Dry heat vs humid heat

[JoeTheJuggler]

Meanwhile, is it not possible that the effectiveness of sweating in dry heat reduces water loss relative to the same person in high humidity (who is uselessly sweating buckets)?

I think it's possible that the effectiveness of sweating might reduce water loss even to less than that experienced by someone in high humidity--at the same temperature.

Experiment - take many pairs of identical twins and get them equally prepared, hydrated, weighed and blood tested etc. Put one from each pair into a chamber at 35° with 30% humidity and a nice fan. Put the other into an identical chamber but with 80% humidity. Let them sweat it out for a decent time and then weigh and blood test them afterwards.

Calling Dr. Mengele?

But yes, that is the sort of evidence we'd need. [ETA: Though I'd leave out the fan, because it complicates things.]

I would again refer to the heat index charts and point out that at 95 F (35 C) 30% humidity results in a heat index of about 96 F. The same temp at 80% humidity results in a heat index of about 130 F!

The experience of these two different conditions isn't just a tiny bit different. It's pretty drastically different.

 

[Skeletor]

As for dry versus humid heat. I lived in Qatar, and during the summer months, it was horrific. During the day the sun was oppressive, and at night it turned into a steamy sweat bath.

Ah, the place that's hosting the World Cup in the summer of 2022? That'll be fun

Oh...but, but they'll have every stadium air conditioned...and air conditioned sidewalks, and...and maybe even a dome over the country!

Yeah, when they announced the World Cup in Qatar, I fully expected the planners to change the schedule to a winter instead of summer playing time. Summer is oppressive in Qatar no matter the time of day, but winter there can be downright chilly at night.

 

[JoeTheJuggler]

In a dry environment you need to constantly be rehydrating and lots of people are not used to that.

Again, the very same thing is true in humid heat.

I might accept the case that in the kind of heat where dehydration isn't so noticeable (a problem especially with older people)--that is, with moderately hot temps--dry heat might be more problematic simply because of the deceptiveness of it. But that's not what you asked about in the OP.

The context was extremely high temperature.

At the same high temperature, dry heat is not more dangerous than humid heat. The heat index chart shows why.

I'm certainly not claiming that 115 F is not dangerous. I'm pointing out that 115 F with very low humidity is about as dangerous as 90 F with extremely high relative humidity.

If we compare 115 F with low humidity to 115 F with high humidity, there is no way that the dryer condition is more dangerous than the humid one.

 

[Ron_Tomkins]

Ah but you forget the human psychology of it. Yes if the people are equally hydrated this might be the case but in my experience people in a dry heat tend to forget to rehydrate adequately and that causes problems.

With all due respect, Travis, that's the stupidest counter-argument I've read in a long time. That you happen to know of some people that are foolish enough that they forget to rehydrate themselves, says nothing about whether Dry Heat is better than Humid Heat. It is not the Heat's fault if some people forget to rehydrate. That does not affect the Heat's "score", but rather the person's.
As far as I understood, we were discussing which type of Heat is better, meaning which one is less miserable, easier to deal with. But now you seem to be moving the goalposts into trying to argue that Dry Heat is more dangerous, based on your personal experience that some people tend to forget to hydrate themselves, but I doubt you'll ever find me a serious statistical study that demonstrates that Dry Heat tends to make people forget to rehydrate themselves, and thus being more likely to kill people.

 

[RussDill]

More dire than hyperthermia?

Ok, lets say you have two people in climates of the same heat index, but opposite humidity extremes, say with temps of 100F and 120F respectively. Since their skin temps are similar, their rate of perspiration and fluid loss should be similar. They don't rehydrate and become dehydrated. I'm thinking the person in the humid climate will stay slicked with sweat much longer than the person in the dry climate.

Also, once both lose all their perspiration, I don't think the heat index applies any longer. The only way either can lose heat is by convection or radiation. I don't have any studies, but I'm pretty sure the person unable to perspire would succumb to hyperthermia much faster in a hot dry environment than a midler, humid environment.

 

[RussDill]

Do you have anything to support this?


To be clear, we're talking about this and not dry, chapped skin.

