2026: Heat Index

[anfurny]

Title text:The heat index is calculated via looking up the "effective temperature" in a table of air temperature and humidity values, and then adding a bunch more degrees because it feels WAY hotter than that.

This is my first time posting a first post and it does seem unnecessarily complex and geeky and time-consuming. Can't we automate this? It's a bit soul-crushing to do something that could obviously be automated.

I've tried to follow the "Seriously People" guidelines with the link and all, but I suggest that any "Seriously People" announcement is an invitation to a solution that is less repetitive and, thus, less soul-crushing.

Edit: Oops forgot Title Text text. Voilà.

[DanAxtell]

Wait. The Heat Index can be lower than the dry-bulb temperature according to the NOAA.gov website. (The “It’s a dry heat” phenomenon.)

Las Vegas, Nevada is a great example. It’s so dry there that the dew point is often below zero degrees Farenheit (-18)—never an experienced temperature there. I believe that no one in Vegas has ever had a drop of sweat fall from the tip of their nose. The place is even more remarkable as a pilgrimage for people with poor math skills that bans non-ignorant people.)

The Heat Index is assuredly not always higher than the dry-bulb temperature any more than pH’s are always above zero! Fluoroantimonic acid can have a pH of -25 and that’s just the record for a pronounceable acid. Mr. Monroe has never addressed this pro-positive bias for pH’s and now he’s extended the pro-positive bias to Heat Index.

The matter covers the quadrafecta of romance, sarcasm, math, and language. (“Heat index” can be something way romantic or way sticky. Either way, it’s a thing.) It needs to be addressed.

We highly-educated white males on this forum demand justice for negative numbers. We can’t imagine any greater injustice in society today.

[Heimhenge]

I live in Arizona and can tell you the "heat index" seems pretty frikkin' arbitrary. I know it's based on a formula, but that formula makes assumptions about average human perspiration and activity level. I for one always feel hotter than the heat index. Especially when it "relatively muggy" at 20% humidity or more. In the winter, when it's less humid, the heat index feels more accurate.

[Pfhorrest]

Is there a combined index that accounts for not just temperature and humidity but also wind chill? Wind chill pulls the perceived temperature down just like humidity pulls it up (for the same reason, aids/impedes perspiration evaporation), so that would be an important factor in calculating how warm it actually feels out.

[Archgeek]

Wait. The Heat Index can be lower than the dry-bulb temperature according to the NOAA.gov website. (The “It’s a dry heat” phenomenon.)

[...]

The Heat Index is assuredly not always higher than the dry-bulb temperature any more than pH’s are always above zero! Fluoroantimonic acid can have a pH of -25 and that’s just the record for a pronounceable acid. Mr. Monroe has never addressed this pro-positive bias for pH’s and now he’s extended the pro-positive bias to Heat Index.

Huh, you'd think they'd notice the extra-low wet-bulb temp and run the wind chill calculation once the index goes sub-dry-bulb. I guess they decided that "wind chill" of over 100 sounded just plain silly.

Is there a combined index that accounts for not just temperature and humidity but also wind chill? Wind chill pulls the perceived temperature down just like humidity pulls it up (for the same reason, aids/impedes perspiration evaporation), so that would be an important factor in calculating how warm it actually feels out.

I think there sort of is? Dorks call it "RealFeel" and may've gone as far as to trademark the mess. It's said to be poorly calibrated.

[qvxb]

TV (totally vapid) personalities use the heat index to scare viewers. "The heat index will be life threateningly high tomorrow. I'll tell you how high when I give the weather forecast later."

[rhhardin]

The heat index is the thermodynamic conjugate of the entropy index.

Disorder minus humidity. It's more disordered than it looks, if it's foggy.

[Thesh]

I think there sort of is? Dorks call it "RealFeel" and may've gone as far as to trademark the mess. It's said to be poorly calibrated.

Apparently that's proprietary. This one is used by the military:

https://en.wikipedia.org/wiki/Wet-bulb_ ... emperature

[SecondTalon]

OP’s commentary was a complaint at needing to provide commentary, confusing the fact that people like to make posts discussing the comic with an opinion that it could be automated because posting a thread for the comic is necessary.

Posting a thread for a comic is not necessary. Posting a thread to discuss a comic is an enjoyable thing people do.

I have changed the OP to someone more in line with this sort of complaint.

[Mikeski]

Posting a comic thread is "soul crushing"? I don't even think "first-world problems" covers that.

...

I think they should use "heat index" effects at the low end, too. 20 degrees F feels colder than 5 degrees F, just because there's still some humidity in the atmosphere to suck the heat out of you faster. (Minnesota-world problems, I know.)

