# The Naked Scientists Forum

### Author Topic: Is a heat index helpful?  (Read 6441 times)

#### Geezer

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##### Is a heat index helpful?
« on: 22/07/2011 17:16:45 »
The Eastern US is experiencing some very high temperatures at the moment. The weather people are talking about "heat index" temperatures as well as the actual temperatures.

Are heat index temperatures really helpful, or do they simply confuse people even more? Wouldn't it be better just to quote the air temperature and relative humidity and let people work out the implications for themselves? Is a "heat index" just another attempt at "dumbing down" science?

#### JP

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##### Is a heat index helpful?
« Reply #1 on: 23/07/2011 14:45:00 »
As a warning system to tell people how dangerous the heat/humidity combo is, I think it's better than just telling them the temperature and relative humidity.  I certainly can't do the calculation in my head.  The formula for it is here:
.
(Source: http://en.wikipedia.org/wiki/Heat_index)

But I agree that the number is pretty useless, since there's no reference for what it means.  Perhaps giving the temperature/humidity along with a warning from low->extreme if the heat index is high might be more useful?

#### Geezer

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##### Is a heat index helpful?
« Reply #2 on: 23/07/2011 17:58:56 »
The formula reinforces my point. There are only two variables, T and R. c1 through c9 are a bunch of constants that are supposed (I think) to represent "typical" conditions, which ends up meaning, in total, they are anything but typical!

I think most people are capable of understanding that if the temperature is high, and the relative humidity is high, it's going to feel very unpleasant. I'm wondering if it would not be better to just let people know whether the RH is going to deviate much from the average for a particular temperature.

#### CZARCAR

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##### Is a heat index helpful?
« Reply #3 on: 23/07/2011 20:28:23 »
Accuweather? refers to heat index as "feels like" including winter windchill factor. Simple enuff 4me

#### Bored chemist

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##### Is a heat index helpful?
« Reply #4 on: 24/07/2011 14:59:37 »
Great! according to that WIKI page
"Here is a formula[8] for approximating the heat index in degrees Fahrenheit, to within ±1.3 °F. It is useful only when the temperature is at least 80 °F and the relative humidity is at least 40%"

So where I live
http://www.metoffice.gov.uk/climate/uk/averages/19611990/sites/sheffield.html
it is, on average, never useful.
And, according to this
http://www.metoffice.gov.uk/climate/uk/ne/
the temperature has practically never been high enough for the heat index to be meaningful.
On the other hand, the wind-chill factors are often important.

#### damocles

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##### Is a heat index helpful?
« Reply #5 on: 24/07/2011 15:23:44 »
Great! according to that WIKI page
"Here is a formula[8] for approximating the heat index in degrees Fahrenheit, to within ±1.3 °F. It is useful only when the temperature is at least 80 °F and the relative humidity is at least 40%"

So where I live
http://www.metoffice.gov.uk/climate/uk/averages/19611990/sites/sheffield.html
it is, on average, never useful.
And, according to this
http://www.metoffice.gov.uk/climate/uk/ne/
the temperature has practically never been high enough for the heat index to be meaningful.
On the other hand, the wind-chill factors are often important.

That should come as no surprise, Bored Chemist. The residents of Sheffield cannot really complain about uncomfortable heat, though they might sometimes find moderate heat and humidity a little uncomfortable because they are not acclimatized.

But where I come in with a little objection is that this sort of heat index works on the supposition that dry heat is fairly innocuous. When the temperature is >40 deg C (=104 deg F) and the humidity is less than 20%, the conditions raise a whole new and different set of problems, as we found here in Melbourne a couple of years ago when these conditions led to hundreds of bushfire deaths, and at least tens of (unrelated) heat exhaustion deaths in our state. (Americans, consider Death Valley, where I believe the heat is very dry).

People might cook in high humidity high temperature conditions, but they dehydrate readily and dangerously in high temperature low humidity conditions.
« Last Edit: 24/07/2011 15:28:48 by damocles »

#### JP

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##### Is a heat index helpful?
« Reply #6 on: 24/07/2011 16:11:25 »
I think most people are capable of understanding that if the temperature is high, and the relative humidity is high, it's going to feel very unpleasant.

I think you overestimate people.  I have a good grasp of temperature and humidity, but the heat index is still more useful to me than temperature and humidity when I'm planning a long run  or bike ride outdoors in the heat.  What may feel tolerable may actually be just slightly above your body's ability to dissipate heat.  While this might not be an issue for a walk outside, it's a huge difference if you're strenuously exercising for an hour.

#### Geezer

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##### Is a heat index helpful?
« Reply #7 on: 24/07/2011 18:34:42 »

I think you overestimate people.  I have a good grasp of temperature and humidity, but the heat index is still more useful to me than temperature and humidity when I'm planning a long run  or bike ride outdoors in the heat.  What may feel tolerable may actually be just slightly above your body's ability to dissipate heat.  While this might not be an issue for a walk outside, it's a huge difference if you're strenuously exercising for an hour.

Well, OK, b-b-b-but in that case wouldn't it be better to just assign arbitrary values like the five categories in the table on the Wiki page?

I'm not too keen on the attempt to decide what a temp/RH combination "feels like" in terms of a particular temperature when there are too many variables that will determine what any individual actually does feel. Not only that, but there is an initial supposition about the RH that determines when a temperature "feels like" the actual temperature! That means the heat index temperature should actually be less than the actual temperature when the RH is low (which, as Damocles points out) can be a situation that is just as dangerous as high RH.