ETA: FWIW, from the NWS data on heat-related deaths in 2012, the state with the highest number (absolute, not per capita) was Missouri with 34. Second was Illinois with 32. Arizona had 4. The total for the U.S. was 155.

http://www.nws.noaa.gov/om/hazstats/heat12.pdf

I don't know if that year was typical, but it sure seems to blow a hole in the theory that dry heat is more dangerous than humid heat.

ETA again: I've scanned a few more years of that data (it's all in pdf files from 2012 back to 1995--just change the last two digits to the appropriate year), and it sure doesn't look like deaths correlate with states that tend to have dry heat. I'd love to see the same figures given per capita and adjusted for age. Ideally would be data ranked by humidity too (as some have noted, parts of the southwest can be humid sometimes).

I'm not sure where they are getting their data, but even you should realize that there is something not right.

http://www.maricopa.gov/publichealth/Services/EPI/pdf/heat/2012annualreport.pdf

Here's a report from Maricopa County (Arizona's most populous county, by far) for 2012. It gives 102 confirmed heat related deaths for 2012.

 

[Dymanic]

I'm thinking the person in the humid climate will stay slicked with sweat much longer than the person in the dry climate.

I snipped out just that portion of your post because it is the very heart of the matter. Staying slick with sweat indicates less evaporation. Less evaporation means higher temperature (hyper) ( thermia).

I'm pretty sure the person unable to perspire would succumb to hyperthermia much faster in a hot dry environment than a midler, humid environment.

Remember the CDC article you linked? The first line reads: "Heat-related illnesses (e.g., heat cramps, heat exhaustion, heat syncope, or heatstroke) can occur when high ambient temperatures overcome the body's natural ability to dissipate heat."

As has been noted, dehydration can occur under any temperature conditions, but as we are concerned only with high temperatures, we can ignore that. Dehydration can have consequences other than hyperthermia as well, but since hyperthermia is the one that typically leads most quickly to death, we can probably ignore that as well in the context of heat-related deaths.

What is most pertinent is that hyperthermia can occur without dehydration. Sweating is the primary mechanism by which the body rids itself of excess heat, and depends on evaporation (it is not meaningful to regard sweating as separable from evaporation; for the mechanism to work requires that both take place). Under hot conditions, dehydration cannot be "more dire" than hyperthermia, because of all of the possible consequences of dehydration, hyperthermia is the very one we are concerned with most.

ETA: Remember James Arthur Ray? The self-help guru convicted on three counts of negligent homicide when some of his... uh... clients couldn't handle the heat in his Sedona sweat lodge? Even if those people were completely dehydrated, they still would have been drenched in moisture from the steam. The problem was that it wasn't cooling them off, because it wasn't evaporating.

Oh yeah. That guy just got out of jail a couple of weeks ago.

 

[RussDill]

I snipped out just that portion of your post because it is the very heart of the matter. Staying slick with sweat indicates less evaporation. Less evaporation means higher temperature (hyper) ( thermia).

Right, which is why there is a higher heat index value for a given temperature when the humidity is higher. The heat index is related to your skin temperature assuming "wet bulb" conditions, aka, the person is sweating. If the heat index is at 120F, in a dry heat, or a humid heat, your body is having a similar amount of difficulty keeping cool in both situations. The difference I was pointing out is the differing danger of dehydration. Once you can't perspire, you are now dealing with "dry bulb" temperature.

Remember the CDC article you linked? The first line reads: "Heat-related illnesses (e.g., heat cramps, heat exhaustion, heat syncope, or heatstroke) can occur when high ambient temperatures overcome the body's natural ability to dissipate heat."

As has been noted, dehydration can occur under any temperature conditions, but as we are concerned only with high temperatures, we can ignore that. Dehydration can have consequences other than hyperthermia as well, but since hyperthermia is the one that typically leads most quickly to death, we can probably ignore that as well in the context of heat-related deaths.

What is most pertinent is that hyperthermia can occur without dehydration. Sweating is the primary mechanism by which the body rids itself of excess heat, and depends on evaporation (it is not meaningful to regard sweating as separable from evaporation; for the mechanism to work requires that both take place). Under hot conditions, dehydration cannot be "more dire" than hyperthermia, because of all of the possible consequences of dehydration, hyperthermia is the very one we are concerned with most.

