Noticing that the seasons are changing rapidly here in So Cal.  After a slight roller coaster ride of warm then cold then warm again we're definitely in the throws of fall/winter.

Anywho, got me thinking as I tend to like it when I see fog rolling in.  I think it's instinctively because it's "cooler" now and cooler just feels better.  However, I also know that in racing (auto) that moist air is not good for oxygen density.

Does the same apply for running?

FWIW, hot & dry hurts.  Cold & dry burns.  Hot & humid just sucks.  So I find cold & moist favorable.

My first outdoor run, when it was 54F and regular humidity, I wanted to cry it felt so amazing. Lol.

I never knew what it was like to live in a steam room until I got here.

Randy - good technical question. I know what you mean about racing - you can re-jet a carb but not a racer.

I guess for normal training runs you wouldn't really notice it but for racing/VO2 you might notice something.

I wonder if training in humidity would be similar to training at higher altitudes.

Hot and humid causes me more asthma attacks than hot and dry or cold and dry, or even cold and humid.  So that one makes a difference to me.

And we won't go on about LV and it's dry fucking weather. 

Caveat:  I have lung damage, so not a "normal" runner.

For me, cold dry air is by far the worst in terms of chest discomfort and breathing. I suffer pretty much all winter running outside, and I usually develop a cough from it. 

Cold, wet air does not seem to effect my breathing, and I don't get a cough after running in cold rain, for example.

I have never been to a desert climate, so I don't know about warm, dry air.

As long as the air quality is ok, I do fine running in warmer, humid weather (although of course then you have issues of having to slow down due to the heat itself).  Air quality is a totally separate issue; if there is a lot of ozone around that tends to bother me no matter what the weather.

I have found that the best running conditions for me are the low to mid 50s with drizzle/mist or fog.

This is a good question and I have a guess but it's just a guess.

 

My guess is that it is irrelevant in terms of the oxygen you can extract from the air.  I say that because your lungs move much more oxygen - 6 to 8 times more IIRC - than your body can actually use.  In other words, most of the oxygen you suck into your lungs is simply exhaled out and does nothing for you.  So, even if the moisture content is higher, my guess is that it doesn't matter even in a race in terms of VO2max.

 

Having said that, humid air has an enormous negative performance impact when racing because one of the major limiting factors in a race is your body's ability to cool itself.  In humid conditions, the sweat on your skin evaporates more slowly so you body doesn't cool itself as efficiently.  When your  body doesn't cool itself as efficiently, you slow down.  Or you blow up and walk.

I was going to mention something about the lung's capacity as I remember reading that before.  Unlike an engine, we only process a fraction of what we breathe in.  That's definitely a significant difference. 

Also, I like the part about sweat.  I do know that in summer it feels horrible when the sweat just overwhelms.  In fact, I think I can sometimes feel the blood practically ready to boil at the surface of my skin when it's horribly hot out and definitely not an issue in the cold, moist climate.

And DEFINITELY gotta take Jenny's word for it.  With special lung considerations in regards to your case, I can definitely feel good about running in cool, moist air. 

I don't get the argument that it doesn't matter because our lungs don't process all the O2 we breathe in - if that were true, altitude would have no effect, and we know that it does. However the variation in O2 due to moisture is quite small as compared to the variation with altitude.  (I'm a meteorologist, in case you didn't know.)  LTH's other argument, though, about cooling one's body, makes sense to me.

I live in the desert (well, it is technically a steppe climate) and 90% of my running is in very dry air, although it never gets very hot (or rather, I run early mornings in the summer so I don't have to run in heat) and I far prefer it to running in humidity.  Even though e.g. 50F here feels much warmer than 50F in a humid climate (I recently visited my family in the DC area and had to wear tights and long sleeves for a run - in the same temperature here I am in shorts and short sleeves!) it is also more enjoyable to me.

I imagine it would depend on dewpoint rather than %RH (that is, on temperature as well as humidity) and also on actual altitude.  E.g. the difference between 5000 and 9000 feet looks huge but in terms of actual oxygen decrease it's not much, because of the exponential curve of air density with altitude.  http://wahiduddin.net/calc/density_altitude.htm gives some discussion of this and does some calculations.  (Some of his examples are rather contrived, though.