I went swimming for the first time this season on Thursday, and I was reminded of a question that has plagued me for nearly ever. When standing or swimming in water at least as deep as I am tall (that is, deep enough so that my face is just above the water level) I’ve noticed that the air directly above the water seems thicker and hotter than normal air. This has made it very difficult for me to catch my breath after swimming long distance, etc. Also, it only seems to occur in pools (not lakes or rivers). Any explanations for this? Or am I just imagining it?
Cave Canem. Beware the Dog.
My guess is that you are swimming in a heated pool. If the temperature of the water is higher than that of the air…instant humidity!
No, I thought of that. That’s why the air is thicker in a bath/shower or a jacuzzi, right? But the pool water was pretty cold. I almost didn’t go in after freezing that first toe. 
Cave Canem. Beware the Dog.
Okay…I’ll try again…Clorine vapors?
Alright! I give up!
Bring in the physicists!
The air above water is always going to be more humid. Water evaporates regardless of the temperature (assuming it’s not absolute zero). It just evaporates quicker when it’s warm.
“The day after tomorrow is the third day of the rest of your life.” -George Carlin
Having spent like most of my life in the sea surfing & pools too, I find this an interesting question.
First, you might sit on the edge of the pool bend down and breath near the surface and see if its chlorine vapors or humidity.
Probably, the water is pressing on your chest when you are in the water so when you stand and breathe it takes more energy to do so.
If your head is just above water level your lungs are about a foot below water level. This is enough that the water pressure on your chest (and thus your lungs) is about 3-4% greater than the air pressure around your face. When you inhale the additional pressure on your chest attempts to force the air back out of your lungs so you have to work slightly harder to inhale.
“You can’t run away forever; but there’s nothing wrong with getting a good head start.” — Jim Steinman
Dennis Matheson — Dennis@mountaindiver.com
Hike, Dive, Ski, Climb — www.mountaindiver.com
I concur with Tanstaafl; when I was learning to dive we had to stand in the deep end of the pool until the water was just below the tips of our snorkels; you can really feel the pressure of the water on your chest when you breathe.
It only hurts when I laugh.