Ok, in theory, wind chill is the temperature you feel when you add the effects of the blowing wind to the ambient (cold) temperature of the area you are in. The heat index is the temperature felt when you add the effect of the humidity to the ambient (hot) temperature.
This seems rather like BS to me. What about the effect of humidity on cold temperatures? What about the effect of the wind on hot temperatures? At what point does the humidity stop interfering with your bodies ability to radiate heat and begin sucking heat from you? At what temperature does the wind stop making you feel colder?
It seems to me that these terms are used as sensational reporting. It may be 103F, but if you can say its feels like 114F, then that’s even better. Or that it may be an unimpressive 40F outside, but with that wind, its actually 30F!
If it were actually 114F outside, a hell of a lot more people would die from heat related causes. At least with wind chill, there is a real danger as your body’s heat is removed far faster that the ambient temperature would suggest. I guess I am just tired of watching the weather on the local news and being told that today it felt like the Sun was about to nova because it was humid outside. I do realize that a high humidity makes is harder for sweat to evaporate, but that hardly justifies telling people that its is 11 degrees hotter than it really is.
Does anyone actually know how the heat index and wind chill are calculated?
Excellent post - this one’s been bothering me for a long time.
To add a few more questions: when the weatherperson says “It’s 92 degrees, but it will really feel like 103 degrees” –
a) I’ve always lived in the Northeast, where heat and humidity always go together. Therefore, to me, it feels like 92 degrees;
b) Compared to what? Zero humidity? Are they saying that it would feel like 103 degrees if you were in a desert? How does that help me understand how hot it is?
Also, windchill is a fabrication. Cars, for instance, have no reckoning of windchill. Say your car won’t start when the temp is below 30F. The temp is 35F with a windchill of 25F. Your car will still start. Also, water won’t freeze in this instance either.
If you live in dry climates in the Western US, you can often experience the phenomenon when the heat index is lower than the temperature.
It’s the cliched “dry heat.”
And, yes, both are figures that are only designed to reflect how humans perceive heat or cold. However, isn’t that what you care about?
If it’s 30F outside, but you are told that the wind chill is 0F, won’t you dress differently than when the temperature is 30F and the wind chill is also 30F?
Wind chill makes a huge difference in determining how quickly you’ll get frostbitten, as Poysyn points out. For the specific formula used to determine wind chill,
Thanks Jeff. I figured that something like this that would be the case.
Bob: I tend to agree with you about the wind chill. Blowing air, like I said in the OP, removes heat faster from your body than a stagnant breeze would. Fine. But it still comes down to how the person standing in the gale feels. If they are a 90 pound Calista Flockhart wannbe, they are gonna be a popsicle. If Rosie O’Donnell is out there, she’s not going to have nearly the problem.
I suspect that the heat index is even more flakey. If I have dry skin, will the high humidity feel better? Pesonally, I am far more comfortable with a decent level of humidity than I am in a “dry heat”. My lips shrivel up like raisins when there’s no moisture in the air, and I definately don’t feel any cooler than I normally would.
Plenty of people don’t like it when its “muggy” out. Some people don’t mind. So I’m curious how such a subjective thing can be quanitfied into a number.
Maybe the news station has a sweaty redneck out back. “Hey Bubba, how hot are ya?” “Bout 114 I reckon.”
Or maybe they measure the amount of sweat dripping off the aforementioned redneck and compare that to a control redneck, who is standing in the shade.
Your close with the sweating rednecks. If the humidity is high, your sweat doesn’t evaporate as fast so you don’t cool off as well. That makes it feel hotter. Since you don’t cool off as well, you are more prone to heat stroke. There is a real effect on health.
If it is cold and wet, then your clothes get wet and conduct heat away from you much better then dry clothes. Even if only the outer layer of clothes gets wet this can have a huge effect. Since it is cold, it is unlikely that much evaporation is going to be happening, so relative humidity doesn’t have much of an impact on how most people feel. The “wet” in “cold and wet” has to be liquid moisture, not vapor, although the water vapor in air will become liquid if it encounters a colder surface or the the temperature drops below the dew point.
As previously stated though, humidity has a huge impact on personal comfort in high heat, but I have found that this is very dependent on what people are used to. I am a So. Cal. guy, and when I travel to Florida I sweat like a pig in even moderate heat. When the Florida people are out here the get nose bleeds and dry skin/lips on what locals consider very comfortable days. Comfort is a very relative thing.
I would take this to the rant in the Pit on bad TV news. If they gave Dry Bulb temps, %Rh, and wind speed then you know what it is going to be like outside. When they give just the predigested figures like Heat Index and WCF we really don’t know.
I got this off of Yahoo.
Heat Index: If you know the relative humidity and the dry air temperature, then you can use the following equation to calculate the heat index.
Note: In order for the Heat Index formula to work correctly, you must use the relative humidity in percent form. In other words, if the relative humidity is 65%, use 65 for RH in the formula, not .65.
From looking at the table it appears they didn’t use a standard %Rh, but defined it some other way. It is on a sliding scale of sorts, but at 70F, %Rh is about 65%. At 90F, it is 30%. At 115F, it is about 15%, and so on. Use a table or a programable calculator.
>> The “baseline” for the comparison is conceptually an unclothed individual moving through calm air at about 4 mph (a pretty brisk walking pace!).
OK, So the standard for wind chill is a naked man walking at a brik pace. So it’s 30 outside but the wind makes it feel like 23 for a naked man walking at a brisk pace… hmmm… a pretty useful information for those people that would go out naked in a winter storm.
Now, for the rest of us they should add a “dress index”. For example: Today it is 100 outside but factoring in the humidity it feels like 120, now we factor in that you are dressed in a suit to go to work and it feels like 140.
Wind chill, at least, does have more than simply sensationalism as its intent. Wind chill indicates, as poyson has already noted, when frostbite might be a factor.
For example, a couple of years ago a (stupid) mother, here, took her kids out to the bus stop with no hand or face protection one winter day. The temperature was 27°F, but the wind was so high that the wind chill was around -14°. The bus driver looked at the kids and simply drove to a nearby hospital. In the few minutes that she had waited for the bus, both kids suffered frostbite. The effect of wind blowing across the skin is not an exercise in imagining naked men in hypothetical winter strolls, but a method to determine when you might surrender a body part to the weather.
Now, just to complicate matters, last year someone determined that errors had been made in the original calculations for wind chill, (which had been roughed out during WWII). So they are now laboriously cross-checking the calculations to present a more accurate chart.
While it might seem “better” to hand out the variables and the formula rather than the matrix results, we’ve got to remember the twits of the world who cannot add up their grocery bill. The point of producing a “magic number” is so that people who are marginally smarter than the mother in the above story will not expose their children to the elements, needlessly.
