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%).
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.
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.
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.
The median cloud cover is 45% (partly cloudy) and does not vary substantially over the course of the month.
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.
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.
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.
Either snow is exceptionally unlikely to fall at this location in December or this station does not reliably report precipitation types.
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.
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).
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).