Impervius
From UnknowableWiki
The Impervius Charm is a spell that causes the object in which it is cast to repel water.
Hermione Granger used it on Harry Potter's glasses during time-out of a Quidditch game against Hufflepuff due to the stormy weather (PA9). It was also cast by the Gryffindor Quidditch team on their faces before a training session under foul weather (OP18).
Hermione suggests that Ron—who is impersonating Magical Maintenance employee Reg Cattermole—should use Impervius to protect Yaxley's office from the rain coming in through the malfunctioning windows in case Finite Incantatem doesn't fix them (DH13).
