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 January. 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 January is characterized by essentially constant daily high temperatures, with daily highs around 82°F throughout the month, exceeding 85°F or dropping below 80°F only one day in ten.
Over the course of January, the length of the day is essentially constant. The shortest day of the month is January 1 with 11:03 hours of daylight; the longest day is January 31 with 11:19 hours of daylight.
The earliest sunrise is at 6:56am on January 2; the latest sunset is at 6:17pm on January 31; the latest sunrise is at 7:00am on January 18; the earliest sunset is at 5:59pm on January 1.
Daylight savings time (DST) is not observed in 2012.
The median cloud cover is 43% (partly cloudy) and does not vary substantially over the course of the month.
On January 16, a representative day, the sky is clear or mostly clear 45% of the time, partly cloudy 30% of the time, and mostly cloudy or overcast 24% of the time.
The probability that precipitation will be observed at this location varies throughout the month. Precipitation is most likely around January 1, occurring in 67% of days. Precipitation is least likely around January 31, occurring in 61% of days.
Throughout January, the most common forms of precipitation are light rain, moderate rain, and heavy rain.
Light rain is the most severe precipitation observed during 59% of those days with precipitation. It is most likely around January 18, when it is observed during 39% of all days.
Moderate rain is the most severe precipitation observed during 20% of those days with precipitation. It is most likely around January 7, when it is observed during 14% of all days.
Heavy rain is the most severe precipitation observed during 17% of those days with precipitation. It is most likely around January 1, when it is observed during 13% of all days.
Either snow is exceptionally unlikely to fall at this location in January or this station does not reliably report precipitation types.
The relative humidity typically ranges from 60% (mildly humid) to 92% (very humid) over the course of a typical January, rarely dropping below 51% (mildly humid) and reaching as high as 97% (very humid).
The air is driest around January 31, at which time the relative humidity drops below 65% (mildly humid) three days out of four; it is most humid around January 11, 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 January, the dew point typically varies from 65°F (mildy humid) to 72°F (very muggy) and is rarely below 61°F (comfortable) or above 75°F (very muggy).
Over the course of January typical wind speeds vary from 0 mph to 17 mph (calm to moderate breeze), rarely exceeding 20 mph (fresh breeze).
The highest average wind speed of 8 mph (gentle breeze) occurs around January 31, at which time the average daily maximum wind speed is 17 mph (moderate breeze).
The lowest average wind speed of 7 mph (light breeze) occurs around January 13, at which time the average daily maximum wind speed is 16 mph (moderate breeze).