When is it foggy in san francisco

Visitor's Guide. Trip Ideas. How Does It Happen? Yes, even in the summer! In the picture below, you can see the clear day with the fog starting to roll in under the Golden Gate Bridge. The hazy look of the bridge is also due to the fog. It isn't windy every day, but it's one more San Francisco weather factor that keeps the city cool most of the year. Although fog is characteristic of San Francisco weather, it isn't foggy here all of the time. So what are the San Francisco fog seasons?

There are two: summer and winter. Each fog season is different as the fog is created by a distinct type of weather pattern. Read on for more details.

Our summer fog is dense and beautiful. This is the fog that is often written about and photographed. The summer fog, officially known as advection fog, typically hangs over the city in the mornings, burns off throughout the day, and then floats back into the city in the evenings from the Pacific Ocean.

The picture above shows you how thick it can get in the summer. I took this while I was on the western end of Crissy Field having a picnic with friends.

The Golden Gate Bridge is less than a half mile away and there is so much fog you can barely see it! The winter fog in San Francisco, known as tule fog, isn't as thick and doesn't get the press that our summer fog gets. In fact, it's often confused with just regular cloud cover since it doesn't always dip down as low on the city and doesn't have the beautiful "billowing" effect.

That's because, rather than coming in under the Golden Gate Bridge from the ocean, this type of fog comes from the inland regions to the east, and can last for days or weeks at a time. It coincides with our rainy season and you will find it anytime between the end of November through early March. In the picture above, which I took in early April from the Marin Headlands on the northern end of the Golden Gate Bridge, you'll notice the difference between winter and summer fog. It isn't even the foggiest place in the US.

That designation goes to Cape Disappointment in Washington. San Francisco is also known as the city built on seven hills.

These hills and valleys, combined with the fog, wind, and surrounding water, are the reason for San Francisco's microclimate—pockets of the city that can vary greatly in temperature, up to 15 degrees from one district to the next! That means, even if it's warm and sunny in Union Square, it might be foggy and windy in Golden Gate Park at the same exact time.

This is one of the reasons that I recommend you wear layers throughout your stay: you can add or remove pieces as you move from neighborhood to neighborhood or sometimes just a few blocks in from the waterfront! The western end is where the city meets the Pacific Ocean. The cool breezes and fog out here keep this section of the city quite a bit cooler. Certain districts, like Noe Valley, get quite a bit of sun and tend to be warmer than other districts just up the street from it, since Twin Peaks to its west blocks some of the fog.

Here is a monthly breakdown of the weather in San Francisco CA. Our distinctive marine layer, known as both Karl and Karla on social media, has been the subject of books and documentaries and even starred as the main ingredient in locally distilled fog-harvested martinis. But as the climate warms, mounting scientific evidence suggests that our beloved — and oft bemoaned — coastal fog is on the decline. That means that changes the way our storms behave, the severity of the storms and other things like fog formation and duration.

Dawson found that, in general, fog seasons are starting later and ending earlier, and the number of foggy hours per day is also dipping dramatically. Like clouds, fog forms when the ambient air temperature drops below the dew point, at which time water vapor condenses out of the air. When this happens in the particulate-laden city air, that condensation becomes the infamous local fog. Some may find this scientific lingo offputting. The warmer an air mass gets, the bigger the moisture bucket gets, and vice-versa.

When the Pacific air blows in and the local air temperature drops, the moisture bucket shrinks, and the fog condenses. In warm weather, inland temperatures along the California coast are quite warm. This heat creates a rising air mass and a temperature and pressure gradient that acts to suck in the marine layer that sits along the coastline. The marine layer is an air mass, but it acts like a fluid. Fluids are lazy and like to flow through the point of least resistance.

In San Francisco, this point happens to be the Golden Gate. This confluence of things — the marine layer, the temperature and pressure gradient, the local geography — predisposes the region to fog. The fog is so integral to the area that much of the local ecosystem relies on the fog. A marine layer is a mass of air that develops over large bodies of water when there is a temperature inversion. A temperature inversion is when the normal thermal lapse rate becomes inverted. Typically, the air cools down as it rises.

The rate at which air cools as it rises is called the lapse rate. In a temperature inversion, the air gets warmer as it rises: a warm air mass is sitting on top of a cool air mass.

As the surface of the ocean cools the air mass above it, that air mass becomes cooler than the air above it and forms a temperature inversion.

In effect, the marine layer is a dense, cool air mass found above a large body of water. The coastline of the San Francisco Bay is a perfect place for the formation of marine layers.