When is it safe to walk on a frozen lake?

[Nameless]

When the ice is uniformly at least 10 cm thick, it's quite safe. We have a river here, a couple of km long, it's the first (or second) biggest ice ring in the world by winter, everybody goes skating on it, they make tests every now and then to make sure it's always safe, just make sure there hasn't been a couple of days over 0 before trhe day you decide to walk on it.

[ruthless rocco]

its safe to walk on when its frozen.