That's an interesting question, as we can pick up and fashion snow into just about anything, and yet too much of it can topple a roof over. Fallen snow can certainly get very heavy, despite as individual flakes being blown all over the place, seemingly lighter than air.

So to see how "heavy" snow can get, we'll have to bring in some math. Let's start with a common example: Shoveling off the driveway. Let's say you had a driveway that was 30 feet long, and 15 feet wide (a somewhat typical driveway), and that it was covered overnight by 10" of fallen snow (certainly possible).

In an average winter scenario, if you would melt 10" of snow into all liquid, you would have 1" of water. Some 'wet snow/ice' situation would be much more water, dry cold snow would be much less. So, 1" of liquid water stacked up across the driveway.

Driveway is 30' x 15' (converted to inches) = 360" x 180" = 64,800 sq" of surface.

Assuming 1" of liquid water was across the surface of the driveway, you'd have an identical number (64,800) of cubic inches of water to deal with.

This comes out to approximately 2244 POUNDS of water. More than 1 Ton !! And you've got to shovel it off the driveway. Assuming you clear a square foot of surface area of the driveway per shovel full, you'll be lifting 5 pounds of snow, 450 times, to clear off the driveway.

...At that rate, you can easily see how you should take MANY rests in the course of clearing off the driveway, and why shoveling more than once is better than waiting until it's all fallen to do something. Clearing off more snow per shovel full, or deeper snow with a higher water content, will add to the literally back-breaking work.

One final note...the roof of a home can only withstand so much fallen snow as well (fortunately most roofs are pitched so heavy snow can slide off). Consider this: 30" of snow on a roof that's 25' x 50' at a normal liquid equivalent would be 18,701 pounds, or more than 9 TONS of snow. That's a lot to push down on a home if it's not shovelled off. But then again, if it's you that has to shovel, someone's moving almost 19,000 pounds.