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Any Pokemon that uses Hail while holding an Icy Rock will have its duration boosted to eight turns instead of the regular five.
Icy Rock is absolutely vital to hail teams in the lower tiers—or it would be, if there were any point to it. As it is, however, without the permanent hail provided by Snow Warning Abomasnow and Snover, hail stall teams cannot function, as the hail will disappear all too quickly. Hail as an offensive strategy, too, is more or less outclassed by the other three weathers, as there are no sweepers that can take advantage of it, other than strong Ice-types with Blizzard.
Route 8 - Talk to the Parasol Lady standing just in front of the entrance to Tubeline Bridge between 9 pm and 5 am.