San Francisco Bay Area Weather
The most pleasant weather usually occurs during April, May and June, and then again during September and October. Summer brings some of the coolest weather and heaviest fog to the Island, which surprises many out-of-town visitors to San Francisco.
Temperatures on Alcatraz Island seldom rise above 75°F (24°C) or fall below 38°F (3°C). It almost never snows on the Island, but winters can be wet and cold. Afternoon winds are common during every season.
Rain occurs frequently during winter and early spring. About half of the Alcatraz tour route is along outside roadways without shelter from the rain, so dress for possible wet weather. (Rain gear is available for purchase at Alcatraz Landing and in the Island bookstores.)
Native San Franciscans have a saying: “Always dress in layers and hope to be pleasantly surprised.”