Change your preferences here.
If hail is the prevailing weather condition, the wielder gets a 25% evasion increase. In addition, if a non-Ice-type Pokemon has this ability, it will not take damage from hail.
God, this ability can be annoying. Anyway, on hail teams, you can expect to see Froslass, who will have Snow Cloak. Along with BrightPowder, this can allow Froslass to survive for longer than she ordinarily would, and thus can get up an extra layer of Spikes or another attack off. Other Pokemon such as Mamoswine will have Snow Cloak too, though they won't necessarily be on a hail team, so you may not have to worry about Snow Cloak unless you are running it yourself.
On October 15th 2012, Snow Cloak was voted to be banned from Smogon's OU metagame as well as all tiers below it, so is currently unusable in all Smogon metagames bar Ubers and Little Cup.
Found in Dream World