I think most out-stations do their sort indoors and don't bring the cans out until they're ready to be loaded. Any snow falling during the loading process, is then probably reduced to rain and just rolls off the cans. And you're quite correct about water getting below decks and damaging sensitive equipment. Why just last week, some water seeped below the main deck on a 727 and almost killed the cat and duck we keep down there to help us navigate and remain shiny side up.