As we move into winter, I've been paying attention to changes in the color of the sky. Over the last few weeks, it seems to have warmed noticably, to the point where the sky is tinted with very lovely and delicate oranges, reds, yellows, pinks, and violets at all times of the day, depending on the cloud cover.
While a warmer colored winter sky might seem counter-intuitive (particularly when one sprinting through the parking lot hoping to out-run the next bone chilling arctic blast), there is at least one good explanation, and a couple of other possibilities.
Here in our upper latitudes, the sun will sit a good deal lower in the sky than during the summer, forcing the light to pass through a greater volume of air, and therefore be subject to greater scattering. This is exactly the same effect that causes reddening of the air at sunrise and sunset, although here it happens on a lesser scale.
Another possibility is that any moisture in the air might be freezing, and the crystals might be refracting light in different ways than drops of liquid water. Beyond a certain height (and I don't know which one) the air temperature is always below freezing, so this would be an effect happening closer to the ground.
One other thing that's occurred to me is that during the summer here, a majority of the landmass is covered with green, and that is not the case during the winter. Some percent of the light hitting the ground must be reflected back up into the atmosphere, and would scatter to some degree. If the reflected light is not green (which is a cooler color), but instead is slanted towards the browns/reds or whites, the effect might be to warm up the reflected light to some degree.
I am, of course, neither a physicist or meteorologist, and I am pretty much just speculating wildly, but these lines of thinking are fun, and also I believe very useful for working artists. If nothing else, it's forcing me to pay closer attention to the natural world.