports over 91F around the Persian Gulf.

Scanning the years records for seven stations around the Persian Gulf, from NOAA’s archive, I found Tw values over 91F at four stations on several different dates in July and August (Note 2). Two reports were over 93F. The longest periods of Tw over 91F were seen at Kish Island, Iran and Bushehr, Iran, both on August 19.

Kish Island lies off the south coast of Iran, with hundred-mile stretches of open water on three sides. Humidity started to build up about 2:00 AM, when the overnight low temperature of 91F was reached. Haze appeared at 6:00 AM and persisted til 1:00 PM, under light, variable winds off the Gulf. Tw rose gradually from a low of 87F at 2:00 AM to a peak of 93.7F at 8:00 AM, when the air temperature reached its high for the day, 102F. From then on, both temperature and Tw declined until 8:00 PM.

At Bushehr, on a peninsula extending west-southwest into the Persian Gulf, Tw was 91F or higher at 9:00 AM, noon and 3:00 PM, cooling in late afternoon but rising over 91F again by 9:00 PM. Observations here were at 3 hour intervals.

A news report also showed a peak Tw of 93F in Jask, Iran on June 27. Jask is on the Gulf of Oman, east of the Persian Gulf.

These were probably the highest Tw values observed on Earth this year, so far. Not lethal, but at or above normal skin temperature, well into the “extreme danger” zone of the heat index.

Higher values have been recorded. During a heat wave in early 2015, Mahshahr reached a wet bulb temperature of 93F one day, followed by a peak value of 94F the next. Mahshahr is near the north coast of the Persian Gulf. Dhahran, Saudi Arabia on the southwest coast experienced a Tw of 97F on July 8, 2003.

Iranians are familiar with the hot, humid air of the Persian Gulf. When it blows their way, it’s called “Sharji.” Sharji is completely normal weather, as engineer Peyman Fatemi described during the 2015 heat wave:

“On Thursday in Mahshahr, we went out for shopping and eating with friends at 7 p.m., when the weather was humid but less hot than noon. Streets were crowded as usual.”

“Our whole skin and shirts got completely wet very quickly — say in less than five minutes — but it is normal and is a matter of taking a shower afterwards,” Fatemi said. “People also drink a lot of cold water, soda-based drinks, malt beverages and so on.” Most people “carry a bottle of cold water, juice, everywhere.”

“..it is very difficult to walk under sunshine between 11 a.m. to 6 p.m. in summer for more than 15 minutes without resting in a shade and drinking a heavy load of water and pouring it over your head.”

What surprised Fatemi about this spell of Sharji was that it happened in early August. Sharji normally comes in September, when prevailing southwest winds blow onshore from the Gulf. But this was a heat wave, like the ones reported in July and August this year, when ridges of high pressure dominated the region’s weather. (Punctuated by rare thunderstorms in Doha on August 15.)

An Israeli columnist, Adam Berkowitz, speculated that this year’s heat waves were divine punishment on Iran for sponsoring terrorism against Israel. He quoted scriptures telling how god had sent heat waves as retribution:

“For behold the day cometh it burneth as a furnace; and all the proud and all that work wickedness shall be stubble..”

“And thy heaven that is over thy head shall be brass, and the earth that is under thee shall be iron.”

Berkowitz could broaden his vision a bit. These heat waves affected the whole Persian Gulf region, not just Iran. As I write, Southern Europe is suffering a severe heat wave, as is the US Southwest. Houston is recovering from a devastating hurricane, while another strong one is building in the Atlantic. If Jehovah’s hand is in this, he may be punishing us all, for our poor stewardship of Mother Earth.

Note 1: Wet bulb temperature “Tw” can be calculated from temperature, atmospheric pressure, and a measure of humidity- either relative humidity or dew point temperature. I calculated values for every cell of the Heat Index chart using average sea level pressure, 1013 millibars. (Full chart here)

This makes sense because Tw depends only slightly on pressure. Pressure differences like those of day to day weather changes will change calculated Tw only by hundredths of a degree. The difference between sea level pressure and typical pressures in Denver or Breckenridge will move it perhaps half a degree Fahrenheit. And the places where Tw gets really high are near sea level.

Note 2: I imported each set of records into a spreadsheet and scanned for dew point temperatures over 32C (89.6F), high enough to make a Tw over 91F likely. Then I looked at the days that fit that criterion, calculating Tw for the hours with a high dew point temperature. I used two online calculators for Tw, comparing the results, and checked the highest values with a skew-T diagram.

“Deadly heat waves projected in the densely populated agricultural regions of South Asia,” Eun-Soon Im,1* Jeremy S. Pal,2* Elfatih A. B. Eltahir3† Science Advances, August 2, 2017: Vol. 3, no. 8, e1603322, DOI: 10.1126/sciadv.1603322

“Global Risk of Deadly Heat,” Nature Climate Change 7, 501–506 (2017)

“An adaptability limit to climate change due to heat stress” Steven C. Sherwooda,1 and Matthew Huberb, 2009, Proceedings of the National Academy of Sciences of the United States of America,

“The Deadly Combination of Heat and Humidity”, New York Times, 2015,

Heat Index: Meteorological Considerations, Wikipedia. Mentions record Tw in Dhahran in 2003.

Historical report from Jask, Iran from Weather Underground,

Report of record Tw’s at Mahshahr, Iran in 2015 from Weather Channel,

“Historic Heat Wave Sweeps Asia, the Middle East and Europe”, Dr. Jeff Masters, Weather Underground, June 6, 2017,

“Rare August Thunderstorms Hit Doha,” Al Jazeera, August 2017,

NOAA Archive of hourly surface weather observations,

“Chart Busting Heat and Humidity in Iran City were Completely Normal, Says Local”, Washington Post, August 3, 2015,

“Is Iran Suffering Divine Retribution Through Soaring Heat Wave?” Adam Eliyahu Berkowitz, BreakingIsraelNews, July 5, 2017

“Dewpoint and Wet-bulb from Relative Humidity”, online calculator from NWS in El Paso, Texas:

“Psychrometric calculator” for Tw and other measures, based on ASHRAE manual: