Average Weather In December For San Francisco, California, USA

Location

This report describes the typical weather at the San Francisco International Airport (San Francisco, California, United States) weather station over the course of an average December. It is based on the historical records from 1974 to 2012. Earlier records are either unavailable or unreliable.

San Francisco, California has a mediterranean climate with dry warm summers and mild winters. The area within 25 mi of this station is covered by oceans and seas (48%), built-up areas (16%), grasslands (14%), forests (13%), and shrublands (8%).

Temperature

The month of December is characterized by gradually falling daily high temperatures, with daily highs ranging from 58°F to 54°F over the course of the month, exceeding 63°F or dropping below 49°F only one day in ten.

Daily High and Low Temperature in December

The daily average low (blue) and high (red) temperature with percentile bands (inner band from 25th to 75th percentile, outer band from 10th to 90th percentile).
Daily low temperatures are around 45°F, falling below 37°F or exceeding 52°F only one day in ten.

Fraction of Time Spent in Various Temperature Bands in December

The average fraction of time spent in various temperature bands: frigid (below 15°F), freezing (15°F to 32°F), cold (32°F to 50°F), cool (50°F to 65°F), comfortable (65°F to 75°F), warm (75°F to 85°F), hot (85°F to 100°F) and sweltering (above 100°F).

Sun

Over the course of December, the length of the day is essentially constant. The shortest day of the month is December 27 with 9:34 hours of daylight; the longest day is December 1 with 9:45 hours of daylight.

Daily Hours of Daylight and Twilight in December

The number of hours during which the Sun is visible (black line), with various degrees of daylight, twilight, and night, indicated by the color bands. From bottom (most yellow) to top (most gray): full daylight, solar twilight (Sun is visible but less than 6° from the horizon), civil twilight (Sun is not visible but is less than 6° below the horizon), nautical twilight (Sun is between 6° and 12° below the horizon), astronomical twilight (Sun is between 12° and 18° below the horizon), and full night.

The earliest sunrise is at 7:06am on December 1; the latest sunset is at 5:01pm on December 31; the latest sunrise is at 7:25am on December 31; the earliest sunset is at 4:51pm on December 9.

Daylight savings time is observed in 2012 but it neither starts nor ends in December, so the entire month is in standard time.

Daily Sunrise & Sunset with Twilight and Daylight Savings Time in December

The solar day over the course of the year 2012 . From bottom to top, the black lines are the previous solar midnight, sunrise, solar noon, sunset, and the next solar midnight. The day, twilights (solar, civil, nautical, and astronomical), and night are indicated by the color bands from yellow to gray. The transitions to and from daylight savings time are indicated by the "DST" labels.

Clouds

The median cloud cover ranges from 57% (partly cloudy) to 64% (partly cloudy). The trend is for the sky to be becoming cloudier with the median cloud cover increasing by 7% from the start to the end of the month.

Median Cloud Cover in December

The median daily cloud cover (black line) with percentile bands (inner band from 40th to 60th percentile, outer band from 25th to 75th percentile).

On December 16, a representative day, the sky is clear or mostly clear 38% of the time, partly cloudy 20% of the time, and mostly cloudy or overcast 43% of the time.

Cloud Cover Types in December

The fraction of time spent in each of the five sky cover categories. From top (most blue) to bottom (most gray), the categories are clear, mostly clear, partly cloudy, mostly cloudy, and overcast. Pink indicates missing data. Outside of the United States clear skies are often reported ambiguously, leading them to be lumped in with the missing data.

Precipitation

The probability that precipitation will be observed at this location varies throughout the month. Precipitation is most likely around December 31, occurring in 39% of days. Precipitation is least likely around December 1, occurring in 34% of days.

Probability of Precipitation at Some Point in the Day in December

The fraction of days in which various types of precipitation are observed. If more than one type of precipitation is reported in a given day, the more severe precipitation is counted. For example, if light rain is observed in the same day as a thunderstorm, that day counts towards the thunderstorm totals. The order of severity is from the top down in this graph, with the most severe at the bottom.

Throughout December, the most common forms of precipitation are light rain, moderate rain, and heavy rain.

Light rain is the most severe precipitation observed during 43% of those days with precipitation. It is most likely around December 14, when it is observed during 16% of all days.

Moderate rain is the most severe precipitation observed during 34% of those days with precipitation. It is most likely around December 31, when it is observed during 13% of all days.

Heavy rain is the most severe precipitation observed during 13% of those days with precipitation. It is most likely around December 29, when it is observed during 5% of all days.

Types of Precipitation Throughout December

Relative frequency of various types of precipitation over the course of a typical December.

Snow

Either snow is exceptionally unlikely to fall at this location in December or this station does not reliably report precipitation types.

Humidity

The relative humidity typically ranges from 58% (mildly humid) to 93% (very humid) over the course of a typical December, rarely dropping below 36% (comfortable) and reaching as high as 100% (very humid).

The air is driest around December 1, at which time the relative humidity drops below 70% (humid) three days out of four; it is most humid around December 31, rising above 90% (very humid) three days out of four.

Relative Humidity in December

The average daily high (blue) and low (brown) relative humidity with percentile bands (inner bands from 25th to 75th percentile, outer bands from 10th to 90th percentile).

Dew Point

Dew point is often a better measure of how comfortable a person will find the weather than relative humidity because it more directly relates to whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid.

Over the course of a typical December, the dew point typically varies from 38°F (dry) to 49°F (dry) and is rarely below 28°F (dry) or above 55°F (very comfortable).

Dew Point in December

The daily average low (blue) and high (red) dew point with percentile bands (inner band from 25th to 75th percentile, outer band from 10th to 90th percentile).

Wind

Over the course of December typical wind speeds vary from 0 mph to 14 mph (calm to moderate breeze), rarely exceeding 26 mph (strong breeze).

The highest average wind speed of 8 mph (light breeze) occurs around December 4, at which time the average daily maximum wind speed is 14 mph (moderate breeze).

The lowest average wind speed of 7 mph (light breeze) occurs around December 31, at which time the average daily maximum wind speed is 13 mph (moderate breeze).

Wind Speed in December

The average daily minimum (red), maximum (green), and average (black) wind speed with percentile bands (inner band from 25th to 75th percentile, outer band from 10th to 90th percentile).
The wind is most often out of the west (38% of the time) and north west (18% of the time). The wind is least often out of the east (4% of the time).

Wind Directions in December

The fraction of time spent with the wind blowing from the various directions over the entire year. Values do not sum to 100% because the wind direction is undefined when the wind speed is zero.

Fraction of Time Spent with Various Wind Directions in December

The fraction of time spent with the wind blowing from the various directions on a daily basis. Stacked values do not always sum to 100% because the wind direction is undefined when the wind speed is zero.