San Francisco Bay sailing is slightly different each day. Even though wind conditions may be similar from one day to the next, when coupled with the current and time of marine layer clearing, a different experience is produced.
Typical summer days have brisk afternoon breezes with a marine layer in the morning that clears between and 10:00 Am and noon. When it really blows around 4:00 pm we try to have it so we are going with that wind rather than into it. At that same time, the east side of Angel island is protected and can be very pleasant.
In the winter, winds tend to be lighter, so between storms can some of the most pleasant days of the year on the bay.
This week ‘s weather:
Synopsis: PZZ500-122315- 229 AM PST Wed Dec 12 2018 .Synopsis for the Central California Coast and Bays Including the Monterey Bay…Greater Farallones…and Cordell Bank National Marine Sanctuaries… Breezy northerly winds behind a passing cold front will gradually subside later this afternoon and evening over the coastal waters. Winds will then become southerly by late Thursday and Friday ahead of a weekend system. A series of west to northwest swell trains will arrive to the waters over the coming days, creating hazardous conditions for small vessels. The most significant long- period swell is forecast to arrive by late Sunday or Monday, which has the potential to bring open water swells of 20 feet or more.
What drives the wind?
San Francisco sailing involves essentially two factors, the gradient wind which is barometric pressure differences and inland heating in the Sacramento Valley which is what drives our Summer wind which is typically the strongest.
In winter the winds tend to be lighter and come from varying directions. The big winds come with storm fronts and strong gradients that often are a part of winter storms. Gradients are the lines on the image above. When they are close together, there is more wind. It sometimes is referred to by the weather guys as pressure wind. But most often there is little barometric pressure difference over large areas , so there is little gradient wind. The central valley is no warmer than the coastal areas so there is no wind at all from that factor.
In Summer, as the hot afternoon air in the Sacramento valley heat up it rises, it creates a sort of vacuum and pulls the cooler air from San Francisco Bay underneath. An old phrase to describe this goes ” The wind doesn’t blow , the valley sucks.” Another factor is a the large high pressure system that sits out in the ocean know as the pacific high. It often moves in close to shore increasing pressure gradients on our coast. When these two thing happen at the same same, we get the afternoon breezes up to 30 knots. These winds combined with the currents of the bay have brought about the old phrase , ” If you can sail San Francisco Bay , you can sail anywhere.”
San Francisco Bay currents are driven by tides, but because such a tremendous amount of water comprising all of San Francisc0 Bay and the delta must push through the narrow Golden Gate the currents can be very strong–often up to 3 knots So when we plan our sailing tours and charters, we take these currents into account.
Gentle Storm II moves through the water at between 6-8 kts. But into the current, if our target is directly upwind, we have to sail off about 45 degrees we only make about 3 knots towards that target. So on a day when we have an incoming tide , or flood, at 3 kts and we are trying to get to the bridge, it becomes a very slow process. But on the opposite tide, we fly out, but get back slowly.