While San Francisco is known for its cool, foggy weather, the city can experience periods of unusually warm temperatures, especially in the fall. In fact, San Francisco’s hottest day on record was a scorching 106°F (41°C) in September 2017.
In This Article
Key Points
- San Francisco’s climate is heavily influenced by the cool waters of the Pacific Ocean and coastal fog
- The city experiences a Mediterranean climate with mild, wet winters and dry summers
- Warm spells can occur, especially in September and October, when high pressure builds and offshore winds blow
The Influence of the Pacific Ocean
San Francisco’s climate is largely shaped by the cold waters of the Pacific Ocean. The California Current brings cool water down the coast, leading to frequent fog and low clouds, especially in the summer.
However, when high pressure builds over the region, it can lead to offshore winds that push the marine layer away from the coast. This allows warmer air to reach the city, causing temperatures to soar, sometimes 20°F or more above average.
Microclimates of the Bay Area
The San Francisco Bay Area is known for its microclimates, with temperatures varying significantly over short distances. This is due to the complex interplay of ocean, bay, mountains, and valleys in the region.
- The city of San Francisco itself can see temperature differences of 10-20°F from one neighborhood to another
- Coastal areas like the Sunset District tend to be cooler and foggier than eastern neighborhoods
- Inland areas like the Mission District and SOMA are often warmer and sunnier
The Role of Coastal Fog
Coastal fog is a defining feature of San Francisco’s climate, especially in the summer. As warm air passes over the cold ocean water, moisture condenses into low clouds and fog.
This fog helps to keep temperatures cool in the city, even when inland areas are sweltering. However, research has shown that coastal fog has declined by 33% since the early 20th century, which could contribute to warmer temperatures.
Fall Heat Waves
While San Francisco’s warmest months are typically September and October, due to the potential for offshore wind events, heat waves can occur.
These heat waves are often caused by high pressure systems that bring warm, dry air from the interior of California. As this air descends the coastal mountains, it warms up even further, leading to hot, sometimes humid conditions in the city.
El Nino and Atmospheric Rivers
El Nino, a periodic warming of the eastern Pacific Ocean, can have a significant impact on San Francisco’s weather. During El Nino years, the city tends to experience wetter, stormier conditions in the winter.
Atmospheric rivers, narrow bands of moisture-rich air, can also bring heavy rain and wind to the Bay Area. These “rivers in the sky” are responsible for up to 50% of the region’s annual precipitation.
FAQ
How often does San Francisco experience heat waves?
San Francisco typically experiences a few heat waves per year, most commonly in September and October. These heat waves can last anywhere from a few days to over a week.
What is the warmest month in San Francisco?
September is usually the warmest month in San Francisco, with an average high of 71°F (22°C). However, individual days in September and October can see temperatures soar into the 80s and 90s°F (27-37°C).
How does coastal fog impact San Francisco’s climate?
Coastal fog helps to keep San Francisco cool and mild, especially in the summer months. As fog has declined in recent decades, the city has seen an increase in warmer, sunnier days.
What is an atmospheric river?
An atmospheric river is a narrow band of moisture-rich air that can bring heavy rain and wind to the West Coast. These “rivers in the sky” are an important source of water for California, but can also cause flooding and mudslides.
How do El Nino conditions affect San Francisco’s weather?
During El Nino years, when waters in the eastern Pacific Ocean are warmer than average, San Francisco tends to experience a wetter, stormier winter season. This can bring much-needed rain to the region, but can also lead to flooding and other weather-related hazards.
