There is a strange phenomenon though that seems to occur at times,
generally when its very cold and clear. During some of those periods,
but not all the time it seems like the band just opens up and I get all
these additional stations, both low and high frequency ones, and some
with 75 to 85% relative signal strength indicated as well. Sometimes
these additional stations will be gone the next day, and other times
they might hang around for a week or so and then, just as quickly as
they appeared they're gone again until the next time they mysteriously
reappear.

Sometimes I have thought that snow might be a factor, that is not when
it's snowing but after it's on the ground and the temperature is cold. I
used to DX VHF TV with just a Vbeam when I was a kid in New York City
but It's been my experience that UHF doesn't usually skip. Could this be
ground wave, but on UHF?

It sounds as if you may be experiencing "tropospheric ducting".

http://en.wikipedia.org/wiki/Troposp...pheric_ducting

http://www.dxinfocentre.com/tropo.html

I've heard of people being able to go up a thousand feet or
so in the Santa Cruz mountains, aim an antenna southwest, and work a
VHF radio repeater system (2-meter, 145 MHz) located at a similar
altitude on one of the islands in Hawaii, when the ducting conditions
are right.

Tropospheric ducting is similar in some respects to the "skip" which
affects lower-frequency transmissions... both are due to the RF signal
being refracted, and thus "bent" out of its usual line-of-sight path.
HF "skip" is commonly an ionospheric phenomenon, while tropospheric
propagation/ducting occurs much lower in the atmosphere and does not
(I believe) require that the air layers in question be ionized.

You could think of tropospheric ducting as sort of a high-altitude
version of the refractive effect that causes the illusion of water on
the ground, when you look out over a hot patch of desert or asphalt.

Clear, cold air probably helps in another way... by reducing the
amount of moisture in the atmosphere it can decrease signal losses.