Reddit mentions of Grizzly G7065 Anvil, 24-Pound

tl;dr 188km/h might do the trick, but possibly more.

I like answering these kinds of questions, but this is going to be very much an approximation (snowballs hate physics). I'll explain why near the end.

Assume a 5cm diameter snowball at a density of 300kg/m^3, which makes for a 157g snowball. Lets be generous and say that when we throw thing thing at someone's head, 50% of the impact energy is transfered (with the remainder being in the snowball breaking apart and flying off).

The impact energy would then be: E = 1/2 mv^2 * 0.5 = 1/2 * 0.157 * v^2 * 0.5.

And as a reference, lets use this 24 pounds/10.9kg anvil dropped from a height of 1 meter directly onto your head.

That anvil would have an impact energy of E = mgh = 10.9 * 9.81 * 1 = 107 Joules.

Equate that to our snowball impact energy, and solve for velocity:

v = sqrt(4E / m) = sqrt(4 * 107 / 0.157) = 52.2 m/s

So your snowball would need to somehow be doing roughly 188km/h to have the same effect as dropping that anvil on your head from a meter up (which, at best, might give you a really bad concussion).

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The problem with snowballs is that they are very variable, and like to break apart, and a lot depends on how you pack them and the quality of the snow you're using. The 188km/h figure is a ballpark estimate at best, and my suspicion is that a lethal snowball would need to be going faster than that. Which, I reiterate, is difficult to do, what with all this atmosphere in the way.