Image: Australian Bureau of Meteorology (12/2019)
It is time for meteorologists—when reporting on extreme world high temperatures—to publish the concurrent data on relative humidity.
The core temperature of the human body is well known to be about 99F (37C) and to be carefully stabilized in maintaining a balance between the heat produced by internal metabolism and that lost to the environment through conduction, radiation, and transpiration. The production of internal heat is essentially constant—unless increased by exercise—and must therefore be continually dissipated, even under ordinary circumstances, to prevent body temperature rise.
Without clothing, in 75 degree air, the skin temperature is about 90F (32C). We perceive an air temperature in the range of 75F as neutral or benign because that fifteen degree difference is enough to allow the body—through conduction, radiation, and transpiration—continually to lose the appropriate amount of heat to retain its balance.
As the ambient air temperature rises emergency measures must come into play. If possible we can limit the internal heat production through resting. If that is insufficient we begin to sweat and perhaps to seek breezes to enhance their evaporative cooling effect. This works well, especially if the air is dry, but not so well in already wet and humid air. And, ominously, it works not at all if the air is already as humid as it can get.
Let me explain:
Behold the Psychrometric Chart:
This diagram (augmented for relevance) shows various physical relationships between air and the amount of moisture it holds under various temperature conditions.
1. The vertical lines are lines of constant ambient air temperature; for example everywhere equal to 80F on the vertical 80F line.
2. The horizontal lines are lines of equal moisture content; for example everywhere equal to 0.016 pounds of water vapor per pound of dry air on the horizontal 0.016 line.
3. The sweeping curves are the loci of equal per-cent relative humidity, i.e., the ratio of the vapor in the air at a point to the most vapor it could possibly hold at 100% relative humidity. At 100% RH we call the air saturated with moisture—it cannot take on any more. Evaporative cooling ceases.
4. The straight sloping lines are more mysterious; they represent the temperature a wet rag would cool down to if held up in the ambient wind as it dried—your wet swim suit in a gale. The interesting—and crucial—thing about this is that from any air temperature starting point on a sloping line—say 80F/60%RH—the evaporative cool-down end-point is always the same, 70F! Check out: 100F/20%RH. This end point is called the wet-bulb temperature because it is measured with a thermometer whose bulb, cooled in the wind, is covered with a wet sock.
When water evaporates it cools, and keeps cooling until the wet-bulb temperature is reached. Thus, the human body can cool itself by sweating only as long as the ambient wet-bulb temperature is cooler than the skin temperature. Once the ambient wet bulb temperature hits 90F evaporative cooling ceases and the skin can no longer be cooled by sweating. In the absence of artificial cooling, the body temperature will then inexorably rise until death ensues—naked, in the shade, with plenty to drink.
For example, referring to the Chart:
When the air temperature reaches 110F/43C—as it has already in parts of the world—and the relative humidity is what we might think of as a modest 46%, the wet-bulb temperature is 90F/32C and an outdoor population faces death by heat stroke, e.g., body temperature over 104F/47C.
The thing is that it’s not strictly the ambient air temperature that counts; it’s your proximity to the sloping 90F wet-bulb line that determines your danger. See .
In terms of the climate crisis existential heat is no joke. Severe temperature events will become more and more common as the world heats. As I write* [01/01/20], today’s record high temperature in Australia is 121.8F/49.9C! (I can find no concurrent %RH data.)
Therefore I think it important in extreme temperature conditions for world weather services routinely to publish concurrent humidity/wet bulb data so that danger can be properly evaluated by the general populace. Therefore, too, I think it important for the general public to have a rudimentary understanding of the psychrometrics of existential heat.
*Today (8/4/20) 53.0C/127.4F in Basra!
An interactive Psychrometric Chart by hvac-eng.com
See  Record wet-bulb: 92F/33C.
See  Cooking: @131F/55C.
See  A good article on this subject.
But wait, there’s more:
In addition to the wet bulb temperature there is another psychrometric entity used by weather services to find the relative humidity—the dew point temperature. It is a more accurate means of determining %RH than by using the wet bulb temperature, but its accurate determination is out of reach for those without special equipment.
It’s as if one sets out a glass of pure water—at the prevailing air temperature—and adds ice cubes (cooling it down) until vapor condensation first appears on the outside—your cold wet glass of beer. The temperature of the water (beer) at that precise point is called the dew point.
On the chart horizontal lines of equal moisture content are parallel to—the “same” as— lines of constant dew point. And at 100.0 %RH (and only then) the air, the dew point, and the wet bulb temperatures are nearly the same.
Now, for example, if it’s 90F/32C out and the weather service says the dew point is 75F/24C then following the 75F horizontal line rightward to its intersection with 90F vertical line shows a %RH of 60.0% (and—going leftward up the sloping line—a wet bulb of about 78F).
It is more common for weather services to cite the dew point than to publish the wet bulb or the %RH, especially in weather record keeping.
And yet more:
In order to popularize an understanding of the relative danger the “Heat Index” has been invented. Its physical basis is arcane, but it purports to hold that for a given index value a given level of danger is present. My beef with it is that it muddles the physics involved. It muddles the concept of degrees Fahrenheit: degrees F temperature or degrees F index? It muddles its interpretation on the psychrometric chart: an “index”, for example, of 100F could be a temperature of 84F and RH of 95%, or a temperature of 96F and RH of 40%— totally different psychrometrics. In its worst interpretation it is an example of the “popularization” of weather information—the ultimate “dumbing down” of science.
 Psychrometrics- Wikipedia
 A new publication of the human effects of enormal temperatures: Thermal comfort indices derived from ERA5 reanalysis.
 Human Survivability Threshold
 Highest wet-bulb temperature ever recorded.
 On a macabre note: Sous-vide cooking 131F/55C. It is not possible to hold your hand, for more than a moment, on a surface at 130F.
 A good general article.