Sweat in Humid vs. Dry Air

I haven't studied the physiology of sweating, but I would expect humid weather (all other variables being constant) to result in greater fluid loss. In dry air sweat evaporates and cools your body, therefore you don't need to sweat as much to regulate your temperature. But in humid air the sweat drips off you and no cooling results. Thus, your body reacts by increasing sweating in an ineffectual attempt to increase cooling. The result would be greater fluid loss.

This is my "seat of the pants" theory. Maybe an exercise physiologist will get on here and give us some facts.

I weigh myself before/after each run and I lose the most weight in the hot sticky humid weather.

Having lived in both a humid climate and in Boulder, CO, I can tell you that you lose a lot more sweat in hot, dry weather compared to humid weather. I had to hydrate quite a bit more in Boulder and it got very annoying. Most people think that since you feel the sweat more on your skin in the humid weather that you are sweating more, but it just isn't evaporating and therefore you aren't sweating as much. That is why your performances get worse in humid weather, because no cooling is taking place from the sweating, it just sits there. Years ago I tried explaining this to a walk-on for Colorado and he just couldn't grasp the concept that he was losing more sweat in Boulder and therefore should be drinking more. He had only been there a few days and I suspect he figured it out.

To clarify my comparison, I am talking about comparing two runs, both in 85 degree weather, one in Boulder with very low humidity, compared to a run in 85 degree weather with a high dew point. The really dry air just dehydrates you so quickly. In perfect conditions, such as fall in the midwest with temps in the 50's or 60's and moderately dry air, of course you won't lose as much fluid as a hot humid day.

Agree that the dry air sucks the water out of you. But is it all sweat, or do you lose more water due to breathing the dry air? This one needs a Mythbusters kind of test.

Since we're using anecdotal evidence, I've also run in both hot dry weather and hot humid weather. I definitely lose more weight running in humid Atlanta, GA or Boone, NC than I did running in dry Bend, OR at similar temps. In fact I'd lose more weight in the humid areas when the temp is in the 70's than I would in Bend when it was in the 90's. The sweat doesn't sit on your skin, it drips off.

However, I will agree that most people don't realize how much water they're losing when the air is dry.

I get way, way more dehydrated in humidity.

I really have a hard time believing people are saying they get more dehydrated in really humid weather, both from my own experience and using logic. I am just wondering if people are mistaking their extra discomfort of running in the humidity compared to dry air. Absolutely running in hot humid weather is more difficult than in hot, dry air. But we are talking about actually hydration levels. Someone mentioned that the sweat doesn't stay on the skin, it just drips off, but that is not true. Much of it is dripping off, but more of it is just staying on the skin, preventing you from sweating more. When you are in hot, dry air, the sweat keeps evaporating and therefore you keep sweating at a rapid rate and getting cooled off.

Anyway, I am not an exercise physiologist and hopefully someone will give us a definite answer, with actual studies. All I know is when I moved to Boulder, I had to drink so much more water, and not just after running, but continuously throughout the day. I eventually had to move away because I couldn't stay hydrated. That never happened in more humid weather.

I agree 100 percent with you Former Altitude...

Living here and running, I have to drink tons of water even though I don't sweat per say AS much as if I was in a humid climate (I've run in Cali, Hawaii and Florida during warm months and sweated like crazy). Don't get me wrong I still sweat where I live, but usually won't notice it until I stop, whereas in a humid climate I sweat immediately.

I've also gone from 'dry' training (in other words, it's still dang hot here in the summer but the humidity is NOT usually as bad unless we have a storm coming then it changes--hot+some humidity equates to horrible running) to a 'humid' race and it was VERY stifling for me, the air felt thick and I felt like I couldn't breathe. It is extremely discouraging training in the dry heat all summer and go to a race that ends up about 90 percent humidity...

I've also gone from the valley to altitude to run and again notice the big difference in the air. Higher altitude I have to drink water ALL the time, I feel like I can't get enough of it and yet I hardly sweat.

