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Sometimes when I bicycle against hard wind, I find it difficult to breathe. Others I have discussed it with have also noticed this effect.

A possible related phenomenon that I heard from an acquaintance who went motorcycling in Arizona was that when it was really hot, he had to drive very slowly in order to be able to breathe. This seems related even though I have never noticed temperature affecting this before.

So:
1. What causes this?
2. Why is it more noticeable when the air is hot? (I assume it would happen to motor-bikers going sufficiently fast in cold air as well.)

EDIT :
When it happens, it's usually when facing the wind directly. If you angle your head so that your mouth is not facing directly forward, it is much less noticeable. Might be psychological, but several other people I asked had experienced the same feeling as well even before I asked them.

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What if we drive along the wind? Are you sure about this observation? –  Mr.ØØ7 Apr 24 '13 at 12:46
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I'm and avid cyclist and haven't noticed number 1. But number 2 is easy. It's harder to breathe in warmer air because it is less dense and there is more moisture. Both meaning less oxygen per breath. –  Jim Apr 24 '13 at 13:03
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Not sure about this observation...When you skydive, you can breathe normally. That fact that most people don't, could be an indication that something psychological, rather than physical, is going on :) –  Rody Oldenhuis Apr 24 '13 at 13:11
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Although you could argue that due to the faster moving air, the pressure on the outside of the mouth will be lower by Bernoulli's principle. This would require more chest expansion for the same amount of air to come in... –  Rody Oldenhuis Apr 24 '13 at 13:14
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@RodyOldenhuis Although I should probably lose all of my reputation for admitting this, I too can attest to this observation. I remember when I was a young kid on multiple occasions sticking my head out of a car window like a dog. The main thing I remember is how difficult it was to breathe. –  OSE Apr 24 '13 at 15:37
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