I shoot a lot below freezing and what you actually want to do is keep your camera cold. If your camera is cold than when snow hits it it just bounces off, if your camera is warm snow will melt then refreeze. I have never shot in sleet or heavy fog at close to freezing so I cant help on that issue but when everything is below freezing keep your camera below freezing and snow won't melt on the camera or glass. If sleet was an issue maybe a rain cover could help.
|AF4_2483 Surfing the Serengeti by DaveInHouston|
from Hot Air Balloon view
|Peregrine Falcon by Psychic1|
from Best Wildlife Photo of the Week - 4