Average Weather in September in San Marcos Costa Rica
Daily high temperatures are around 71°F, rarely falling below 68°F or exceeding 75°F.
Daily low temperatures are around 59°F, rarely falling below 56°F or exceeding 61°F.
For reference, on March 28, the hottest day of the year, temperatures in San Marcos typically range from 60°F to 75°F, while on October 2, the coldest day of the year, they range from 58°F to 71°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 Marcos experiences essentially constant cloud cover, with the percentage of time that the sky is overcast or mostly cloudy remaining about 95% throughout the month.
The clearest day of the month is September 12, with clear, mostly clear, or partly cloudy conditions 6% of the time.
For reference, on June 5, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 96%, while on January 2, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 63%.
Cloud Cover Categories in September
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In San Marcos, the chance of a wet day over the course of September is essentially constant, remaining around 50% throughout.
For reference, the year's highest daily chance of a wet day is 53% on October 7, and its lowest chance is 8% on March 16.
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 Marcos is gradually increasing, starting the month at 6.2 inches, when it rarely exceeds 10.8 inches or falls below 2.4 inches, and ending the month at 6.7 inches, when it rarely exceeds 12.1 inches or falls below 2.8 inches.
Average Monthly Rainfall in September
Over the course of September in San Marcos, the length of the day is gradually decreasing. From the start to the end of the month, the length of the day decreases by 15 minutes, implying an average daily decrease of 32 seconds, and weekly decrease of 3 minutes, 41 seconds.
The shortest day of the month is September 30, with 12 hours, 3 minutes of daylight and the longest day is September 1, with 12 hours, 18 minutes of daylight.
Hours of Daylight and Twilight in September
The latest sunrise of the month in San Marcos is 5:27 AM on September 1 and the earliest sunrise is 2 minutes, 24 seconds earlier at 5:24 AM on September 30.
The latest sunset is 5:44 PM on September 1 and the earliest sunset is 18 minutes earlier at 5:27 PM on September 30.
Daylight saving time is not observed in San Marcos during 2018.
For reference, on June 21, the longest day of the year, the Sun rises at 5:17 AM and sets 12 hours, 41 minutes later, at 5:58 PM, while on December 21, the shortest day of the year, it rises at 5:47 AM and sets 11 hours, 34 minutes later, at 5:21 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 Marcos is gradually increasing during September, rising from 33% to 38% over the course of the month.
The highest chance of a muggy day during September is 38% on September 28.
For reference, on June 1, the muggiest day of the year, there are muggy conditions 44% of the time, while on January 24, the least muggy day of the year, there are muggy conditions 2% 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 Marcos is gradually increasing during September, increasing from 2.6 miles per hour to 3.1 miles per hour over the course of the month.
For reference, on October 8, the windiest day of the year, the daily average wind speed is 3.2 miles per hour, while on December 15, the calmest day of the year, the daily average wind speed is 1.7 miles per hour.
Average Wind Speed in September
Wind Direction in September
San Marcos 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 Marcos is essentially constant during September, remaining around 83°F throughout.
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 Marcos 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 Marcos are increasing during September, increasing by 389°F, from 3,636°F to 4,025°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 Marcos is essentially constant during September, remaining within 0.1 kWh of 3.8 kWh throughout.
Average Daily Incident Shortwave Solar Energy in September
For the purposes of this report, the geographical coordinates of San Marcos are 9.660 deg latitude, -84.020 deg longitude, and 5,266 ft elevation.
The topography within 2 miles of San Marcos contains large variations in elevation, with a maximum elevation change of 2,411 feet and an average elevation above sea level of 5,035 feet. Within 10 miles contains large variations in elevation (9,304 feet). Within 50 miles also contains extreme variations in elevation (12,490 feet).
The area within 2 miles of San Marcos is covered by trees (42%), grassland (30%), shrubs (16%), and cropland (13%), within 10 miles by trees (64%) and grassland (19%), and within 50 miles by trees (43%) and water (30%).
This report illustrates the typical weather in San Marcos 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 is only a single weather station, Juan Santamaría International Airport, in our network suitable to be used as a proxy for the historical temperature and dew point records of San Marcos.
At a distance of 43 kilometers from San Marcos, closer than our threshold of 150 kilometers, this station is deemed sufficiently nearby to be relied upon as our primary source for temperature and dew point records.
The station records are corrected for the elevation difference between the station and San Marcos according to the International Standard Atmosphere , and by the relative change present in the MERRA-2 satellite-era reanalysis between the two locations.
Please note that the station records themselves may additionally have been back-filled using other nearby stations or the MERRA-2 reanalysis.
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.