When I saw NOAA’s San Francisco weather forecast this morning I smiled. Northwest winds in the afternoon, blowing straight off the Pacific ocean.
That’s bad news for the tourists shivering in their shorts at Fishermans Wharf, but great news for a bicycle commuter like me who rides to the southeast end of the city to catch her train home. The 20 mph afternoon sea breeze that pushes me to my destination is a familiar weather pattern for most ocean communities, especially in summer. This pattern is caused when temperatures in the inland valleys warm up in the sun, causing air to rise and expand, creating a low-pressure region. Meanwhile, air over the cool ocean sinks and compresses, creating a relatively high-pressure region. Air flows from high to low pressure, creating what cyclists and tourists experience as wind. The greater the pressure difference, the stronger the wind.
In San Francisco, this natural air-conditioner can keep the city downright chilly compared to Sacramento or San Jose—thirty degrees difference is not uncommon. It’s also why Mark Twain was reputed to have said “The coldest winter I ever spent was a summer in San Francisco.” According to Snopes website though, Twain may have never said it… but it sure does ring true to the tourists who come here expecting “typical” California weather.