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.
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.)
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.