San francisco weather patterns

In San Francisco, the average percentage of the sky covered by clouds experiences significant seasonal variation over the course of the year. The clearer part of the year in San Francisco begins around May 22 and lasts for 5. The cloudier part of the year begins around October 23 and lasts for 7. A wet day is one with at least 0.

The chance of wet days in San Francisco varies significantly throughout the year. The wetter season lasts 5. The month with the most wet days in San Francisco is February , with an average of 8. The drier season lasts 6. The month with the fewest wet days in San Francisco is July , with an average of 0. Among wet days, we distinguish between those that experience rain alone , snow alone , or a mixture of the two. The month with the most days of rain alone in San Francisco is February , with an average of 8.

To show variation within the months and not just the monthly totals, we show the rainfall accumulated over a sliding day period centered around each day of the year. San Francisco experiences significant seasonal variation in monthly rainfall. The rainy period of the year lasts for 7.

The month with the most rain in San Francisco is February , with an average rainfall of 3. The rainless period of the year lasts for 4. The month with the least rain in San Francisco is July , with an average rainfall of 0. The length of the day in San Francisco varies significantly over the course of the year. In , the shortest day is December 21 , with 9 hours, 33 minutes of daylight; the longest day is June 20 , with 14 hours, 47 minutes of daylight.

The earliest sunrise is at AM on June 13 , and the latest sunrise is 1 hour, 54 minutes later at AM on November 6. The earliest sunset is at PM on December 5 , and the latest sunset is 3 hours, 45 minutes later at PM on June The figure below presents a compact representation of key lunar data for The horizontal axis is the day, the vertical axis is the hour of the day, and the colored areas indicate when the moon is above the horizon.

The vertical gray bars new Moons and blue bars full Moons indicate key Moon phases. We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid.

Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night. This section discusses the wide-area hourly average wind vector speed and direction at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.

The average hourly wind speed in San Francisco experiences mild seasonal variation over the course of the year. The windier part of the year lasts for 5. The windiest month of the year in San Francisco is May , with an average hourly wind speed of 9.

The calmer time of year lasts for 6. The calmest month of the year in San Francisco is October , with an average hourly wind speed of 7.

The wind is most often from the west for 9. The wind is most often from the north for 2. San Francisco is located near a large body of water e.

This section reports on the wide-area average surface temperature of that water. The average water temperature experiences some seasonal variation over the course of the year. The time of year with warmer water lasts for 2. Fog is wet. It generates a cold, damp wind as it comes in.

Don't expect to go swimming, either The ocean temperatures are far too cold unless you have an insulated wetsuit. What to pack: Summer fog in San Francisco is colder than you can imagine. If you're going from May through July, pack all the layers you think you'll need, then pack one more.

Or be prepared to make an emergency sweatshirt purchase. Average humidity isn't all that high compared to other places, but the fog can turn your carefully-smoothed hairstyle into something that looks more like a haystack.

If your hair tends to frizz, pack extra products to keep it tamed. Like spring, fall in San Francisco is generally sunny and rain-free. Check the forecast before you pack to be sure your clothing covers the range of expected temperatures. You can get more detailed information about San Francisco's fall weather in the guide to San Francisco in September , October , and November. It can be chilly in San Francisco in the winter at least by California standards , and it's also the rainy season.

What to pack: You might need an umbrella or raincoat. And pack lots of layers in case things change fast. Winter is a fickle season, and just looking at the averages doesn't tell the whole story because weather patterns vary a lot from year to year. Your best bet to stay dry and warm is to check the forecast just before you go.

You can get more detailed information about San Francisco's winter weather in the guide to San Francisco in December , January , and February. The average rainfall is particularly deceiving anywhere in California. Rain depends on what happens in the Pacific Ocean. When the ocean experiences El Nino conditions, it can rain a lot. In other years, you'll hardly see a drop all winter. And when it does rain, a whole month's rainfall often comes in just a day or two.

June Gloom is real. If you don't know Pacific coastal weather patterns, this may sound wrong — but it's true. The isohyet contours of equal rainfall analysis developed by Rantz depicts a maximum 22 " contour over the higher terrain in the south central portion of San Francisco. This is primarily due to the orographic effects resulting from uplift of the airflow from the Pacific striking the Coast Range. The 20" contour is along the western edge and also across the northeastern quadrant of San Francisco, with the 18" contour just touching the waterfront in the extreme northeast.

Snow is extremely rare in San Francisco, with only 10 documented instances of measurable snow at the official observing site in the past seasons. Snow has fallen on a number of other occasions, but usually only in trace amounts or at the higher elevations. Additionally, some of these occurrences are not true snow events but were the result of either small hail or ice pellets Pericht, Spring and fall are transition periods for San Francisco.

These seasons usually produce the most cloud-free days between the overcast days of summertime stratus and the rain laden clouds of winter. San Francisco's hottest days are typically during the spring and fall when high pressure builds into the Pacific Northwest and Great Basin, and dry offshore winds replace the Pacific seabreeze.

The occurrence of rainfall during the early spring and fall is infrequent, with only about 5 days per month on the average. While most storms during these periods produce light precipitation, the occasional coupling of polar and subtropical airmasses can produce heavy rainfall events.

The diversity of San Francisco's microclimates in general and its rainfall patterns in particular must be considered when utilizing the data from a single site. In a relatively flat region, without the influences of the ocean and topography, there is little discernible change in annual rainfall averages with distance. However, within a distance of only a few miles in San Francisco there can be as much as a 20 percent difference in average annual rainfall i.

Byers, Horace R. University of California Publications in Geography, Vol. Null, E. Jan , Climate of San Francisco.