And should there be a "wind heating factor" at the far high end, as well? I can't imagine a breeze helps you cool down if it's 102F and 90%+ humidity. Putting more hot air/water molecules in contact with you has to warm you up faster, convection-oven style, doesn't it?

[Soupspoon]

If the air is more humid than the actual liquid-exuding sweaty skin, and hotter too, it could be worse (though, stationary, it'd at least reach equilibrium in all ways) but then you're probably being broiled and far from comfort regardless.

If air is capable of taking up moisture by evaporation, however slightly, then it'll still be a cooling breeze. Merely to Hot As Hell rather than Hot As A Boil-In-A-Bag In Hell.

[dis astranagant]

I live in Arizona and can tell you the "heat index" seems pretty frikkin' arbitrary. I know it's based on a formula, but that formula makes assumptions about average human perspiration and activity level. I for one always feel hotter than the heat index. Especially when it "relatively muggy" at 20% humidity or more. In the winter, when it's less humid, the heat index feels more accurate.

Heat index works off an implied 20% humidity and baseline dewpoint of 57 dungarees Frankenstein. The main problem with it is that all stated temperature measurements are taken in the shade (or in specially designed boxes that make their own shade) above grass while most people move around in the sun over concrete or asphalt. This can lead to vast differences in actual temperature and heat index even on relatively mild days. It's mostly accurate for someone resting under a shade tree.

[Solandri]

Wait. The Heat Index can be lower than the dry-bulb temperature according to the NOAA.gov website. (The “It’s a dry heat” phenomenon.)

I never actually looked up how it's calculated. But I always assumed it was based on (1) the rate of heat flow from your body to the air due to temperature differential (call it Q1), and (2) the rate of heat flow from your body to the air due to evaporative cooling of your sweat (which would vary with humidity - call it Q2). So Q(t, h) = Q1(t) + Q2(h).

Then, you establish some baseline "normal" humidity, and calculate the heat flux from your body to the air via these two mechanisms at any temperature and "normal" humidity. Q(Tn, Hn) = Q1(Tn) + Q2(Hn). These are your heat index temperatures.

Then when humidity deviates from "normal", it'll increase or decrease the amount of evaporative cooling (Q2 changes).

Q(t, h) = Q1(t) + Q2(h)
Q(Tn, Hn) = Q1(Tn) + Q2(Hn)

So to get a "feels like" heat index, you just set the two rates of heat flow the same (increase or decrease Q1 to compensate for the decrease or increase in Q2).

Q1(t) + Q2(h) = Q1(Tn) + Q2(Hn)
Q1(Tn) = Q1(t) + Q2(h) - Q2(Hn)

So you plug in the actual temp t to find Q1(t), the actual humidity to find Q2(h), subtract the idealized rate of evaporative cooling Q2(Hn). And can solve for Q1(Tn), the idealized "feels like" temperature at which your body would cool at the same speed, or the heat index.

Wind chill would work the same way. The increase in evaporative cooling due to higher airflow over your skin (or a wet bulb thermometer) results in it "feeling like" the actual temperature (Q1) is lower.

[dis astranagant]

There isn't an actual formula. Just a huge table compiled in a 70s paper and a number of rather ugly approximate curves and this algorithm used by the calculator on the NWS website, where T is the dry bulb temp in F and H is % relative humidity


Which is off by 1 for a couple values at 90F.

[cellocgw]

Yeah, yeah, yeah, complain all you want about heat index calculations.

Here in New England we're just a few short months away from the dreaded "Wind Chill Index" "It's 25 Fahrenheit today, but with a wind of 5 mph it feels like -7000 Fahrenheit" .

[Vroomfundel]

I believe that no one in Vegas has ever had a drop of sweat fall from the tip of their nose.

That's a surreal experience, at least for someone who hasn't been in a desert. First time I was in Arizona it was 42C (108F) outside and I was like, how badass am I going to be if I go for a run now - I'll have bragging rights for eternity. So I did 6K and to my astonishment my t-shirt was dry when I came back. Have in mind that I'm a prolific perspirator.

Even if dry heat feels more comfortable one should be careful though. You only feel cool because you evaporate efficiently so you tend to dehydrate ultra fast. On that run, for instance, I consumed a litre of ice water.

We highly-educated white males on this forum demand justice for negative numbers.

So do we now have a token gay guy? What a diverse bunch we are!

[pogrmman]

Even if dry heat feels more comfortable one should be careful though. You only feel cool because you evaporate efficiently so you tend to dehydrate ultra fast. On that run, for instance, I consumed a litre of ice water.

Yeah, it’s stupid easy to dehydrate in the desert. Earlier this summer, I did some day hikes in the 4-10 mi range in Utah and sucked down 3-5 liters of water each hike. It was a pain to carry all that water, but at least it got used up!