Perhaps I have a bit of confirmation bias here. Living in AZ, all of the heat exhaustion/stroke cases I've seen are a result of someone not drinking enough water and not being able to perspire sufficiently.

ETA: Remember James Arthur Ray? The self-help guru convicted on three counts of negligent homicide when some of his... uh... clients couldn't handle the heat in his Sedona sweat lodge? Even if those people were completely dehydrated, they still would have been drenched in moisture from the steam. The problem was that it wasn't cooling them off, because it wasn't evaporating.

Oh yeah. That guy just got out of jail a couple of weeks ago.

ridiculous.

 

[jimbob]

More humidity at any given temperature is definitely worse. And high humidity doesn't prevent dehydration. You sweat just as much, it just doesn't cool you off as well.

The problem with the comparison, though, is that relative and even absolute humidity are often lower when the temperature gets extremely high, so quite frequently we aren't interested in comparisons of the same temperature at different humidity levels, since the factors that lead to high humidity (ie, lots of water evaporation) also act to slow temperature increase. And it's obvious that even compared to 90% relative humidity and 100 F (which is admittedly miserable), there is some temperature at which even 0% humidity is still worse. So there's some tradeoff, and the heat index is intended to capture the basics of that tradeoff. If one were to draw out a chart of temperature versus humidity, there should be sloped lines of equi-misery: if one increases temperature but decreases humidity by the proper amounts (or vice versa), one should be able to maintain a constant level of discomfort. But I doubt that chart is truly universal, it probably varies between individuals based on their particular physiology. The heat index is just an attempted approximation of an average.

This

You sweat, it is just that it doesn't provide much benefit.

As an aside, I remember reading that Amazonian Indians tend to sweat less than most other people for a given temperature, which would make sense as it wouldn't benefit them much in a still, humid rainforest environment.

 

[luchog]

When I was in training in the Army, they were very strict to differentiate between simple heat and heat+humidity. They relied heavily on "Wet-bulb Temperature" to determine the relative risk factor created by the heat and humidity. My training was predominantly in the spring and summer in South Carolina, and summer and fall in Oklahoma. The temperature and humidity in South Caroline were roughly 95F/90%, and in Oklahoma was 110F/30%. There was a substantial difference between the two. The former was commonly risk category 4-5, while the latter was generally only category 3, rarely peaking at 4.

 

[lomiller]

When I was in training in the Army, they were very strict to differentiate between simple heat and heat+humidity. They relied heavily on "Wet-bulb Temperature" to determine the relative risk factor created by the heat and humidity. My training was predominantly in the spring and summer in South Carolina, and summer and fall in Oklahoma. The temperature and humidity in South Caroline were roughly 95F/90%, and in Oklahoma was 110F/30%. There was a substantial difference between the two. The former was commonly risk category 4-5, while the latter was generally only category 3, rarely peaking at 4.

Are you sure it was as high as 95 @ 90%?

I ran that though a wet bulb temperature calculator and it comes to 92 (33.5 C), which isn’t common anywhere on earth.

From the wiki link

A sustained wet-bulb temperature exceeding 35 °C is likely to be fatal even to fit and healthy people, unclothed in the shade next to a fan; at this temperature we switch from cooling the skin (losing heat to the environment), to warming it.[7] As of 2010 wet-bulb temperatures only very rarely exceeded 30 °C anywhere.

 

[luchog]

Are you sure it was as high as 95 @ 90%?

I ran that though a wet bulb temperature calculator and it comes to 92 (33.5 C), which isn’t common anywhere on earth.

A little bit of hyperbole; but it was hot and humid enough we weren't even allowed to wear full uniform or do anything at all strenuous at times. Most of the day we sat around in the shade drinking icewater.

You can see the official ratings here: http://www.ddeamc.amedd.army.mil/wetbulb.aspx

And yes, it did break 90 several times, peaking at about 92. For several weeks straight it rarely ventured below category 4; and almost never below category 3 most of the time I was there. We tended to do all of our strenuous work very early morning, before the heat could really hit. By contrast, Oklahoma's 100+F days with low humidity was comfortable.

