1. What area of field did we need to make a snowball 2m across?
Snow on the ground has a density roughly 30% that of water.
The snow is 5cm deep
Snow made into a snowball is roughly 60% as dense as water.

2. And this sphere must have a volume of 4/3πr3, which is…
Surface snow is only half as dense as snowball snow, so we’d need twice as much in volume.
About 4.2 cubic metres?
Did you bring a calculator?

3. So… we’d need 8.4m3 of surface snow?
At a depth of 0.05m, the area would have to be… m2
Can’t you just enjoy the snow for once instead of doing maths the whole time?
You did not just do that in your head.