# Heat index of 103 F and 99% humidity = hotter than boiling

I played around with a heat index calculator on Google and plugging in 103 degrees Fahrenheit and 99% humidity yields a heat index of 216 Fahrenheit - hotter than boiling water.
How can that be - that being at 103 Fahrenheit could cook you like a lobster? Would the ambient air literally scald and burn you like boiling steam itself?

Addendum: I know it’s impossible for 103 F and 99% humidity to exist together in a natural setting, but still wondered if this means that such hypothetical conditions in a lab would literally boil you.

And according to the calculator, a mere 150 F, combined with 99% humidity, takes you to 792 degrees F!

Checking a heat index calculator, I find this caveat:

The temperature and humidity combination you selected are very far outside the chart,so the calculation is evidently meaningless.

The heat index is empirically derived, from a reasonable number of data points a function is derived that both yields the original data points, and smoothly interpolates that data in a useful manner. Which yields the calculator. In this case the formula is a polynomial, with coefficients chosen for no other reason than that they match the above requirements. Critically, they have no other meaning. They don’t relate to physical processes. Used outside the domain of values for which it was generated, the formula will continue to create values, just values that have no relation to reality.

No. The temperature of the air is still just 103 degrees. It can’t make you hotter than it actually is.

Environment Canada uses a similar but different scale called the humidex, but the principle is the same, and to add to what was just said above, you can see in this chart from Occupational Health and Safety that the humidex scale, like the US heat index scale, is only valid in a limited range of commonly experienced temperature and humidity ranges. Furthermore, the calculations are subjective and rather arbitrary and the subjective feel varies with the individual. As such, the index is, at best, only very, very roughly related to equivalent temperature, and is really more of a unit-less index. But the main point is that it doesn’t mean anything at all when temperature and/or humidity are outside of a nominal range.

Perhaps I’m over simplifying, but the heat index signifies how hot you feel, not how hot it actually is. In order for someone to physically boil, the actual heat needs to be at or near the boiling point of 212F/100C.

Well there isthis sales pitch

I haven’t heard of people cooking inside a steam room using the ambient air.

The steam room advertising does what the weather reports should do - “The temperature today is 95, but it feels much hotter”. (instead of trying to specify a number)

The heat index indicates how hot you feel based on your comfort (and also the danger of heat-related illness), which is related to how well your body can shed heat. Sweat evaporation is important to shedding heat, so higher humidity at a given temperature reduces evaporation and makes you feel as hot as a higher temperature with less humidity. It is not the same as though your skin were exposed to the heat-index equivalent.

The reverse is true of wind chill. If you have a temperature of 35F and wind of 40 MPH, the wind chill is 20F but water won’t freeze under those conditions.

This environment would cause you to become hotter than 103 degrees, because you have to export body heat. Does that count as “make you hotter than it actually is”?

You were contradicting their apparent claim that it would scald and burn, and of course you are right. They might better have made their output default to something less dumb outside the operating range.

But, those conditions would be worse than uncomfortable, they’d be fairly quickly fatal. It would be bad if they failed to point that out.

Conditions like this could certainly occur in enclosed spaces in not very implausible circumstances, such as a closed vehicle that had gotten very wet inside in a flood.

Using temperature units to report the combined effects of temperature and humidity (or temperature and wind speed) is an unfortunate practice that causes confusion. I think the news likes to report the extreme “temperature” values provided by heat index and wind chill because they draw more attention. Too many times I have heard newscasters and friends reporting index or wind chill as if they are the actual temperature. They also seem to report wind chill for the speed of wind gusts to exaggerate.

“Temperature” already means something. Some other units should be used for the feels-like effects that depend on wind speed, humidity, sunlight, if you are wearing a hat, how oily or hairy your skin is, what kind of fabric you are wearing, what body part we are talking about, your level of physical activity… Or just report the temperature, humidity, and wind speed.

Or better yet, report temperature dewpoint and wind speed. Much more indicative of how it’s going to feel.

Assuming Humidity in this context means relative humidity.

The body cools itself by losing heat to air by direct cooling and evaporation of sweat. Assuming a body temperature of 98.6 F, direct cooling is not possible when the air temperature is above 98.6F. The body must now cool itself by evaporation alone.

**Air at 103F and 99 % humidity will condense water on skin at 98.6F. ** - no evaporation is possible and the body will overheat. The maximum humidity for 103 F air at which evaporation of water from 98.6F skin is still possible is about 86%

For different temperatures, here is a table of maximum humidity at which evaporation will not happen from human skin at 98.6F

T, F % Rel Humidity
100 95%
105 81%
110 69%
115 59%
120 50%

Hope this helps.

We probably get that 103 and 86% humidity in some places in the world on occasion…

Thanks, good info y’all

I was in a steam room just yesterday and the sign on the wall said the temperature is maintained between 45C and 50C (113-122F). Presumably the humidity was 100%, given that I couldn’t see from one side of the room to the other with the steam.

Plugging 113F and 100% humidity into that calculator I get a heat index of 307F. I didn’t boil alive, in fact it really didn’t feel uncomfortable. Obviously, you wouldn’t want to stay in there for too long, but 15-20 minutes was perfectly bearable.

This describes my bath.

But your own body heat can make your body hotter, and since when the high humidity prevents evaporative cooling. (the skin is damp.) you rely on conduction.

Its your own body making you hot, but the high humidity is a factor in that making you feel its a hot day…

I don’t get how the websites got the algorithm wrong.
99% humidity makes it feel about 10 degrees hotter. 75% is probably only 2 degrees hotter.

Well its all subjective and highly variable, because one reason you feel hot is that you are covered in sweat and work out that is because of body temperature… well your body has to be at a fairly constant temperatre, and it sweats a lot trying to ensure that is the case… But when dehydrated, sweat can come and go in burst of sweating… And some might react quite differently… eg only sweat a little bit, while the other is saturated. So you can’t literally translate relative humidity to rate of sweat production.

This has me wondering - those conditions, even if not very uncomfortable, are technically fatal ones if people stay there too long. How do the operators/managers of such steam rooms not feel paranoid about potential customer deaths? They’re essentially having customers walk in and out of a room that is at fatal conditions and betting that the customers will be able to physically exit before things get too bad.