 

[JoeTheJuggler]

I'm not sure where they are getting their data, but even you should realize that there is something not right.

http://www.maricopa.gov/publichealth/Services/EPI/pdf/heat/2012annualreport.pdf

Here's a report from Maricopa County (Arizona's most populous county, by far) for 2012. It gives 102 confirmed heat related deaths for 2012.

Obviously they're using two different definitions of heat-related deaths. The data you give don't provide any comparison to other states in other parts of the country. [ETA: And so it does not provide any evidence to support the claim that dry heat is more dangerous than humid heat all other things being equal.]

That's why I went with numbers given by a national entity.

If you are going to use these numbers for Arizona, what numbers for other states will you compare them to? You can't reject some of the numbers from the NWS site and accept others in order to make a comparison.

So yet again I ask, what evidence do you have to support the claim that dry heat is more dangerous?

Does that evidence (which I haven't seen at all yet) outweigh the research behind heat index tables? Does it refute the fact that sweat is more effective at cooling the body in dry heat than it is in humid heat, ceteris paribus?

 

[JoeTheJuggler]

If the heat index is at 120F, in a dry heat, or a humid heat, your body is having a similar amount of difficulty keeping cool in both situations.

Yes. But in dry heat, you're at that heat index at a higher temperature than you are in humid heat.

If we're answering the question the OP poses, we should compare the same temperature at two different relative humidity levels. In this comparison, dry heat is not more likely to dehydrate you than the humid heat. In this situation, there will be two different heat index values, with the humid heat having the higher one.

 

[RussDill]

Obviously they're using two different definitions of heat-related deaths. The data you give don't provide any comparison to other states in other parts of the country. [ETA: And so it does not provide any evidence to support the claim that dry heat is more dangerous than humid heat all other things being equal.]

That's why I went with numbers given by a national entity.

If you are going to use these numbers for Arizona, what numbers for other states will you compare them to? You can't reject some of the numbers from the NWS site and accept others in order to make a comparison.

You are making an assumption that data is being collected by a consistent standard or even in a vaguely similar way state by state. I'm not sure, but they may only be collecting fatalities from "weather events", eg, heat waves, and ignoring normal days. In any case, its pretty clear you are cherry picking. The only vague information given is:

The U.S. Natural Hazard Statistics provide statistical information on fatalities, injuries and damages caused by weather related hazards. These statistics are compiled by the Office of Services and the National Climatic Data Center from information contained in Storm Data, a report comprising data from NWS forecast offices in the 50 states, Puerto Rico, Guam and the Virgin Islands.

Here's some data fram the CDC, who is very clear about how they collect their data and who's data is actually fully available:

http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5425a2.htm
http://www.cdc.gov/mmwr/preview/mmwrhtml/mm4822a2.htm
http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5529a2.htm

So yet again I ask, what evidence do you have to support the claim that dry heat is more dangerous?

How about using the CDC wonder engine. I'll do years 1999-2010, ages 15-44. (cause / number of deaths / population / deaths per 100,000)

Missouri:

X30 (Exposure to excessive natural heat) / 20 / 28,690,918 / 0.1

Arizona:

X30 (Exposure to excessive natural heat) / 103 / 29,097,133 / 0.4

http://wonder.cdc.gov/ucd-icd10.html

Does that evidence (which I haven't seen at all yet) outweigh the research behind heat index tables? Does it refute the fact that sweat is more effective at cooling the body in dry heat than it is in humid heat, ceteris paribus?

Look, no one is comparing a dry 100°F to a humid 100°F. I think it's clear to everyone which is more dangerous/miserable. I think what is being compared is similar heat index numbers. Which is more miserable or dangerous, 120° heat index with low humidity, or 120° with high humidity. I don't think anyone would call 88°F with low humidity a "dry heat", but the same temperature combined with a high dew point would be considered hot.

 

[JoeTheJuggler]

You are making an assumption that data is being collected by a consistent standard or even in a vaguely similar way state by state. I'm not sure, but they may only be collecting fatalities from "weather events", eg, heat waves, and ignoring normal days. In any case, its pretty clear you are cherry picking.

It's a reasonable assumption that in the NWS reports of number of heat related deaths per state they used the same standard.

