I think there’s some justification to the “heat index” notion, because it’s based on your body’s ability to cool itself, which is a function of humidity – and the conditions are essentially impossible to escape (assuming that you’re outside).
The “feels like” stuff on the cold end of the scale, though, I think is far more dubious, since it’s based on windchill, and it’s easy enough to find shelter from wind. If it’s 35 degrees and windy, why not just say that? It’s adds no value at all to say that it “feels like 22 degrees”. It’s 35 degrees, everyone knows that it feels colder if you stand fully exposed to the wind, but if you stand on the lee side of a stand of trees then it it doesn’t.
HI = c[sub]1[/sub] + c[sub]2[/sub] T + c[sub]3[/sub] R + c[sub]4[/sub] T R + c[sub]5[/sub] T[sup]2[/sup] + c[sub]6[/sub] R[sup]2[/sup] + c[sub]7[/sub] T[sup]2[/sup]R + c[sub]8[/sub] T R[sup]2[/sup] + c[sub]9[/sub] T[sup]2[/sup] R[sup]2[/sup]
where
H I = heat index (in degrees Fahrenheit)
T = ambient dry-bulb temperature (in degrees Fahrenheit)
R = relative humidity (percentage value between 0 and 100)
c[sub]1[/sub] = -42.379 , c[sub]2[/sub] = 2.04901523 , c[sub]3[/sub] = 10.14333127 , c[sub]4[/sub] = -0.22475541 , c[sub]5[/sub] = -6.83783 × 10[sup]-3[/sup] , c[sub]6[/sub] = -5.481717 × 10[sup]-2[/sup] , c[sub]7[/sub] = 1.22874 × 10[sup]-3[/sup] , c[sub]8[/sub] = 8.5282 × 10[sup]-4[/sup] , c[sub]9[/sub] = -1.99 × 10[sup]-6[/sup].
It’s not purely subjective. The key is that temperature isn’t the important quantity at all, because humans do not have any biological processes that respond to the temperature of the environment, nor can we detect temperature. What we can actually detect, and what matters to our biology, is the rate at which heat is leaving or entering the body. Now, this mostly depends on the temperature, but it also depends on other factors such as humidity and wind speed (properly speaking, it’d also depend on barometric pressure, but I don’t know if that’s significant enough to be worth taking into account). That is, if, under two sets of conditions, a human body will lose heat at the same rate in both of them, then those two conditions have the same heat index.
Thanks all, for the replies. I’ve learned a thing or two today. I was unaware that the heat index can actually go into a negative mode so to speak, whereas, if the humidity gets low enough, one would subtract from ambient temperature. Perhaps this phenomena is what I’ve experienced in the before mentioned desert areas.
I will however mention that this subtraction factor appears to be minimal… My 100 degree day in the desert may have “felt like” 95, (or so) but it was pleasant. Right now here at home, the RH is about 80% and the temp is only 84F, equating to a NWS index “feels like” temperature of 94F… And it is purely nasty outside.
As others have alluded to, the other major difference is that if you are sitting outside in, say, Las Vegas, with 100 degree temp and low humidity, and you’re in the shade with even a very slight breeze, you’re going to be feeling great. Go out walking in the sun for a mile or two and the sunlight alone (to say nothing of the exertion) is going to have a very, very quick effect on you.
Humidity does have a way of making even the shade miserable.
ETA: And by the way, you know how perspiration works, right? When you sweat, the water on your skin evaporates. As the sweat goes away, your skin is cooled. In dry weather, sweat evaporates more quickly; in a humid climate, more slowly. So you get “more bang for the buck” from sweating in a dry setting than a humid setting, in a very simple sense.
“The night was hot, wait no, the night, the night was humid. The night was humid, no wait, hot, hot. The night was hot. The night was hot and wet, wet and hot. The night was wet and hot, hot and wet, wet and hot; that’s humid. The night was humid. . . . The night was dry, yet it was raining.”
The wiki article on heat index is surprisingly good at getting into the background and the limitations of trying to create single number for perceived thermal uncomfortableness.
The article on wind chill is similar. And also brings out that different nation’s weather agencies have different formulas for both these things. Which underlines the point that it’s all empirical with a healthy dollop of subjectivity. Well-reasoned subjectivity, not random caprice, but subjectivity nevertheless.
The subjectivity then comes in in choosing what humidity to assume as standard, what mass/surface area ratio to assume for your standard human, what type & volume of clothing to assume as standard, etc. Whether we assume a bald head or one covered with a dense layer of tightly curled hair makes a big difference.
US NWS Wind chill talks about perception on exposed skin. IOW, it’s how cold you’d feel standing out there buck naked. Their intent is good: to provide numbers applicable to exposed faces and hands of outdoor workers. But it effectively overstates the gross impression a bundled up person gets while hurrying from car to building.
i.e. …
They made a subjective decision that an *exposed worker *index would be more useful to their target audience than a *hurrying from car to building *index would be. Once they made that subjective decision they applied objective science to build the best exposed worker index they could. Even then they probably assumed the exposed skin was male. The same index designed for female heat transfer would probably be a warmer number for any given physical situation.
Of course 100 in Phoenix is going to feel different than 100 in Houston. That’s the *point *of the heat index after all.
What it’s saying is that an unremarkable summer day Houston with a temp of 95 and relative humidity of 48% (dew point of 72 degrees) feels about equally uncomfortable as a unremarkable 104 degree day in Phoenix with relative humidity of 25% (dewpoint of 58 degrees).
They’re definitely not the same sensation- Houston would feel like you’re being braised, while Phoenix would feel much more like being baked.
Agree “subjective” isn’t the ideal term. But to me at least, “arbitrary” carries the connotation of “without specific cause” or “random” or even “thoughtless”.
IOW …
Throwing a dart at the phone book is an arbitrary way to pick today’s lunch stop. Asking the group if they’d prefer Chinese or Indian is a subjective way to pick today’s lunch stop.
Most real world problems have that mix of objective factors and, for lack of a better word, subjective factors. The subjective factors might or might not be quantifiable, but there’s definitely no agreed conversion function to render even the quantifiable one directly comparable. e.g. …
How much one values speed of service vs. cuisine vs. price vs. distance from the office will vary from person to person. 3 of the 4 are quantifiable in principle, but nobody would assert there’s a clear and universally accepted conversion function between price and speed of service.
I’m all ears if you or anyone else has a better term than “subjective.” “Arbitrary” is a decent counteroffer, but I don’t think it works. At least not for me.
I’ve told this before, but it made such an impression on me that I have to repeat it.
When I lived in Houston, several summers ago, the weatherguy on the nightly news started his segment with, “We’ve had lower humidity today, so it has been feeling cooler than the thermometer tells us”. Later in the same segment, he was going through the numbers and said “…and the temperature is 88 degrees, but it feels like 92”.
In the same newscast, the same person said it both felt cooler and warmer than the actual temperature. Within minutes (two, at most). I guess, in Houston, lower than normal humidity is higher than normal humidity everywhere else.
