There’s probably a simple explanation to this, but…
Why, when the air temperature rises to, say, near body temperature, do I start to feel uncomfortably hot after maybe half an hour, whereas I could* lie down and soak in a hot bath (at a considerably higher temperature) for hours (*could - the opportunity to do so seldom arises).
Has to do with heat exchange? While you’re lying in the admittedly hot tub, it’s still receiving heat from your body? Whereas if you’re sitting on the porch in 98 degree weather, the hot air circulating isn’t as efficient at extracting heat from your body as water is?
[sub]goes away to mull over person who hops into a nice hot bath when it’s 98 in the shade[/sub]
The weather here in the UK runs the gamut of temperatures; Hot baths in winter, cooler ones (or showers in summer), but I will take hot baths on a hot day if there’s a particularly large amount of grime to shift, and yet even a hot bath on a hot day is refreshing.
If the water temperature is above body temp (which it usually is), wouldn’t there be a tendency for the water to lose heat into me, rather than the other way around?
But what about the bath thing?
My WAG is that comfortable bathing temps are usually below body temp. Even if you started out with 100[sup]o[/sup]F water, because the tub is uninsulated, and has a large surface area for heat exchange, the water cools rapidly. And even if the water is slightly below body temp, in contrast to air that is slightly below body temp, it will feel refreshing. That’s because water is a more efficient medium for heat exchange. This effect was driven home to me one time that I fell asleep in the tub. I woke up an hour later. The water in the tub couldn’t have been less than 75[sup]o[/sup]F (ambient), and yet I was freezing.
In addition, your body can lose heat to the tub base. If you have the standard porceleanized metal tub, the heat exchange will be fairly rapid. This phenomenon is so pronounced that in bathtub scald injuries, the body part in contact with the tub will not be burned.
Another factor could be the rapid evaporative cooling of any unsubmerged body parts.
I’ll have to get the thermometer out; I’d say that the baths I take are above body temp. (wouldn’t a body temperature bath feel only lukewarm?)
::wipes off hands::
I just stuck my hand in the 37[sup]o[/sup]C (98.6[sup]o[/sup]F) water bath. The water feels like a comfortable bathing temp; not too hot, nor too cool. Mosts baths probably begin at a higher temp. However, it should cool rapidly. Since your body constantly acclimates, you may not acutely perceive the temp change.
Definitely take a thermometer into the bath with you. Start out with the hottest water you can safely tolerate. After an initial reading, take temp readings every five minutes or so (try to splash around a bit before reading the thermometer to ensure the water temp is uniform).
I wish the experiments I have planned for tonight were as fun as yours. 
[sub] You’re a guy, aren’t you? If not, maybe I can come over and help to agitate the bath water a bit? [/sub]
I’m pretty sure a body temperature bath would be too lukewarm for me. Our bath has an electronic control and it keeps the water at 42 degrees C, according to its display.
I suspect that our body can get rid of a lot of heat through the mounth (or lungs, to be exact).
Given the fact that bathing comfort is rather subjective - some like it hot and others don’t, I think the comfort zone is quite narrow.
I maintain our hot tub at 98° which is fine for a good long soak. But if I turn it up to 103° I get really limp and sleepy right away, and have to get out after only a few minutes.