best thing to do for your head in the rain is find the nearest coffeeshop and sit a spell!

no, seriously, it won't be raining that much, a shower cap should be sufficient. after all day in a rain, there's water inside your coat wether you have a hood on or not, usually. i wouldn't sweat it.

if you want you can also purchase a goretex cycling cap but doubt it wouldn't get worn much in the summer... but if you want that cap, check at Gregg's Greenlake Cycle, saw some last time i was there!