Average Weather In December For San Juan, Puerto Rico

Location

This report describes the typical weather at the Luis Muñoz Marín International Airport (San Juan, Puerto Rico) weather station over the course of an average December. It is based on the historical records from 1992 to 2012. Earlier records are either unavailable or unreliable.

San Juan has a tropical rainforest climate with no dry season. The area within 25 mi of this station is covered by oceans and seas (48%), forests (30%), grasslands (12%), marshes (6%), and croplands (3%).

Temperature

The month of December is characterized by essentially constant daily high temperatures, with daily highs around 83°F throughout the month, exceeding 87°F or dropping below 80°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 73°F, falling below 71°F or exceeding 76°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 11:01 hours of daylight; the longest day is December 1 with 11:07 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 6:40am on December 1; the latest sunset is at 5:59pm on December 31; the latest sunrise is at 6:56am on December 31; the earliest sunset is at 5:46pm on December 2.

Daylight savings time (DST) is not observed in 2012.

Daily Sunrise & Sunset with Twilight 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.

Clouds

The median cloud cover is 45% (partly cloudy) and does not vary substantially over the course 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 41% of the time, partly cloudy 32% of the time, and mostly cloudy or overcast 23% 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 average probability that some form of precipitation will be observed in a given day is 68%, with little variation over the course of the month.

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, heavy rain, and moderate rain.

Light rain is the most severe precipitation observed during 52% of those days with precipitation. It is most likely around December 30, when it is observed during 38% of all days.

Heavy rain is the most severe precipitation observed during 22% of those days with precipitation. It is most likely around December 8, when it is observed during 17% of all days.

Moderate rain is the most severe precipitation observed during 19% of those days with precipitation. It is most likely around December 27, when it is observed during 14% 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 64% (mildly humid) to 92% (very humid) over the course of a typical December, rarely dropping below 54% (mildly humid) and reaching as high as 98% (very humid).

The air is driest around December 1, at which time the relative humidity drops below 69% (mildly humid) three days out of four; it is most humid around December 29, rising above 89% (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 67°F (muggy) to 73°F (very muggy) and is rarely below 63°F (mildy humid) or above 76°F (very muggy).

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 16 mph (calm to moderate breeze), rarely exceeding 19 mph (fresh breeze).

The highest average wind speed of 8 mph (light breeze) occurs around December 30, at which time the average daily maximum wind speed is 16 mph (moderate breeze).

The lowest average wind speed of 7 mph (light breeze) occurs around December 11, at which time the average daily maximum wind speed is 15 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 east (39% of the time) and north east (17% of the time). The wind is least often out of the north west (1% of the time), west (1% of the time), south west (1% of the time), and north (3% 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.