Seattle is proof that the phrase "good weather" is a matter of perspective. Outdoor activities continue all year long, despite the cloud cover. Summers are cool and usually dry. Though high temperatures average in the mid-70s F/24 C, occasional August days reach near 90 F/32 C. Nights can be cool, so take a sweater or jacket.
Winters are drizzly and overcast but not harsh. January temperatures run 35-45 F/3-8 C. Any snow that falls typically does not last (although when it does, the entire city shuts down). It may occasionally rain hard in Seattle November-February, but a steady mist is more common. (It's usually accompanied by thick clouds that can hide the spectacular mountain scenery for days and, sometimes, weeks at a time.)
No matter when you visit, pack a raincoat and rain hat—umbrellas are not considered trendy and are useless when the wind is blowing. The most popular months for tourists are May-August, but the city is even more beautiful when the leaves change color in the fall. The mild weather, sans crowds, is perfect for hiking, river rafting, mountaineering and other sports.