Yesterday in "Weather Eye" we had an example of how "everywhere you go you always take the weather with you", how the very presence of humans in an enclosed environment alters the indoor climate of that space.
One indoor area, however, where human influence is negligible compared to the imposed conditions is inside a sauna. Typically, the temperature is about 110C, well above the boiling point of water, a figure which invites the question as to why the occupants do not end up as stew.
The first scientist to have given serious thought to this conundrum is reputed to have been Sir Charles Blagden who, in the spring of 1775, created something of a stir at the Royal Society in London.
To that distinguished gathering he recounted his Scandinavian adventures of being in a room with wooden walls and ceiling in which the temperature had been increased to 126 C. He described how he had remained within the chamber for a full 45 minutes and how a raw steak which he had taken in with him was cooked to perfection by the time his experiment was over.
It amazed the assembled academics that an identical fate did not befall Sir Charles. It was left to later generations, however, to discover that the key to his survival, in what we now know as a sauna, was the dryness of the heat and, hence, the very low humidity.
Humans can survive only if their internal body temperature is maintained within a degree or two of 37C. But the temperature of the outer layers of skin can stray from this figure quite considerably without ill effect: the threshold for pain, for example, and hence the possibility of harm, is around 45C or 46C, depending on the individual.
Moreover, like any object of significant mass, the human body has a thermal inertia, a capacity to absorb a certain amount of heat before the "deep body" temperature begins to rise. Thus, for a limited period, external temperatures significantly above the magic 37 C are tolerable.
However, most importantly, when exposed to extreme heat the body starts to sweat profusely, and in a very dry environment the rapid evaporation is very effective as a cooling mechanism. It is noticeable, indeed, that if water is thrown on a hot surface in the sauna, it vapourises almost instantaneously and increases the humidity significantly; perspiration quickly loses its effectiveness and anyone inside will feel an immediate intensification of the heat.