Dry heat vs humid heat

[Ladewig]

Along the Gulf Coast, there are enough trees that some home construction is actually done in the shade. I have never seen home construction done in the shade in Las Vegas.

 

[Dymanic]

I remember hearing somewhere that in dry heat the person is not aware of how much they are sweating since the sweat evaporates quickly. They do not see the sweat and thus do not replenish the body with water accordingly.

There is, of course, this thing called "thirst" that sometimes kicks in to provide a helpful reminder to those who are that lacking in awareness.

Whereas in humid heat, the sweating is more noticeable and thus leads to that person to drinking more fluids to compensate.

So they stay hydrated, and that's a good thing. Unfortunately, it is of limited value as far as hyperthermia goes, because the whole purpose of all that sweating is to dissipate body heat through evaporative cooling. If the sweat isn't evaporating effectively due to high ambient humidity, that important mechanism may not be working well enough to prevent hyperthermia. It is possible to be fully hydrated and hyperthermic at the same time.

 

[Dymanic]

I live in Northern California, have a six figure salary, and don't have AC for my home, and almost never have (and rarely used it when I did). My house is tucked around behind several other houses, so I can see all four sides without violating anyone's property rights, and none of the five houses closest to me have any AC either. Neither do the other two units in my house. A good percentage of my friends don't bother with AC either--and the majority of the ones that do live on the other side of the coastal hills, and are isolated from the sea/bay. This is a temperate climate, which gets uncomfortably hot so rarely that many of us find little reason to bother with an air conditioner in our houses.

It sounds as though you live within spitting distance of that enormous air conditioning unit known as the "Pacific Ocean". Gloat much?

 

[Travis]

According to NOAA's table of heat index values, 100 degrees F at 40% relative humidity should subjectively feel the same as 90 degrees F at 75% relative humidity.

Unfortunately that chart doesn't go up to 115F. I'd be interested to know what 115F with 20% humidity feels like.

It's easier to dehydrate in wet heat than dry because you need to sweat more to get the same cooling. Basically it’s always better to have a dry heat than wet heat, but this still doesn’t mean the dry heat will be comfortable.

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.

I think I can help you.
I have studied enough physiology to know that humans have an overwhelming need to complain. Therefore there is nothing you can do or say to stop people from whining about how much more uncomfortable they are.

A friend of mine has decided the best way to deal with excessive complaining (and kooky conspiracy theories and petrified bigotry) is to just say "imagine that" and change the subject.

And of course I started this all off by complaining for complaining's sake.

Heat stroke occurs at lower temps in high humidity.

But can still occur in low humidity.

There can be so many explanations for that. Perhaps the more tolerable dry heat causes people to spend more time outside in dry areas, whereas in humid areas people tend to stay indoors or otherwise shield themselves?

As I note above people tend to not rehydrate properly in a dry heat. My suspicion is that they don't realize they are sweating as much as they are.

http://www.srh.noaa.gov/images/oun/wxsafety/summerwx/heatindex.gif

Above 80% humidity 90f "feels like" 115f. Regardless of humidity 115f is in the realm of freakishly hot.

Well the hottest I've experienced was Death Valley in June when it was 121F with humidity somewhere between 20-30%. I defy anyone to tell me that isn't really hot.

It looks to me like 90F at 80% feels like 105F, which I would agree with. Am I reading the chart wrong?

Edit: Or 110F, those lines are hard to follow.

I think it hits feeling like 115F at 85% humidity.

 

[xtifr]

It sounds as though you live within spitting distance of that enormous air conditioning unit known as the "Pacific Ocean". Gloat much?

Yes. Yes, I do. But that's not my point here. Also, people in LA or NYC live next to big oceans as well (in one case, the same big ocean), but don't generally dispense with AC. So it's more complicated than that. But in any case, my point was that "living in California" by itself does not necessarily equate to "probably owns an air conditioner". In fact, it may suggest a lower probability of air conditioner ownership than most of the rest of the US. And since the thread starter mentioned living in California....

And back to the original question: even if dry heat is statistically more deadly, my experience is that I have absolutely no problem remembering to stay hydrated in a dry heat. If I were old and starting to lose my wits, or an alcoholic, I might find myself with some concerns along those lines, but since I'm neither, I'll still pick dry heat over humid.

But, of course, too much heat is never fun, and I sympathize with anyone who's experiencing too much heat right now. Been there, done that. It is indeed somewhat silly to make a pissing match out of who is suffering more, because both conditions are unpleasant and potentially dangerous.

 

[TriangleMan]

As someone who lives in 40C+ heat for around four months of the year I can tell you that humid heat is far worse to deal with than dry heat at the same temperature and everyone dreads when humid days occur (in Qatar you can have bone-dry days and very humid days, I think depending on whether the wind is blowing from the southern deserts or from the Gulf).

During a humid day you will be drenched in sweat and condensation after walking maybe 75 metres, walking outside is more bearable on a dry day. I wouldn't say it was great or pleasant, but still easier than on a humid day.