I guess I’m so used to having some humidity (dew point ~70°) in the summer that a drier heat still feels weird to me. People keep their houses too cold around here — lots of people seem to like keeping the AC at ~72°, which is absurdly cold. The last time it was that cool outside was July 5th! I’m definately a warm weather guy — low 90s and dry is perfect, or the mid 80s with the normal humidity we’ve got around here.

[Heimhenge]

... And should there be a "wind heating factor" at the far high end, as well? I can't imagine a breeze helps you cool down if it's 102F and 90%+ humidity. Putting more hot air/water molecules in contact with you has to warm you up faster, convection-oven style, doesn't it?

Air that is hotter than your skin and in contact with your skin will lose heat to your body, so that cools the air down, sure. Then if "new" hot air arrives via the wind it's able to transfer its heat as well. So I see what you're saying about a "convection oven" effect. But intuition tells me such an effect would be overwhelmed by Brownian motion within the thin layer of air in contact with your skin, with the net result being no additional heating compared to still air. Evaporative cooling 1, convection oven effect 0.

[Pfhorrest]

Living as I do in coastal Southern California, the normal weather here most days of the year is hot and sunny but dry and slightly breezy. I still sweat when I got out for hikes, and my clothes still feel damp from sweat when I get back, but never soaked like I ran through a sprinkler, and it's a rare, weird day when it's so humid that the sweat is dripping off of me in drops instead of just lightly coating my skin.

From my perspective, there being so much moisture in the air that perspiration doesn't work properly is the weird situation, and what you all are gawking at about Las Vegas sounds like normal, properly-functioning weather.

This also reminds me of an earlier xkcd where different locations were organized by whether they're good for cold-haters or hot-haters or both or neither, which bothered me for not taking into account humidity at all, which makes an enormous difference for whether heat is hatable or not. If my body's naturally evolved evaporative cooling system can function as intended, then heat is great. If you've got some kind of thick muck of water vapor in the air jamming up the works, then fuck that noise.

[rmsgrey]

Shouldn't the comic title be quoted?

I'm not sure heat index is a thing here in the UK - we get wind chill, but not standardised humidity adjustments...

[Keyman]

I'm not sure heat index is a thing here in the UK - we get wind chill, but not standardised humidity adjustments...

Really?!?!?! Then how do your TV "weather terrorists" earn their living in the summer?

[Soupspoon]

I'm not sure heat index is a thing here in the UK - we get wind chill, but not standardised humidity adjustments...

Really?!?!?! Then how do your TV "weather terrorists" earn their living in the summer?

Showing weather-related (scenic) pictures sent in by the public?

[dis astranagant]

Shouldn't the comic title be quoted?

I'm not sure heat index is a thing here in the UK - we get wind chill, but not standardised humidity adjustments...

It was meant as a refinement of Canada's Humidex, which gives a dimensionless number given a dew point and a dry bulb temp. It's more an indicator of heat stroke risk than a real temperature equivalence.

[Eoink]

I'm not sure heat index is a thing here in the UK - we get wind chill, but not standardised humidity adjustments...

Really?!?!?! Then how do your TV "weather terrorists" earn their living in the summer?

9 years out of 10 in the UK they get to talk about wind chill in the summer.

[Vroomfundel]

Yeah, I didn't mention that part to not embarass myself but this comic was the first time I heard of heat index. It's not a thing in Europe. I guess the humidity in each area is in the same ballpark throughout the year so they just don't bother - people know what 32C feels like.

Something slightly related, I got reminded about a discussion with a classmate back in high school. He insisted that drinking hot tea in hot weather cools you down because, duh, arabs in the desert do it so it should obviously work. I demanded at least a semblance of explanation of how this might work in order to accept it, which I didn't get so promptly rejected the claim and never considered drinking hot tea in hot weather. It seemed stupid to overload my cooling system which is already struggling by artificially raising my body temperature, even if it bring some subjective sense of cooling off.

The satisfying resolution came a decade later when I figured out how this could work. Drinking a hot beverage raises your body temperature and triggers your overheating system. This in turns kick-starts perspiration, which does indeed cool you off, not just subjectively.

There are two caveats though:
1) You cool down by sweating more, which might not be what you're after e.g. in the office where you are trying to avoid the wet-shirt-armpit effect.
2) This only works if you weren't sweating already. If you have droplets of sweat rolling over your skin then it will only make you feel more miserable and might actually be dangerous in extreme heat.

So, for me the matter was settled - drinking a hot beverage was useless advise as there was no situation when I would like to sweat more, even if it brings a temporary relief.

Then, there was another revelation - the dry heat! Now I knew full well that high humidity makes the heat even more unbearable but little did I know that there actually are places on the surface of this planet where it's so dry that you don't actually form droplets of sweat on your skin.

