Weather Talk: Snow is hard to forecast because it's mostly air
Please cut the snow forecasters a little slack.
True, winter precipitation is a lot less variable than that which falls from summer thunderstorms. But snow accumulation is very hard to predict because most of snow accumulation is not snow, but air.
Whereas rainfall is a volume that can be easily caught and measured, accumulated snow is made from irregular and variably shaped crystals surrounded by air.
Most northern Plains snow accumulation is about one part precipitation surrounded by 10 to 15 parts air. Some of the dry, fluffy flakes we get in cold weather are one part water to 30 to 40 parts air. Even the wettest, half-melted snow accumulations contain three or four times more air than water and ice.
The shape of the crystals, the way wind breaks them up, and the way they settle into an accumulation are often much more important in determining accumulation than how much ice falls from the clouds.