The coastal areas of central and southern California have a Mediterranean climate, which occurs on the west coasts of continents at mid-latitudes throughout the world. These regions are characterized by temperate, wet winters contrasted with warm, dry summers. The average rainfall in central California ranges from 15 to 55 inches per year, with almost all precipitation occurring between November and April. Winter storms typically yield one to three inches of rain over the course of several days and are separated by periods of clear, mild weather. Rain is rare during the summer drought, which can last up to seven months.
California's mountains and valleys create microclimates in the state's coastal region. Most notably, mountains parallel to the coast produce rain shadows and drier interior valleys. Gaps in these coastal ranges permit ocean fog to penetrate inland, providing some relief from summer heat and drought. During the summer, San Francisco receives significant moisture and cooling and from the coastal fog. Consequentially, the warmest weather of the year is often found during fall and spring.
The complex climate of the San Francisco Bay Area has a significant impact on the region's ecology. Indeed, local plants and animals have adapted to the extended summer drought; moreover, the localized climatic zones have led to the evolution of numerous endemic species with very limited geographic ranges. These climatic conditions--coupled with the local geology--contribute to the high species diversity on the San Francisco Peninsula and to the number of rare or endangered species at the Presidio, particularly in the plant kingdom.