Oakland's temperatures typically stay within a comfortably narrow range. The average low in January and February, which tend to be the coldest months in Oakland, gets down to just under 45 degrees. The average high in September, typically the hottest month, is around 75 degrees. In other words, the variation in average temperatures for the entire year is just about 30 degrees. Los Angeles ranges from 48.5 in January to a whopping 84.8 in August – a variation of around 36 degrees. Boston's range is even more dramatic at almost exactly 60 degrees, from about 22 in January to about 82 in July.
This means that if you are not a fan of extreme temperatures – either high or low – Oakland could offer the perfect climate. You do not need completely separate wardrobes for various seasons. Wear a light shirt or tank top with jeans in summer, and add a sweater or raincoat in winter, and you're all set. We locals have the luxury of being able to complain about the weather being “freezing” when it's 45 or 50 degrees and “burning hot” at 75 or 80 degrees.
Oakland gets about 23 inches of rain annually, spread out across approximately 60 days. Snow is almost unheard-of – though it can occasionally be seen for a day or two on nearby Mount Diablo. Even this is unusual enough to typically make the local news when it happens. Expect brief bouts of hail once or twice a year, with individual pieces rarely measuring more than 1/4” across.
Rain often comes in stretches that last several days, interspersed with days that are cloudy, foggy, clear, or even sunny. It's normal to get days of sunshine and gentle warmth even in winter. Thanks to the consistently mild temperatures throughout the year, the rain is more of an uncomfortable nuisance than a serious problem. The downside to our consistently gentle climate is that many local drivers seem to have no idea what to do in heavy rain, so be very careful if you're driving during a storm.
As you might guess from Oakland's proximity to San Francisco's notorious layer of fog, the weather is often overcast and foggy even when it isn't actually raining. The hills to the east of Oakland and Berkeley trap the fog here rather than letting it blow further inland. This becomes dramatically clear if you drive from Oakland into the suburbs on the other side of the hills on a foggy day. In doing so, you will go through the Caldecott Tunnel. There's a good chance that as soon as you exit the tunnel, you will find yourself emerging into warmth and sunshine.
On many days that start with high fog or just being overcast, the sun comes out before noon. If you want to do something that benefits from a clear view – such as climbing a mountain, hiking in the hills, or going up the Berkeley Campanile – plan to do it no earlier than 11 AM or noon. This will give the fog a chance to burn off.