The amount of water on your skin has nothing to do with whether or not you continue to sweat. It's entirely dependent on your body temperature. Sweat that doesn't evaporate doesn't do anything to cool you off, so it doesn't matter if the sweat is on your body or on the ground. If your body isn't getting a cooling effect from the evaporating sweat, you will sweat at an even greater rate.If it's 73 degrees and the air is so humid you can see it, your sweat won't cool you at all. You will sweat heavily for the entire run and lose a boat load of fluid. In such conditions, I could lose 7 lbs on a 10 mile run. But in 30% humidity, moderate sweating will cool you. You will continue to sweat but only at a moderate rate. A 10 mile run in such conditions might result in 3-4 lbs of fluid loss.My wife just pointed out that you may have felt dehydrated because your mouth felt dry all the time. Breathing dry air will do that.

Its certainly true that hot humid weather results in more sweat on your body at the end of the run. Perhaps with dry weather since the sweat does evaporate you do sweat the same amount, but don't really see it.

I think dehydration happens equally, but I feel my body sweating a whole lot more in humid weather. Recently I have had to wring out my clothes five or six times after running.

The real question is which is harder on your breathing. Hot humid weather (85-95 degrees and 90% humidity) or cool dry air at altitude (lets say 5500 ft-6500 ft.)??

From experience I would argue the humid air is more difficult to breathe, because the lungs are forced to deal with large amounts of water in every breath. I once trained at 800 ft. in hot humid weather and averaged 6:40/mile (a solid effort) then I went up to altitude (Reno and Lake Tahoe) and noticed that with the same effort I was rolling 6:20s.

sorry about the typos.

[quote]Blowing.Rock Master wrote:

The amount of water on your skin has nothing to do with whether or not you continue to sweat. It's entirely dependent on your body temperature. Sweat that doesn't evaporate doesn't do anything to cool you off, so it doesn't matter if the sweat is on your body or on the ground. If your body isn't getting a cooling effect from the evaporating sweat, you will sweat at an even greater rate.

If it's 73 degrees and the air is so humid you can see it, your sweat won't cool you at all. You will sweat heavily for the entire run and lose a boat load of fluid. In such conditions, I could lose 7 lbs on a 10 mile run. But in 30% humidity, moderate sweating will cool you. You will continue to sweat but only at a moderate rate. A 10 mile run in such conditions might result in 3-4 lbs of fluid loss.

My wife just pointed out that you may have felt dehydrated because your mouth felt dry all the time. Breathing dry air will do that (quote]

You bring up good points that sound logical. I am still not sure I agree with them or not. I just know from my own experience that I absolutely dehydrated more at altitude than in the Midwest in the summer. And I was definitely dehydrated, not just a dry mouth. I drank a ton of water, but never had to go to the bathroom excessively; in other words, I needed all that water. Some others seems to have the same experience as me, others haven't. I feel confident that I have read the sweat is not evaporating in the humid weather, so you are not losing as much fluid, even though it seems you are. Your explanation makes sense however, so I really don't have an answer except my own experience and I am not sure where I have read my information.

I live and train in the swampish hellhole of SW Florida, and have spent several weeks in drier western climates. Maybe I'm just used to Florida weather, but in my experience, I've never been more dehydrated than I was in Phoenix and Denver. It came on me so fast -- I couldn't believe how thirsty I was. Again, my problem could have been that my body is not used to the sweat-less water loss. But I very rarely get that thirsty here... not like you guys out west.

Way less comfortable here in Florida, but way more dehydrating in the west -- I'll go with that answer...

That is exactly my point, if I had to REALLY chose between 'dry' versus 'humidity' to race, I would pick 'dry'...but of course the ULTIMATE choice would be "Cold and Dry" versus "Hot and Humid"... :)

Altitude is a factor as well. Lower pressures increase evaporation rates.

This thread is amazing in the fact that even though people have different opinions on this and have tried to present their case, nobody has called anyone else a moron or idiot or anything. It would be nice if more threads were like this, a nice discussion about something that some people may be interested in.

One thing for me with the dehydration living at altitude probably had as much to do with just living in the warm, dry climate. All day long I was sweating more, having the sweat evaporate and therefore keeping me cool and comfortable, all the while losing fluids that I had to replace to keep myself cool and comfortable. It was the daily living that brought about my constant dehydration as much as my daily running was.

Also, I do fully expect to be called an idiot in the next response based on my thoughts above.

Moron.

Just kidding. Yes, I agree that it's been amazingly civil. Maybe I'm just biased, because I've spent more time in humid areas than dry ones. I'm not an expert, by any means.

Former altitude runner wrote:

but more of it is just staying on the skin, preventing you from sweating more.

If this were true then swimmers would never sweat while in the pool.