Now, I believe the matter is settled for good. If you are in Las Vegas, and not in an office (or another environment where sweating would be undesirable, like a televised presidential debate or a dense poker table) - feel free to help yourself to a cup of hot tea, ideally while wearing a robe.

[Keyman]

I'm not sure heat index is a thing here in the UK - we get wind chill, but not standardised humidity adjustments...

Really?!?!?! Then how do your TV "weather terrorists" earn their living in the summer?

Showing weather-related (scenic) pictures sent in by the public?

Yep, OK...that's a thing.

Actually sent one in myself - a full double rainbow over downtown after a storm went by.
They didn't use it, the bastards!!

[Ranbot]

It's not a thing in Europe.

And it shouldn't be a thing in the US either because it's pseudo-scientific statistical bullshit. The only reason for silly heat index and wind chill factor "values" is to give weather reporters an extra 10-30 seconds of talking time in the 24-hour news cycle and make weather look more extreme than it really is. You'll get more [dumb] people listening/watching your weather report if you can say 85F "feels like" 105F with heat index; or 35F "feels like" -10F with wind chill. One might prepare for the weather differently, but there's no change to the actual temperature.

[Archgeek]

It's not a thing in Europe.

And it shouldn't be a thing in the US either because it's pseudo-scientific statistical bullshit. The only reason for silly heat index and wind chill factor "values" is to give weather reporters an extra 10-30 seconds of talking time in the 24-hour news cycle and make weather look more extreme than it really is. You'll get more [dumb] people listening/watching your weather report if you can say 85F "feels like" 105F with heat index; or 35F "feels like" -10F with wind chill. One might prepare for the weather differently, but there's no change to the actual temperature.

I don't know, heat index might be a bit doopy (if the dewpoint's over 70F and it's over 100 out, I know to wear thin, loose, short clothing and stick to the shade, thank you very much), but wind chill can be genuinely useful. If it's 35 out with a wind chill of -10, then I know there's an evil north wind that'll slice through my pants like they aren't even there, and I might want to consider an extra layer even though it's above freezing.

[Ranbot]

It's not a thing in Europe.

And it shouldn't be a thing in the US either because it's pseudo-scientific statistical bullshit. The only reason for silly heat index and wind chill factor "values" is to give weather reporters an extra 10-30 seconds of talking time in the 24-hour news cycle and make weather look more extreme than it really is. You'll get more [dumb] people listening/watching your weather report if you can say 85F "feels like" 105F with heat index; or 35F "feels like" -10F with wind chill. One might prepare for the weather differently, but there's no change to the actual temperature.

I don't know, heat index might be a bit doopy (if the dewpoint's over 70F and it's over 100 out, I know to wear thin, loose, short clothing and stick to the shade, thank you very much), but wind chill can be genuinely useful. If it's 35 out with a wind chill of -10, then I know there's an evil north wind that'll slice through my pants like they aren't even there, and I might want to consider an extra layer even though it's above freezing.

Like I said and you agree, you prepare differently, but the temperature is not any different. I grew up in a place that where thermostats regularly dipped below zero degrees Fahrenheit for multiple days. Even then I stood outside waiting for bus, made snow forts and snow men, went skiiing, etc. because I had the appropriate clothes that stopped wind and cold. A wind chill of -30F is a non-factor with the appropriate clothes. However on a weather report it looks way cooler [no pun intended] to show a 25F as -5F with wind chill.

[Leovan]

Considering the whole point of the weather report is to know how to prepare for the day, this makes it rather significant. And it's easier to understand when they say it's above freezing but feels like way below than if they just say what the temperature is and then two minutes later they tell you there's a strong wind from the north. People won't connect the two and will neglect to wear the heavy jacket and suffer. And blame the weather report for claiming it's above freezing.
In essence, they're converting a multi unit problem to a close approximation in one unit, which can be very useful for comparing and quick judgement.

[DanD]

Considering the whole point of the weather report is to know how to prepare for the day, this makes it rather significant. And it's easier to understand when they say it's above freezing but feels like way below than if they just say what the temperature is and then two minutes later they tell you there's a strong wind from the north. People won't connect the two and will neglect to wear the heavy jacket and suffer. And blame the weather report for claiming it's above freezing.
In essence, they're converting a multi unit problem to a close approximation in one unit, which can be very useful for comparing and quick judgement.

2nded. Some of you may live in an area where the humidity doesn't vary so much, but in the US North East, it can. And yes, I can do a combined humidity/RH estimate, but a heat index of 120 tells me I don't want to bike no matter what. I don't care if it's 96 and 65% RH or 88 and 100% RH.