It relates to the measurement of relative humidity. When RH = 100% the air is fully saturated with water and wet towels, sweat etc. cease to have any cooling effect because their water content cannot evaporate.fountainhall wrote: ↑Wed Aug 21, 2019 4:15 pmI happened to notice this in the Hong Kong paper this morning. The 2nd para is of interest. I assume it refers to external body temperature. I wonder if anyone knows what precisely this means? Should we be carrying around wet towels to keep the skin moist in extreme heat?
RH is conventionally measured by comparing the readings on two thermometers in an air stream. One is wet, the other dry. If the temperatures are the same, then RH is 100%. If they are not (it will always be the wet bulb that is cooler) then RH is worked out from the difference in a way that I cannot remember, and am too lazy to look up.
35 degrees wet bulb temperature is just that - the wet bulb shows 35 degrees. This can be 35 degrees in the shade with an RH of 100% or some higher temperature with an RH lower than 100% with sweating still having some cooling effect.
The machine I have used was rather fun to a schoolboy, which I was at the time. It was for all the world like an old fashioned football rattle with two mercury thermometers, one with its bulb wrapped in a wet rag, which you waved for a minute or or two around your head before taking the readings.