Heat index - threat or menace?

[dreamshark]

We all know that it is miserably hot and humid; do we really need a made up number to tell us exactly how miserable we are? Well, maybe we do, because if we didn't have weather drama why would anybody even watch network TV anymore?

But I have yet to see a coherent explanation of what "feels like 117" actually means. Does 97 degrees at 80% humidity "feel like" 117 with zero humidity? Or like 117 with some kind of average humidity? Or like 117 in one of those states where they keep saying "But it's okay because it's a dry heat?" Because I'm not sure that even in the desert there is such a thing as zero humidity.

Personally, I think they just made up a random formula because Big Weather was whining about not having a number as dramatic as the Wind Chill Index that they could throw around in the summer to alarm people.

Comments

humidity

[tagwn.livejournal.com]

They actually do have a formula. I've been sensitive to humidity all my life and having the sweat sucked out of me by hot arid desert winds is very different than being slowly smothered in Florida. Both can be deadly, of course, but 90 with 80 dew point will probably kill me faster than 90 in Las Vegas (except for the disgustingness of the Las Vegas part).

Re: humidity

[dreamshark.livejournal.com]

Oh, I have no doubt that they have a formula, but what is it based on? The wind-chill index is apparently based on how long it takes for exposed flesh to freeze solid (anyway, that's how weather guys always describe it). This is something that can be scientifically measured (although I hate to imagine just how they tested that). But how do you measure how hot it "feels"?

The dew point is a perfectly good measurement with a clear scientific meaning. But if you have the dew point and the temperature, I think you've pretty much covered the bases.

Re: humidity

[sunshinenilkcub]

I kind of like it that they figure it out for me, then I don't have to because my brain is fried. Plus it's validation. My apartment is 90 inside and it feels 15 degrees warmer outside and they say the heat index is 117, so hey, I'm pretty good! Plus knowing what the heat index is tells me how to prepare. If they tell me that the heat index on Wednesday is going to be 110 to 115, then I know to wear a tank top, close my bedroom door and turn the AC on full blast in there, have my son sleep on a cot in my bedroom because the night is not going to cool down much.

I didn't know that about the frozen flesh. I'd be curious how they figured that out, unless they happened to do it on poor souls who died out there. I'd also be curious how they came up with the heat index. How do they figure out just how hot it feels like it is out there?

[minnehaha.livejournal.com]

Wikipedia explains both the mathematics and the physiology pretty well. And, yes, it is kind of like the Wind Chill Index for summer.

B

[dreamshark.livejournal.com]

It's a little hard to extract all the numbers in the right order from the Wikipedia entry, but it looks like the temperature/humidity combo that the formula norms against is 77F/50% humidity. That answers my most basic question about what this formula is based on, but does not allay my skepticism about this formula giving anything like accurate information about what a particular combination of temperature and humidity "feels like."

Why pick that combination to norm against? Our heat index yesterday may "feel like" 117F somewhere, but it probably doesn't feel anything like 117F in places like Arizona, where real temperatures of 117 are not uncommon but the humidity is almost never as high as 50%.

Personally, I think the temperature the last few days feels like 145. So there.

[skylarker.livejournal.com]

Hee. It does seem like a drama-based measurement, but I like my weather drama. And I like your assessment of 'feels like 145.' Personally, it feels like walking into a hot, wet sponge to me.

[quadong.livejournal.com]

If I've managed to type it all in properly, the Wikipedia formula says that for temperature T to have a heat index equal to T, the humidity must be:

80F: 44%
85F: 45%
90F: 40%
95F: Below 40%, which the page says the formula is not valid for
100F: Same deal.

I am skeptical.

[quadong.livejournal.com]

http://www.hpc.ncep.noaa.gov/html/heatindex.shtml
at least broadly agrees with the Wikipedia formula.

Since 40% humidity is nearly the bottom of what we normally get in Minnesota in the summer, it hardly seems reasonable to set it as the reference. As you say, it just allows weather reporters to quote shockingly high heat indexes all the time, since the heat index is *always* higher than the temperature.

[dd-b.livejournal.com]

I have to say, the Wikipedia explanation doesn't give me any confidence that the numbers at all accurately represent either comfort or any sort of risk.

The basic idea -- that it's less comfortable at 94F/90% than at 94F/60%, for example -- I strongly agree with. I'm not sure which is more dangerous -- dehydration would be faster at lower humidity, wouldn't it? So we can race dehydration against actual heat stroke and see who "wins".

Remember, people, thirst is not a reliable guide to needing to take in more fluids in hot conditions!

[cakmpls.livejournal.com]

As I have been saying all along, that "feels like" is crap. I've experienced 110+ degrees in the desert, and it feels NOTHING like what yesterday felt like here. I have no objection to the existence of a "heat index," if it communicates maybe the dangerousness of the temps, but "feels like," no.