Average Weather in September in Earlham Iowa, United States
Daily high temperatures decrease by 10°F, from 81°F to 71°F, rarely falling below 58°F or exceeding 90°F.
Daily low temperatures decrease by 12°F, from 61°F to 49°F, rarely falling below 37°F or exceeding 70°F.
For reference, on July 20, the hottest day of the year, temperatures in Earlham typically range from 67°F to 85°F, while on January 21, the coldest day of the year, they range from 16°F to 32°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 Earlham experiences essentially constant cloud cover, with the percentage of time that the sky is overcast or mostly cloudy remaining about 31% throughout the month.
The clearest day of the month is September 14, with clear, mostly clear, or partly cloudy conditions 71% of the time.
For reference, on February 13, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 54%, while on August 26, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 71%.
Cloud Cover Categories in September
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Earlham, the chance of a wet day over the course of September is rapidly decreasing, starting the month at 31% and ending it at 25%.
For reference, the year's highest daily chance of a wet day is 43% on June 7, and its lowest chance is 8% 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 Earlham is decreasing, starting the month at 3.3 inches, when it rarely exceeds 6.5 inches or falls below 1.2 inches, and ending the month at 2.6 inches, when it rarely exceeds 4.6 inches or falls below 0.9 inches.
Average Monthly Rainfall in September
Over the course of September in Earlham, the length of the day is rapidly decreasing. From the start to the end of the month, the length of the day decreases by 1 hour, 19 minutes, implying an average daily decrease of 2 minutes, 44 seconds, and weekly decrease of 19 minutes, 6 seconds.
The shortest day of the month is September 30, with 11 hours, 48 minutes of daylight and the longest day is September 1, with 13 hours, 7 minutes of daylight.
Hours of Daylight and Twilight in September
The earliest sunrise of the month in Earlham is 6:42 AM on September 1 and the latest sunrise is 30 minutes later at 7:12 AM on September 30.
The latest sunset is 7:49 PM on September 1 and the earliest sunset is 50 minutes earlier at 6:59 PM on September 30.
Daylight saving time is observed in Earlham during 2018, but it neither starts nor ends during September, so the entire month is in daylight saving time.
For reference, on June 21, the longest day of the year, the Sun rises at 5:42 AM and sets 15 hours, 11 minutes later, at 8:53 PM, while on December 21, the shortest day of the year, it rises at 7:39 AM and sets 9 hours, 10 minutes later, at 4:49 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 Earlham is very rapidly decreasing during September, falling from 34% to 4% over the course of the month.
For reference, on July 22, the muggiest day of the year, there are muggy conditions 56% of the time, while on January 1, the least muggy day of the year, there are muggy conditions 0% 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 Earlham is increasing during September, increasing from 9.1 miles per hour to 10.6 miles per hour over the course of the month.
For reference, on April 1, the windiest day of the year, the daily average wind speed is 12.9 miles per hour, while on August 3, the calmest day of the year, the daily average wind speed is 8.2 miles per hour.
Average Wind Speed in September
Wind Direction 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).
The growing season in Earlham typically lasts for 5.9 months (179 days), from around April 17 to around October 13, rarely starting before March 30 or after May 5, and rarely ending before September 24 or after November 1.
The month of September in Earlham is more likely than not fully within the growing season, with the chance that a given day is in the growing season decreasing from 100% to 80% over the course of the month.
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 Earlham are increasing during September, increasing by 439°F, from 2,700°F to 3,139°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 Earlham is decreasing during September, falling by 1.2 kWh, from 5.7 kWh to 4.5 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 Earlham are 41.492 deg latitude, -94.124 deg longitude, and 1,076 ft elevation.
The topography within 2 miles of Earlham contains only modest variations in elevation, with a maximum elevation change of 171 feet and an average elevation above sea level of 1,076 feet. Within 10 miles also contains only modest variations in elevation (384 feet). Within 50 miles contains only modest variations in elevation (830 feet).
The area within 2 miles of Earlham is covered by cropland (99%), within 10 miles by cropland (92%), and within 50 miles by cropland (89%).
This report illustrates the typical weather in Earlham 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 6 weather stations near enough to contribute to our estimation of the temperature and dew point in Earlham.
For each station, the records are corrected for the elevation difference between that station and Earlham 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 Earlham is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Earlham and a given station.
The stations contributing to this reconstruction are: Perry Municipal Airport (27%, 38 kilometers, north); Des Moines International Airport (25%, 40 kilometers, east); Creston Municipal Airport (14%, 58 kilometers, south); Osceola Municipal Airport (13%, 61 kilometers, southeast); Audubon County Airport (11%, 70 kilometers, west); and Atlantic Municipal Airport (9%, 78 kilometers, west).
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.