I am not cherry picking. I'm also not offering this of evidence that humid heat is more dangerous, but pointing to it as evidence that weakens the claim that dry heat is more dangerous. (That's why I prefaced this with "FWIW".)

Here's some data fram the CDC, who is very clear about how they collect their data and who's data is actually fully available:

http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5425a2.htm
http://www.cdc.gov/mmwr/preview/mmwrhtml/mm4822a2.htm
http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5529a2.htm

OK. . . you're getting warmer, but while the Arizona page lists a total number of heat-related deaths, the Missouri page does not. It only describes 4 case studies.

The Arizona page points out that the risk of heat-related death is greater in Arizona than the national average.

This supports the claim that Arizona is hotter than the average of the U.Ṡ state. But I still don't see anything to support the claim that dry heat is more dangerous than humid heat.

Look, no one is comparing a dry 100°F to a humid 100°F. I think it's clear to everyone which is more dangerous/miserable.

That's not what the OP is asking.

I think what is being compared is similar heat index numbers. Which is more miserable or dangerous, 120° heat index with low humidity, or 120° with high humidity.

In fact, that actually supports the comparison that the OP is criticizing. (For example, 120 heat index with low humidity means a temperature very near 120 F. But 120 heat index with very high humidity could be a temperature below 100 F.)

If you're only defending the claim that at the same heat index, the dry heat causes a greater danger of dehydration, I wouldn't argue against it. But that's not the same as the claim that dry heat is more dangerous than humid heat, all other things being equal.

In fact, by using same heat index values, you're already assuming that at the same temperature, humid heat is more dangerous and uncomfortable than dry heat.

ETA: Please read my first couple of posts in this thread. It sounds like you and I agree that at the same temperature humid heat is more uncomfortable and more dangerous.

 

[Red Baron Farms]

Look, no one is comparing a dry 100°F to a humid 100°F. I think it's clear to everyone which is more dangerous/miserable.

It is my contention that while a humid 100°F is more miserable, (higher heat index), a dry 100°F is more dangerous.

You don't need a fancy scientific study to prove this. Just go to an ecosystem with relatively many humid 100°F +/- days as compared to an ecosystem with relatively many dry 100°F +/- days. Look around. Which ecosystem has more life? Humidity is water and water = life in abundance. Only in our artificial world does that potentially change. So sure, in some cases taking into account the artificial world we live in, humid could sometimes be more dangerous, but as a general rule not. It is a lot more than just how we feel. With enough humidity you won't dry out your eyes nose mouth lungs or crack your skin. I would be willing to bet money that if you took 2 people. Put one in a controlled dry 100°F environment and the other in a controlled humid 100°F environment. Gave them both 5 gallons of water. Then just sat back and waited to see who survived the longest. The one in the humid environment would survive longer almost every time. But if you ordered them to do forced labor, and didn't care if they got over heated, then in that scenario the one in the humid environment might die quicker, certainly more likely to get heat stroke quicker.

 

[lomiller]

It is my contention that while a humid 100°F is more miserable, (higher heat index), a dry 100°F is more dangerous.

I don’t agree.

Look around. Which ecosystem has more life? Humidity is water and water = life in abundance.

Don’t confuse availability of groundwater to the effect of heat/humidity.

I would be willing to bet money that if you took 2 people. Put one in a controlled dry 100°F environment and the other in a controlled humid 100°F environment. Gave them both 5 gallons of water. Then just sat back and waited to see who survived the longest. The one in the humid environment would survive longer almost every time.

100 def F with humidity over 82.5 humidity isn’t just uncomfortable it’s over the 95 deg wet bulb temperature which is lethal to healthy individuals.

At this temperature and humidity you don’t survive long enough to dehydrate and sweating becomes irrelevant as even wet skin can’t transport heat out of your body.
See the wiki quote from above

A sustained wet-bulb temperature exceeding 35 °C is likely to be fatal even to fit and healthy people, unclothed in the shade next to a fan; at this temperature we switch from cooling the skin (losing heat to the environment), to warming it.

 

[Ron_Tomkins]

IOK. . . you're getting warmer,

I see what you did there...

 

[John Jones]

I'm sorry, reality disagrees

http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5529a2.htm

Never mind. I see I took the bait.