Even wizards get wet when they walk in the rain. Unless they have an umbrella Or this Wizard Rain Cloak.
Created by Ole Jensen (in collaboration with a bunch of folks), the rainy day outfit is basically a wizard's robe complete with a pointy hat magically fashioned into a single-piece, full-body, rain-repelling coat. With glittery splash paint work all over. That way, you can walk ominously in the rain like you're on your way to a ritual sacrifice.
The Wizard Rain Cloak is made from breathable micropolyester, with full wind and water resistance for conjuring magic spells in stormy weather. In case the rain stops, you can take off the outfit and fold everything right into the hood, allowing you to wear just a witch hat in the summer. The product page says there's an "extra extension for backpacks" -- not sure what that means, but I assume you can wear a rucksack under (where you can hide your pig's blood and sacrificial chicken) and have it fit.
If this thing came at the height of the Harry Potter craze, it would have sold a gazillion pieces. For now, it's just a limited edition with 400 examples available. No word on pricing, but every witch and wizard without the power to evade the common cold should probably get one.