Bay Area Weather


*"The coldest winter I ever spent was a summer in San Francisco."*

*-- Mark Twain (unverified)*

A damp chill in the air and the sound of foghorns in the distance describes a typical summer day in San Francisco. Summer fog is so thick it often blankets the beautiful Golden Gate Bridge where you're guaranteed to find a few tourists in shorts and T-shirts shivering as they take pictures. At the southern end of the city, just a few miles away, the weather can be sunny and warm. And if you take the Bay Bridge and head East out of San Francisco, temperatures can be as much as thirty degrees warmer over the hills in the interior valleys.

Winter here is a different story. As I walked around Union Square earlier this year, I overhead a woman on her cell phone exclaim to her friend, "It's 65 degrees and sunny in January! Can you believe it?" I was surprised at how shocked she seemed. The temperature in San Francisco hardly varies year round. Most San Franciscans, like me, are fair weather folks and feel that temperatures below 50 degrees are freezing. If it's above 70 degrees, we think we must be having a heat wave.

San Francisco and Bay Area weather is what draws approximately seven million people to live here despite being one of the most expensive places to live in the nation. It's Mediterranean climate classification, Csa (Dry-Summer Subtropical), and unique geographical features that include the Pacific coastal ranges and the San Francisco Bay provide us with pleasant weather throughout the seasons.

During the summer months, a semi permanent High over the Pacific Ocean brings northwesterly winds to California. When the west and northwest winds hit the coastal range, they don't always make it over the mountaintop and are forced to move south parallel to the hills and mountains. The winds then push the surface ocean water south and southeast. As the surface water moves south, the water from the deeper levels replaces the top layer of the ocean. This upwelling of the cold coastal waters along the Pacific Coast help to create the inversion or marine layer. As warmer air moves over the cold water, the marine layer develops and brings almost perpetual fog and low stratus clouds along the coast. The marine layer hugs the coast all summer long and is kept in place by the coastal ranges.

*The Coastal Ranges*

The Pacific Coast Ranges generally run north and south, almost parallel to the coastline. In the North Bay, Mount Tamalpais (2604 ft) is the defining peak. The East Bay has Mount Diablo (3849 ft). To the east of San Jose lies Mount Hamilton (4213 ft). Also in the South Bay are the Santa Cruz Mountains with its summit reaching a height of 3232 feet. These coastal ranges act as a barrier by protecting the interior areas from the prevailing winds from the Pacific Ocean. Therefore, the interior areas are somewhat isolated from the cooler weather due to the ocean breeze and the upwelled cold ocean waters.

The mountains and hills are not only a barrier to the interior areas, but they are the borders for the different weather on each side. The western sloping hills, which face the Pacific Ocean, receive a lot of moisture. Western slopes get fog during the summer and precipitation from orographic lifting mainly during winter months. In contrast, the eastern sloping hills face typical conditions of the lee side of the mountain. The eastern side is drier and the rainshadow effect reduces the amount of rainfall. For example, if you live on the western side of the Santa Cruz Mountains, you see over 50 inches of rain each year. If you live in San Jose, east of the Santa Cruz Mountains, you will get an average of only 15 inches a year.

The tall coastal ranges are almost impenetrable by the coastal fog except for one area.

*The San Francisco Bay*

Throughout the entire California Coastal Range, you will find only one spot where there is a complete gap or breach down to sea level. This is the one spot where the ocean breeze can make its way past the coastal range without having to climb over any mountains. That spot is the mouth of the San Francisco Bay, marked by the famous Golden Gate Bridge. The bridge marks the gateway between the Pacific Ocean and the inland areas. It is the battle zone of the cool maritime air versus the continental air.

The San Francisco Bay draws marine air eastward past the Golden Gate Bridge and into East Bay cities such as Oakland and Hayward. Summer months are when we see more intense continental heating during the day that then pulls the cool marine air into the Bay. The Bay also funnels the ocean breeze bringing us strong winds in the afternoons when the temperature and pressure gradients are strongest. It is easiest to see the confluence of air masses by watching the fog roll in and out of the Bay. Sometimes fog will roll in out and out several times in one day especially during the months of June, July, and August.

Some say that the inversion or marine layer provides natural air-conditioning for the cities along the Bay. It keeps temperatures moderate during the summers for cities near the Bay. For example, the East Bay cities experience mild and very comfortable summers with temperatures in the 70s. While cities just over the East Bay hills can get exceedingly hot with temperatures easily into the 90s.

Ironically, the gorgeous coastline and beaches are not ideal for summer picnics or wearing bathing suits. Coastal communities stay in the upper 50s and low 60s all summer long and they don't get much sun. And in the rare case it is actually sunny along the coast, you still can't forget about the upwelling waters because you'll be reminded the second your toes touch the water. It's why surfers wear 5mm wetsuits in middle of summer in the frigid Pacific Ocean.

During the cooler months and evenings, the Bay acts as a heating influence. Because of water's high specific heat, it can prevent lows from plunging during the overnight hours. The overnight lows stay fairly mild and are usually in the mid 40s during the summer and winter for San Francisco and cities near the Bay. Once you get away from the Bay and over the hills and into the valleys, you can occasionally find below freezing temperatures during cold and clear nights.

*Summary and Conclusion*

The Bay Area is a great place to live because you can almost find something to your liking. The weather in the Bay Area can cover a range of precipitation and temperatures even within a short distance away. The weather in each location is known as a microclimate in the Bay Area. The fog, the mountains, the cool ocean breeze, and the Bay are what bring us the unique microclimates. In the Bay Area, one can go to the beach if a temperature on 60 degrees suits you. Or if you are a sun seeker, you can head inland where temperatures can climb over 100 degrees. Also, we can get to snow pretty easily, by taking a four-hour drive to Lake Tahoe for some skiing. Unfortunately, San Francisco is hardly fog free during the summer and 4th of July fireworks are sometimes impossible to see. However, you can't beat swimming outdoors even during winter months. And don't forget, if you don't want to look like a tourist, always bring a sweater or coat no matter what season we're in.