(I can see I'm not going to win this argument  )

#### CZARCAR

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##### Is a heat index helpful?
« Reply #8 on: 24/07/2011 18:53:05 »
google Smartvent computerized ventilator which compares in & outdoor humidity so to decide when to ventilate

#### JP

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##### Is a heat index helpful?
« Reply #9 on: 24/07/2011 23:22:16 »
Geezer, I completely agree that the number itself is pretty meaningless.  The heat index makes it slightly easier for me to figure out when it's safe to go for a long run or bike ride, but the number is still pretty arbitrary.  The number also doesn't have much relation to actual temperatures, as you point out. It would be more useful if they'd just tell you in words how dangerous the heat is if you're doing strenuous exercise.

#### Geezer

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##### Is a heat index helpful?
« Reply #10 on: 25/07/2011 00:05:19 »
Geezer, I completely agree that the number itself is pretty meaningless.  The heat index makes it slightly easier for me to figure out when it's safe to go for a long run or bike ride, but the number is still pretty arbitrary.  The number also doesn't have much relation to actual temperatures, as you point out. It would be more useful if they'd just tell you in words how dangerous the heat is if you're doing strenuous exercise.

I never pay any attention to the "feels like" temperature here because a) I try to avoid doing anything strenuous; and b) it's seldom very humid here. Do you tend look at the heat index number only, or do you think you look at the actual temp and the heat index and do a sort of tradeoff?

In your case, you are probably not far from the norm that was established to determine the "feels like" values, so it's probably fairly accurate for you. However, a combination that feels like 80°F for you might feel a lot more like 100°F for my pal Fat Bastard

#### JP

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##### Is a heat index helpful?
« Reply #11 on: 26/07/2011 19:16:46 »
Geezer,

I can certainly tell you that I was shocked to learn that the daily high in Singapore was around 30 C, since it felt a lot hotter than that.  Of course, the humidity was around 85%.  Moving there, I had no idea what 30 C and 85% humidity would feel like.  Hearing that the heat index is around 40 C says a bit more to me.  In general, I find that for whatever reason I can't tell much about how it will feel just from relative humidity and temperature.

(Actually as an american, 40 C means a trip to a calculator to find out that it's around 104 F.)

As to your question, when I go out to exercise, the most important things to me are temperature, how it feels when I step outside, and if it's sunny.  I sometimes check heat index, especially if I'm planning a workout in advance and am relying on the forecast.  I almost never check relative humidity except as an afterthought.

#### Geezer

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##### Is a heat index helpful?
« Reply #12 on: 27/07/2011 07:50:10 »
Well, that pretty much seems to wrap this one up. The heat index is helpful (to JP at least ).

#### JP

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##### Is a heat index helpful?
« Reply #13 on: 27/07/2011 12:18:54 »
But not to Geezer!

So... how about wind chill?  :)

#### Geezer

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##### Is a heat index helpful?
« Reply #14 on: 27/07/2011 17:50:35 »

So... how about wind chill?  :)

It's equally dodgy. A bunch of different standards, and quite arbitrary. It would be much more honest just to use a scale of one through five or something similar.

My objection to these things is that they attempt to assign a degree (!) of precision that has little basis.

#### JP

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##### Is a heat index helpful?
« Reply #15 on: 27/07/2011 17:59:07 »
I completely agree.  The number has no precision to me, but it's a useful scale in both cases.  A warning scale from 1 to 5 would be far more useful.

#### Geezer

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##### Is a heat index helpful?
« Reply #16 on: 27/07/2011 18:15:06 »
If we didn't have so much important work to do here on TNS, we could could propose a new international standard

#### imatfaal

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##### Is a heat index helpful?
« Reply #17 on: 27/07/2011 18:46:40 »
weather in london is so mundane compared to those of you in more exotic climes; basically comes down to two yes/no questions - is it raining? and do I need a coat?

#### Geezer

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##### Is a heat index helpful?
« Reply #18 on: 27/07/2011 20:22:56 »
weather in london is so mundane compared to those of you in more exotic climes; basically comes down to two yes/no questions - is it raining? and do I need a coat?

Aren't both of those questions pretty much redundant?

#### CliffordK

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##### Is a heat index helpful?
« Reply #19 on: 27/07/2011 20:32:33 »
I spent some time in Missouri.
No AC in my vehicles.

Anyway, I assume the humidity was reasonably constant.  So, while 95°F might be different between Missouri and Oregon, it was reasonably representative for comparing day-to-day temperatures in one location.

Wind chill?  Of course I would drive with the windows down, and it wasn't bad, except for those moments when a cell phone call would come in and I had to roll up the windows.

Anyway, as long as the temperature kept below 100°F, then it was ok.

I went to Wichita once with 105°F weather, I think.  That was plain HOT.  The problem is that when you roll down the windows, it just blows warm air at you.  But, rolling up the windows doesn't help either.

Anyway, Heat Index might help somewhat in comparing vastly different environments.  But, it is no longer fully representative if you can artificially alter the air flow, for example rolling down the window, or using a fan, at which point, the moving air brings you back to needing a true comparison between body temperature and ambient temperature, i.e. just temperature.

Wind Chill in the winter?
Perhaps it is the same concept.  Wind speed might be useful.  But, as a human we can alter the parameters.  So, when you don your winter coat, you are separating the wind from your skin.  If you choose to ride your bicycle in 30°F weather...  then one is making one's own wind chill.

Anyway, I don't pay any attention to the calculated values.

#### The Naked Scientists Forum

##### Is a heat index helpful?
« Reply #19 on: 27/07/2011 20:32:33 »