Average Weather in September in Decatur Michigan, United States
Daily high temperatures decrease by 10°F, from 77°F to 67°F, rarely falling below 55°F or exceeding 86°F.
Daily low temperatures decrease by 10°F, from 59°F to 49°F, rarely falling below 38°F or exceeding 69°F.
For reference, on July 19, the hottest day of the year, temperatures in Decatur typically range from 63°F to 82°F, while on January 29, the coldest day of the year, they range from 19°F to 31°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 Decatur experiences gradually increasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy increasing from 32% to 37%.
The clearest day of the month is September 1, with clear, mostly clear, or partly cloudy conditions 68% of the time.
For reference, on December 29, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 68%, while on August 26, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 69%.
Cloud Cover Categories in September
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Decatur, the chance of a wet day over the course of September is essentially constant, remaining around 31% throughout.
For reference, the year's highest daily chance of a wet day is 35% on July 30, and its lowest chance is 17% on February 3.
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 Decatur is essentially constant, remaining about 3.2 inches throughout, and rarely exceeding 5.3 inches or falling below 1.3 inches.
Average Monthly Rainfall in September
Over the course of September in Decatur, 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, 21 minutes, implying an average daily decrease of 2 minutes, 47 seconds, and weekly decrease of 19 minutes, 31 seconds.
The shortest day of the month is September 30, with 11 hours, 47 minutes of daylight and the longest day is September 1, with 13 hours, 8 minutes of daylight.
Hours of Daylight and Twilight in September
The earliest sunrise of the month in Decatur is 7:09 AM on September 1 and the latest sunrise is 30 minutes later at 7:39 AM on September 30.
The latest sunset is 8:17 PM on September 1 and the earliest sunset is 50 minutes earlier at 7:27 PM on September 30.
Daylight saving time is observed in Decatur 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 6:08 AM and sets 15 hours, 15 minutes later, at 9:23 PM, while on December 21, the shortest day of the year, it rises at 8:08 AM and sets 9 hours, 6 minutes later, at 5:15 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 Decatur is very rapidly decreasing during September, falling from 26% to 4% over the course of the month.
For reference, on July 28, the muggiest day of the year, there are muggy conditions 40% 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 Decatur is increasing during September, increasing from 8.9 miles per hour to 10.7 miles per hour over the course of the month.
For reference, on January 15, the windiest day of the year, the daily average wind speed is 13.7 miles per hour, while on August 4, the calmest day of the year, the daily average wind speed is 8.1 miles per hour.
Average Wind Speed in September
Wind Direction in September
Decatur 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 Decatur is rapidly decreasing during September, falling by 7°F, from 71°F to 63°F, over the course of the month.
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).
The growing season in Decatur typically lasts for 5.8 months (178 days), from around April 25 to around October 19, rarely starting before April 8 or after May 14, and rarely ending before September 30 or after November 9.
The month of September in Decatur 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 90% 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 Decatur are increasing during September, increasing by 392°F, from 2,322°F to 2,714°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 Decatur is decreasing during September, falling by 1.3 kWh, from 5.5 kWh to 4.2 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 Decatur are 42.108 deg latitude, -85.974 deg longitude, and 758 ft elevation.
The topography within 2 miles of Decatur is essentially flat, with a maximum elevation change of 75 feet and an average elevation above sea level of 759 feet. Within 10 miles is essentially flat (387 feet). Within 50 miles contains only modest variations in elevation (574 feet).
The area within 2 miles of Decatur is covered by cropland (63%), trees (21%), and artificial surfaces (12%), within 10 miles by cropland (63%) and trees (32%), and within 50 miles by cropland (51%) and water (21%).
This report illustrates the typical weather in Decatur 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 5 weather stations near enough to contribute to our estimation of the temperature and dew point in Decatur.
For each station, the records are corrected for the elevation difference between that station and Decatur 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 Decatur is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Decatur and a given station.
The stations contributing to this reconstruction are: Hree Rivers Municipal Dr Haines Airport (23%, 36 kilometers, southeast); South Haven Area Regional Airport (23%, 36 kilometers, northwest); Ross Field-Twin Cities Airport (21%, 38 kilometers, west); Kalamazoo-Battle Creek International Airport (21%, 38 kilometers, east); and Michigan City Municipal Airport-Phillips Field (13%, 52 kilometers, southwest).
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.