December Weather in San Juan Puerto RicoDaily high temperatures are around 84°F, rarely falling below 80°F or exceeding 87°F. Daily low temperatures are around 73°F, rarely falling below 70°F or exceeding 77°F. For reference, on August 20, 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. The figure below shows you a compact characterization of the hourly average temperatures for the quarter of the year centered on December. 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. Mombasa, Kenya (7,348 miles away) is the far-away foreign place with temperatures most similar to San Juan (view comparison). CloudsThe month of December in San Juan experiences rapidly decreasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy decreasing from 34% to 21%. The clearest day of the month is December 31, with clear, mostly clear, or partly cloudy conditions 79% of the time. For reference, on May 29, 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%. PrecipitationA 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 December is very rapidly decreasing, starting the month at 35% and ending it at 22%. For reference, the year's highest daily chance of a wet day is 50% on September 25, and its lowest chance is 20% on January 9. RainfallTo 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 December in San Juan is rapidly decreasing, starting the month at 2.8 inches, when it rarely exceeds 6.7 inches or falls below 0.4 inches, and ending the month at 1.4 inches, when it rarely exceeds 3.3 inches or falls below 0.1 inches. SunOver the course of December in San Juan, the length of the day is essentially constant. The shortest day of the month is December 20, with 11 hours, 1 minute of daylight and the longest day is December 1, with 11 hours, 6 minutes of daylight. The earliest sunrise of the month in San Juan is 6:40 AM on December 1 and the latest sunrise is 16 minutes later at 6:56 AM on December 31. The earliest sunset is 5:46 PM on December 1 and the latest sunset is 12 minutes later at 5:59 PM on December 31. Daylight saving time is not observed in San Juan during 2024. For reference, on June 20, the longest day of the year, the Sun rises at 5:48 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 6:52 AM and sets 11 hours, 1 minute later, at 5:53 PM. The figure below presents a compact representation of the sun's elevation (the angle of the sun above the horizon) and azimuth (its compass bearing) for every hour of every day in the reporting period. The horizontal axis is the day of the year and the vertical axis is the hour of the day. For a given day and hour of that day, the background color indicates the azimuth of the sun at that moment. The black isolines are contours of constant solar elevation. MoonThe figure below presents a compact representation of key lunar data for December 2024. The horizontal axis is the day, the vertical axis is the hour of the day, and the colored areas indicate when the moon is above the horizon. The vertical gray bars (new Moons) and blue bars (full Moons) indicate key Moon phases. The label associated with each bar indicates the date and time that the phase is obtained, and the companion time labels indicate the rise and set times of the Moon for the nearest time interval in which the moon is above the horizon.
HumidityWe 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 gradually decreasing during December, falling from 96% to 93% over the course of the month. For reference, on June 11, the muggiest day of the year, there are muggy conditions 100% of the time, while on March 8, the least muggy day of the year, there are muggy conditions 81% of the time. WindThis 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 gradually increasing during December, increasing from 11.3 miles per hour to 11.8 miles per hour over the course of the month. For reference, on July 13, the windiest day of the year, the daily average wind speed is 12.5 miles per hour, while on October 9, the calmest day of the year, the daily average wind speed is 9.3 miles per hour. The highest daily average wind speed during December is 11.8 miles per hour on December 31. The hourly average wind direction in San Juan throughout December is predominantly from the east, with a peak proportion of 88% on December 31. Water TemperatureSan 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 December, remaining within 1°F of 81°F throughout. Growing SeasonDefinitions 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. 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 rapidly increasing during December, increasing by 835°F, from 10,030°F to 10,865°F, over the course of the month. Solar EnergyThis 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 essentially constant during December, remaining within 0.1 kWh of 4.9 kWh throughout. TopographyFor the purposes of this report, the geographical coordinates of San Juan are 18.466 deg latitude, -66.106 deg longitude, and 36 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%). Data SourcesThis report illustrates the typical weather in San Juan, based on a statistical analysis of historical hourly weather reports and model reconstructions from January 1, 1980 to December 31, 2016. Temperature and Dew PointThere 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. The stations contributing to this reconstruction are:
To get a sense of how much these sources agree with each other, you can view a comparison of San Juan and the stations that contribute to our estimates of its temperature history and climate. Please note that each source's contribution is adjusted for elevation and the relative change present in the MERRA-2 data. Other DataAll data relating to the Sun's position (e.g., sunrise and sunset) are computed using astronomical formulas from the book, Astronomical Algorithms 2nd Edition , 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 airports and weather stations are provided by AskGeo.com . Maps are © OpenStreetMap contributors. DisclaimerThe 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. Please review our full terms contained on our Terms of Service page. |