The real difference occurs in the evening. If it is a dry day then the temperature decreases noticeably after the sun sets and the evenings are relatively pleasant, at least enough that you can walk around outside with only a bit of discomfort. On humid days it's actually worse in the evenings, it's like the moisture in the air somehow releases the heat after the sun sets so even though it's dark it is hot and sticky and you sweat like you're in a sauna.

Given Qatar can switch from very humid to dry within a day it's easy to compare -- humid heat is worse.

 

[JoeTheJuggler]

Unfortunately that chart doesn't go up to 115F. I'd be interested to know what 115F with 20% humidity feels like.

[ETA: According to the chart ApolloGnomon posted, the heat index for 115 F temp at 20% relative humidity is about 120 F--about the same as 90 F at 90% humidity.]

Well, the heat index certainly won't be lower than 115F. And 115F heat index is dangerously hot.

But it doesn't matter if that 115 heat index happens in actual 115 F temperature with low humidity or a lower temperature with high humidity.

And 115 F would result in a higher heat index if the humidity were higher.

You did ask for the relationship between heat and humidity, and that subjective experience is described by the heat index.

ETA: As I said in my first reply, there is a trade off where a higher temp at lower humidity is preferable (feels less hot) than a lower temp at higher humidity, and the limits of that trade off are obvious from a comparison of heat index values.

 

[lomiller]

I live in Northern California, have a six figure salary, and don't have AC for my home, and almost never have (and rarely used it when I did). My house is tucked around behind several other houses, so I can see all four sides without violating anyone's property rights, and none of the five houses closest to me have any AC either. Neither do the other two units in my house. A good percentage of my friends don't bother with AC either--and the majority of the ones that do live on the other side of the coastal hills, and are isolated from the sea/bay. This is a temperate climate, which gets uncomfortably hot so rarely that many of us find little reason to bother with an air conditioner in our houses.

(Cars and office buildings are another matter.)

So seeing "California" and leaping to the conclusion AC will be the norm is somewhat unjustified, unless you specify what part of California. (La-la land is a whole 'nuther matter.)

IIRC some parts of California have remarkably stable temperatures that are almost never particularly hot or cold. If anything we should expect to see less climate control that you would in more seasonal climates, at least in some areas.

 

[JoeTheJuggler]

Along the Gulf Coast, there are enough trees that some home construction is actually done in the shade. I have never seen home construction done in the shade in Las Vegas.

On an unrelated note: they're about to finish a new bridge spanning the Mississippi at St. Louis, and they have to do the final bits at night when the materials are less expanded due to the heat. (I would say when they're contracted, but even at night, it's nothing like the cold they'll face in the winter.)

 

[JoeTheJuggler]

IIRC some parts of California have remarkably stable temperatures that are almost never particularly hot or cold. If anything we should expect to see less climate control that you would in more seasonal climates, at least in some areas.

So if the claim that most people don't have AC is based on the fact that most of the people in question don't need AC, then the claim is irrelevant. (In considering whether dry heat or humid heat is more difficult to deal with, the fact that people who don't live in either don't have air conditioning doesn't matter.)

I will safely say that most of the people that live in the U.S. in places where it gets very hot have AC. And "most" means just that--at least 50%.

The original claim has been clarified, though: he was talking about most people in the world. So even if we're talking about only the people where it gets very hot, the claim is true.

Also, a trivial observation that supports the point he was making: even if you have AC in the home, it doesn't help you deal with high humidity if you work or do other activities outside.

 

[lomiller]

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.

I don’t really buy this. If anything I’d give more credibility to the opposite, because there at least you could argue that evaporation in a dry heat would cause your nose, throat and mouth to feel dry triggering an urge to drink more. I’d need more convincing that either environment inherently triggered more hydration.

What I will concede about a dry heat is that it’s almost away going to be clear and therefore sunny, which can create localized temperatures much greater than the reported air temperature which could be a serious problem.

 

[JoeTheJuggler]

As I note above people tend to not rehydrate properly in a dry heat. My suspicion is that they don't realize they are sweating as much as they are.

Or maybe there's a confounding factor. Older people tend to dehydrate more easily (for the same reason--they're not aware of it). I'll speculate that maybe more older people live in dry, hot climates!



But seriously, do you have any evidence that people dehydrate more readily in dry heat?

I've never heard any such a thing, and it doesn't really make much sense. I understand they might not realize they're sweating as much because the sweat is evaporating, but when that happens, it cools the body temp, and they in fact sweat less (than someone sweating unproductively in the same temp at higher humidity).

 

[RussDill]

Kinda makes you wonder why anyone lives in Indio, or Las Vegas, or Phoenix, if you really think about it.

During the recent heat wave some weeks ago, Death Valley drew a large number of visitors when the word went out that there was a possibility that it would break its own record for the highest reliably reported air temperature in the world -- 134 °F (56.7 °C) -- on July 10, 1913. This suggests that among the things you'd have to control for is the mental health of the individual. And sure enough, that factor appears on a table in the article you linked.


