I think there are a lot of factors here. One is the relative humidity
of the ambient air. On a dry day, the air in your lungs will probably
not have time during a single breath to approach 100% relative
humidity very closely, whereas it might get closer on a humid day.
Then when you exhale, surrounding air gets entrained in the air you
just breathed, lowering the absolute humidity, which means, again,
you're less likely to see your breath on a dry day. Theoretically, if
you hold your breath long enough to reach 100% RH, some small amount
of condesation should be possible at any temperature below that of
your body. (There might be some issue with overcoming the energy
associated with surface tension in order to form a stable spherical
droplet, but I think that's minor). I think the reason you don't see
your breath on an 80 degree day is 1) the rapid entrainment and mixing
which decreases the humidity, and 2) even if there is a little
condensation, you don't see it because the droplets are too few. You
can get around both these by holding a piece of glass right in front
of your lips and breathing on it. Even on a fairly warm day, you can
fog it up. Four things make it easier to fog up glass than see your
breath in the air: 1) rapid cooling, because of the conduction of heat
from the air to the glass; 2) minimized mixing, because of the
proximity of the glass to the source of the warm moist air; 3)
nucleation sites on the surface of the glass facilitate condensation;
4) the two dimensional surface of the glass concentrates water
droplets in one plane, and refraction of light as it passes through
them make the glass appear foggy, so they're easier to see than
droplets suspended in air. |
