It's always darkest, and coldest, before the dawn

Ever notice how you shiver a little just before the sun comes up? I need to remember to document this (check the dang thermometer) but enough of my friends have said they notice it, too, that I assume it is a genuine phenomenon. A little research proved me right.

Early Morning Wonder Valley, by Sandra Lytch

The chill is the effect of two things: the solar semi-diurnal tide and the latent heat of vaporization.

Gail, at Ask A Scientist!, writes about solar semi-diurnal tides, "The heated, expanded atmosphere literally pushes air ahead of the rising Sun in a bi-modal wave, causing peaks and valleys of pressure, and resulting in a slightly decreased pressure a few hours before sunrise; the decrease in pressure can translate to a slightly decreased air temperature."

Gail goes on to say that this effect is nearly imperceptible.

Joepoidog, at Answers.com writes regarding latent heat of vaporization, "When the Sun's rays first hit dew or frost on the ground, but before the radiant heating of the Earth has begun, the Sun's rays actually begin the process of evaporation and it is this process of evaporation that removes heat from the environment thus registering a noticeable drop in temperature."

Or, it could be that's the time I'm hightailing it to the outhouse and I just notice how damn cold it is.