The innacuracy of those numbers suggests that there are some things you don't know about Phoenix. To be fair, it's been quite a while since I lived there myself, and my understanding is that weather patterns there have changed some in recent years. With that disclaimer in mind, my answer to your question -- with respect to Phoenix specifically -- is that while dry heat prevails through much of the summer (which is at least seven months long, as I recall), from somewhere around mid-July to mid-September, there's this thing called the "monsoon season" during which high temperatures are not infrequently accompanied by high humidity -- coming, yes, in waves.

My bad, I screwed up the low temp, I meant on some days it doesn't drop below 90F. 2012 though, I do count 119 days at 100F or higher.

"High humidity" is pretty subjective here, this past July, the highest the dew point got was 73F, and the high dew point days are not the days with the high temps.

 

[RussDill]

Or maybe there's a confounding factor. Older people tend to dehydrate more easily (for the same reason--they're not aware of it). I'll speculate that maybe more older people live in dry, hot climates!



But seriously, do you have any evidence that people dehydrate more readily in dry heat?

I've never heard any such a thing, and it doesn't really make much sense. I understand they might not realize they're sweating as much because the sweat is evaporating, but when that happens, it cools the body temp, and they in fact sweat less (than someone sweating unproductively in the same temp at higher humidity).

Or maybe its that their sweat actually does allow them to keep being physically very active, where as in a humid heat, you can't cool off so you slow down. As soon as dehydration sets in though, you'd fall off a cliff. What's worse, 100F with high humidity covered in sweat, or 115F with no humidity, dehydrated, and no sweat?

Or better question, which will kill you faster, being dehydrated in 115F heat (dry), or being dehydrated in 100F heat (with humidity)? Does the dew point really matter if you can't sweat?

 

[JoeTheJuggler]

What I will concede about a dry heat is that it’s almost away going to be clear and therefore sunny, which can create localized temperatures much greater than the reported air temperature which could be a serious problem.

I'd buy that. Heat index doesn't take into account radiative heating (solar gain).

Personally, when I run in hot weather, ceteris parabis, the difference between a cloudy and sunny day is pretty stark.

 

[JoeTheJuggler]

Or maybe its that their sweat actually does allow them to keep being physically very active, where as in a humid heat, you can't cool off so you slow down.

That's getting to be a pretty indirect cause. I would say in that case you're not comparing apples to apples if you're comparing someone more active in dry heat to someone less active in humid heat.

I said it jokingly, but you could as well consider a higher median age to be "caused" by the dry heat.

ETA: Also, isn't that sort of like arguing that extreme cold is very safe since few people die in the Antarctic compared to other places in the world? It's similar to saying that few people live there because in fact it's so dangerously cold. Similarly, arguing that people are less active in humid vs dry heat at the same temperature is another way of saying humid heat feels worse and that it would be more dangerous to be equally active in it.

 

[RussDill]

Along the Gulf Coast, there are enough trees that some home construction is actually done in the shade. I have never seen home construction done in the shade in Las Vegas.

Heck, in Phoenix, for many kinds of construction, they don't bother even trying to do anything during they day, they work at night and use lights.

 

[RussDill]

ETA: Also, isn't that sort of like arguing that extreme cold is very safe since few people die in the Antarctic compared to other places in the world? It's similar to saying that few people live there because in fact it's so dangerously cold. Similarly, arguing that people are less active in humid vs dry heat at the same temperature is another way of saying humid heat feels worse and that it would be more dangerous to be equally active in it.

Almost gets to what I'm saying, but I think the idea that sweat works really really well in a dry heat so long as you stay hydrated is the main point. I've seen people suffering from heat stroke/exhaustion many times (in AZ). The reason has never been that its too hot, the reason has always been that they aren't drinking enough water, everyone else doing the same activity in the same area is fine.

So I guess that a dry heat can be more dangerous in that its easy for people who aren't prepared or keeping hydrated to get into trouble. If the question is which one is more miserable, no matter how much you hydrate or prepare, I'll give the point to the mugginess.

 

[JoeTheJuggler]

Almost gets to what I'm saying, but I think the idea that sweat works really really well in a dry heat so long as you stay hydrated is the main point.

Someone else has pointed out that those are two different issues. Sweat works well to cool the body if it evaporates whether or not you rehydrate (unless of course you've dehydrated to the point where you stop sweating--and you will in fact feel acutely thirsty long before that happens whether or not your skin feels dry or sticky).

And since it is working to cool your body, you would sweat less (and therefore require rehydration less) than someone in the same temp in higher humidity.

 

[desertgal]

Being from the Nevada desert, and having spent the last ten years in Virginia, Alabama, and now Charleston, I'd pick dry heat over humid. True, hot is hot, but wet and hot is more uncomfortable.