Average Weather in September in San Juan Puerto Rico
Daily high temperatures are around 88°F, rarely falling below 86°F or exceeding 91°F.
Daily low temperatures are around 77°F, rarely falling below 75°F or exceeding 80°F.
For reference, on August 21, the hottest day of the year, temperatures in San Juan typically range from 78°F to 89°F, while on January 25, the coldest day of the year, they range from 72°F to 83°F.
Average High and Low Temperature in September
The figure below shows you a compact characterization of the hourly average temperatures for the quarter of the year centered on September. The horizontal axis is the day, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.
Average Hourly Temperature in September
The month of September in San Juan experiences essentially constant cloud cover, with the percentage of time that the sky is overcast or mostly cloudy remaining about 68% throughout the month. The highest chance of overcast or mostly cloudy conditions is 69% on September 26.
The clearest day of the month is September 1, with clear, mostly clear, or partly cloudy conditions 34% of the time.
For reference, on May 30, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 73%, while on January 11, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 82%.
Cloud Cover Categories in September
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In San Juan, the chance of a wet day over the course of September is essentially constant, remaining around 49% throughout.
For reference, the year's highest daily chance of a wet day is 50% on September 26, and its lowest chance is 20% on January 9.
Probability of Precipitation in September
To show variation within the month and not just the monthly total, we show the rainfall accumulated over a sliding 31-day period centered around each day.
The average sliding 31-day rainfall during September in San Juan is gradually increasing, starting the month at 4.1 inches, when it rarely exceeds 8.3 inches or falls below 1.1 inches, and ending the month at 4.6 inches, when it rarely exceeds 8.6 inches or falls below 1.2 inches.
The highest average 31-day accumulation is 4.8 inches on September 18.
Average Monthly Rainfall in September
Over the course of September in San Juan, the length of the day is gradually decreasing. From the start to the end of the month, the length of the day decreases by 30 minutes, implying an average daily decrease of 1 minute, 2 seconds, and weekly decrease of 7 minutes, 12 seconds.
The shortest day of the month is September 30, with 11 hours, 59 minutes of daylight and the longest day is September 1, with 12 hours, 29 minutes of daylight.
Hours of Daylight and Twilight in September
The earliest sunrise of the month in San Juan is 6:09 AM on September 1 and the latest sunrise is 5 minutes later at 6:14 AM on September 30.
The latest sunset is 6:38 PM on September 1 and the earliest sunset is 25 minutes earlier at 6:13 PM on September 30.
Daylight saving time is not observed in San Juan during 2018.
For reference, on June 21, the longest day of the year, the Sun rises at 5:49 AM and sets 13 hours, 14 minutes later, at 7:03 PM, while on December 21, the shortest day of the year, it rises at 6:51 AM and sets 11 hours, 1 minute later, at 5:53 PM.
Sunrise & Sunset with Twilight in September
We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.
The chance that a given day will be muggy in San Juan is essentially constant during September, remaining around 100% throughout.
For reference, on June 12, the muggiest day of the year, there are muggy conditions 100% of the time, while on March 9, the least muggy day of the year, there are muggy conditions 81% of the time.
Humidity Comfort Levels in September
This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.
The average hourly wind speed in San Juan is decreasing during September, decreasing from 10.9 miles per hour to 9.6 miles per hour over the course of the month.
For reference, on July 14, the windiest day of the year, the daily average wind speed is 12.5 miles per hour, while on October 10, the calmest day of the year, the daily average wind speed is 9.3 miles per hour.
Average Wind Speed in September
Wind Direction in September
San Juan is located near a large body of water (e.g., ocean, sea, or large lake). This section reports on the wide-area average surface temperature of that water.
The average surface water temperature in San Juan is essentially constant during September, remaining around 84°F throughout.
The highest average surface water temperature during September is 84°F on September 27.
Average Water Temperature in September
Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere).
