San Francisco is known for its cool summers and iconic fog. This unique atmospheric phenomenon results from a complex interplay of scientific principles and specific geographical conditions. Understanding why the city experiences such consistent fog involves exploring how fog forms, the distinct environmental elements present, and how local terrain shapes its movement.
The Science of Fog Formation
Fog is essentially a cloud that forms at or very near the ground level, composed of countless tiny water droplets suspended in the air. These droplets become visible when water vapor, an invisible gas, cools and condenses. Condensation occurs when air cools to its dew point, the temperature at which it can no longer hold all of its moisture, forcing the excess water vapor to transform into liquid water around microscopic particles like dust or salt.
San Francisco’s most common fog type is advection fog. This form develops when warm, moist air moves horizontally over a cooler surface. As humid air flows across cold ground or water, it rapidly loses heat. This cooling causes water vapor to condense, creating visible fog. Advection fog can persist and move with the wind.
San Francisco’s Unique Ingredients: Ocean, Air, and Land
San Francisco’s fog is heavily influenced by the Pacific Ocean’s cold waters, California’s interior atmospheric conditions, and resulting pressure differences. The California Current brings cold water from the North Pacific. Coastal upwelling further enhances this, maintaining sea surface temperatures between 52 and 58 degrees Fahrenheit year-round.
During warmer months, intense solar heating over California’s Central Valley creates a low-pressure area. This thermal low draws cooler, denser air from the Pacific Ocean inland. The pressure gradient between high pressure over the cold ocean and low pressure inland drives a strong onshore flow of moist marine air.
As cool, moist marine air moves inland, it encounters warmer, drier air above it. This atmospheric layering creates a temperature inversion, where warmer air sits above a cooler air mass. This inversion acts like a lid, trapping the cool, moist air close to the surface and preventing it from rising. This trapped marine layer is where the fog forms, facilitating its density and persistence.
How Local Geography Intensifies the Fog
San Francisco’s topography plays a significant role in amplifying the fog’s presence. The Golden Gate Strait, a narrow opening, acts as a natural funnel. Prevailing northwest winds push the marine layer and fog through this gap, concentrating it as it enters the Bay Area. This funneling effect is why the Golden Gate Bridge is frequently shrouded in mist.
Once fog passes through the strait, the city’s hills influence its behavior. As moist air encounters elevated terrain, it is forced to rise. This upward movement causes the air to cool further, enhancing condensation and thickening the fog. Hills can also act as barriers, trapping fog within certain neighborhoods, leading to distinct microclimates. The fog can also spill over these hills, sometimes appearing like a cascading wave before dissipating on the warmer, eastern side of the city.
The Fog’s Daily and Seasonal Patterns
San Francisco’s fog follows predictable daily and seasonal rhythms. During summer months, from June through August, the fog is most prevalent. This is due to the increased intensity of the Central Valley’s thermal low, which strengthens the pressure gradient and draws more cool, moist air inland. This period is sometimes called “June Gloom” or “Fogust.”
On a typical summer day, fog often rolls in during the late afternoon or evening. As inland areas cool after sunset, the temperature difference between the coast and interior remains, allowing marine air to continue its push. By morning, the city can be enveloped. As the sun warms the land, the fog tends to “burn off” or dissipate, often clearing by midday.
However, some days, especially during peak summer, the fog can persist throughout the day, particularly in neighborhoods closer to the ocean or in the path of the Golden Gate flow.