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 September. 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 September is characterized by essentially constant daily high temperatures, with daily highs around 88°F throughout the month, exceeding 91°F or dropping below 86°F only one day in ten.
Over the course of September, the length of the day is essentially constant. The shortest day of the month is September 30 with 11:58 hours of daylight; the longest day is September 1 with 12:28 hours of daylight.
The earliest sunrise is at 6:10am on September 2; the latest sunset is at 6:38pm on September 1; the latest sunrise is at 6:15am on September 30; the earliest sunset is at 6:13pm on September 30.
Daylight savings time (DST) is not observed in 2012.
The median cloud cover is 37% (mostly clear) and does not vary substantially over the course of the month.
On September 15, a representative day, the sky is clear or mostly clear 49% of the time, partly cloudy 30% of the time, and mostly cloudy or overcast 16% of the time.
The average probability that some form of precipitation will be observed in a given day is 69%, with little variation over the course of the month.
Throughout September, the most common forms of precipitation are thunderstorms, light rain, heavy rain, and moderate rain.
Thunderstorms are the most severe precipitation observed during 41% of those days with precipitation. They are most likely around September 14, when it is observed during 29% of all days.
Light rain is the most severe precipitation observed during 33% of those days with precipitation. It is most likely around September 5, when it is observed during 25% of all days.
Heavy rain is the most severe precipitation observed during 13% of those days with precipitation. It is most likely around September 1, when it is observed during 10% of all days.
Moderate rain is the most severe precipitation observed during 13% of those days with precipitation. It is most likely around September 29, when it is observed during 10% of all days.
Either snow is exceptionally unlikely to fall at this location in September or this station does not reliably report precipitation types.
The relative humidity typically ranges from 63% (mildly humid) to 90% (very humid) over the course of a typical September, rarely dropping below 55% (mildly humid) and reaching as high as 96% (very humid).
The air is driest around September 4, at which time the relative humidity drops below 67% (mildly humid) three days out of four; it is most humid around September 25, rising above 87% (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 September, the dew point typically varies from 72°F (very muggy) to 77°F (oppressive) and is rarely below 70°F (muggy) or above 79°F (oppressive).
Over the course of September typical wind speeds vary from 0 mph to 16 mph (calm to moderate breeze), rarely exceeding 20 mph (fresh breeze).
The highest average wind speed of 7 mph (light breeze) occurs around September 1, at which time the average daily maximum wind speed is 16 mph (moderate breeze).
The lowest average wind speed of 6 mph (light breeze) occurs around September 30, at which time the average daily maximum wind speed is 15 mph (moderate breeze).