Temperatures in San Juan are sufficiently warm year round that it is not entirely meaningful to discuss the growing season in these terms. We nevertheless include the chart below as an illustration of the distribution of temperatures experienced throughout the year.
Time Spent in Various Temperature Bands and the Growing Season in September
Growing degree days are a measure of yearly heat accumulation used to predict plant and animal development, and defined as the integral of warmth above a base temperature, discarding any excess above a maximum temperature. In this report, we use a base of 50°F and a cap of 86°F.
The average accumulated growing degree days in San Juan are very rapidly increasing during September, increasing by 925°F, from 7,189°F to 8,114°F, over the course of the month.
Growing Degree Days in September
This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.
The average daily incident shortwave solar energy in San Juan is gradually decreasing during September, falling by 0.6 kWh, from 5.4 kWh to 4.8 kWh, over the course of the month.
Average Daily Incident Shortwave Solar Energy in September
For the purposes of this report, the geographical coordinates of San Juan are 18.466 deg latitude, -66.106 deg longitude, and 0 ft elevation.
The topography within 2 miles of San Juan contains only modest variations in elevation, with a maximum elevation change of 194 feet and an average elevation above sea level of 5 feet. Within 10 miles contains only modest variations in elevation (984 feet). Within 50 miles contains very significant variations in elevation (4,390 feet).
The area within 2 miles of San Juan is covered by water (78%) and artificial surfaces (19%), within 10 miles by water (58%) and grassland (16%), and within 50 miles by water (75%) and trees (14%).
This report illustrates the typical weather in San Juan year round, based on a statistical analysis of historical hourly weather reports and model reconstructions from January 1, 1980 to December 31, 2016.
Temperature and Dew Point
There are 2 weather stations near enough to contribute to our estimation of the temperature and dew point in San Juan.
For each station, the records are corrected for the elevation difference between that station and San Juan according to the International Standard Atmosphere , and by the relative change present in the MERRA-2 satellite-era reanalysis between the two locations.
The estimated value at San Juan is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between San Juan and a given station.
All data relating to the Sun's position (e.g., sunrise and sunset) are computed using astronomical formulas from the book, Astronomical Tables of the Sun, Moon and Planets , by Jean Meeus.
All other weather data, including cloud cover, precipitation, wind speed and direction, and solar flux, come from NASA's MERRA-2 Modern-Era Retrospective Analysis . This reanalysis combines a variety of wide-area measurements in a state-of-the-art global meteorological model to reconstruct the hourly history of weather throughout the world on a 50-kilometer grid.
Land Use data comes from the Global Land Cover SHARE database , published by the Food and Agriculture Organization of the United Nations.
Elevation data comes from the Shuttle Radar Topography Mission (SRTM) , published by NASA's Jet Propulsion Laboratory.
Names, locations, and time zones of places and some airports come from the GeoNames Geographical Database .
Time zones for aiports and weather stations are provided by AskGeo.com .
Maps are © Esri, with data from National Geographic, Esri, DeLorme, NAVTEQ, UNEP-WCMC, USGS, NASA, ESA, METI, NRCAN, GEBCO, NOAA, and iPC.
The information on this site is provided as is, without any assurances as to its accuracy or suitability for any purpose. Weather data is prone to errors, outages, and other defects. We assume no responsibility for any decisions made on the basis of the content presented on this site.
We draw particular cautious attention to our reliance on the MERRA-2 model-based reconstructions for a number of important data series. While having the tremendous advantages of temporal and spatial completeness, these reconstructions: (1) are based on computer models that may have model-based errors, (2) are coarsely sampled on a 50 km grid and are therefore unable to reconstruct the local variations of many microclimates, and (3) have particular difficulty with the weather in some coastal areas, especially small islands.
We further caution that our travel scores are only as good as the data that underpin them, that weather conditions at any given location and time are unpredictable and variable, and that the definition of the scores reflects a particular set of preferences that may not agree with those of any particular reader.