Average Weather in September in Portland Maine, United States
Daily high temperatures decrease by 10°F, from 73°F to 63°F, rarely falling below 55°F or exceeding 82°F.
Daily low temperatures decrease by 9°F, from 57°F to 48°F, rarely falling below 37°F or exceeding 65°F.
For reference, on July 25, the hottest day of the year, temperatures in Portland typically range from 62°F to 78°F, while on January 29, 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
frigid 15°F freezing 32°F very cold 45°F cold 55°F cool 65°F comfortable 75°F warm 85°F hot 95°F sweltering
The month of September in Portland experiences gradually increasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy increasing from 36% to 40%. The lowest chance of overcast or mostly cloudy conditions is 36% on September 1.
The clearest day of the month is September 1, with clear, mostly clear, or partly cloudy conditions 64% of the time.
For reference, on May 16, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 57%, while on September 1, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 64%.
Cloud Cover Categories in September
0% clear 20% mostly clear 40% partly cloudy 60% mostly cloudy 80% overcast 100%
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Portland, the chance of a wet day over the course of September is gradually increasing, starting the month at 25% and ending it at 28%.
For reference, the year's highest daily chance of a wet day is 33% on June 13, and its lowest chance is 22% on February 5.
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 Portland is increasing, starting the month at 3.0 inches, when it rarely exceeds 5.6 inches or falls below 1.2 inches, and ending the month at 3.9 inches, when it rarely exceeds 6.4 inches or falls below 1.4 inches.
Average Monthly Rainfall in September
Over the course of September in Portland, 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, 25 minutes, implying an average daily decrease of 2 minutes, 57 seconds, and weekly decrease of 20 minutes, 38 seconds.
The shortest day of the month is September 30, with 11 hours, 45 minutes of daylight and the longest day is September 1, with 13 hours, 10 minutes of daylight.
Hours of Daylight and Twilight in September
The earliest sunrise of the month in Portland is 6:05 AM on September 1 and the latest sunrise is 33 minutes later at 6:37 AM on September 30.
The latest sunset is 7:15 PM on September 1 and the earliest sunset is 53 minutes earlier at 6:22 PM on September 30.
Daylight saving time is observed in Portland during 2020, but it neither starts nor ends during September, so the entire month is in daylight saving time.
For reference, on June 20, the longest day of the year, the Sun rises at 4:59 AM and sets 15 hours, 27 minutes later, at 8:26 PM, while on December 21, the shortest day of the year, it rises at 7:11 AM and sets 8 hours, 56 minutes later, at 4:07 PM.
Sunrise & Sunset with Twilight in September
The figure below presents a compact representation of key lunar data for September 2020. 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.
Moon Rise, Set & Phases 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 Portland is decreasing during September, falling from 11% to 1% over the course of the month.
For reference, on July 29, the muggiest day of the year, there are muggy conditions 26% 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
dry 55°F comfortable 60°F humid 65°F muggy 70°F oppressive 75°F miserable
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 Portland is increasing during September, increasing from 5.9 miles per hour to 7.1 miles per hour over the course of the month.
For reference, on February 20, the windiest day of the year, the daily average wind speed is 10.2 miles per hour, while on July 30, the calmest day of the year, the daily average wind speed is 5.3 miles per hour.
Average Wind Speed in September
The hourly average wind direction in Portland throughout September is predominantly from the west, with a peak proportion of 34% on September 1.
Wind Direction in September
Portland 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 Portland is decreasing during September, falling by 5°F, from 61°F to 56°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 Portland typically lasts for 5.7 months (173 days), from around April 24 to around October 14, rarely starting before April 7 or after May 10, and rarely ending before September 28 or after October 31.
The month of September in Portland 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 86% over the course of the month.
Time Spent in Various Temperature Bands and the Growing Season in September
frigid 15°F freezing 32°F very cold 45°F cold 55°F cool 65°F comfortable 75°F warm 85°F hot 95°F sweltering
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 Portland are increasing during September, increasing by 320°F, from 1,779°F to 2,099°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 Portland is decreasing during September, falling by 1.4 kWh, from 5.4 kWh to 4.0 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 Portland are 43.661 deg latitude, -70.255 deg longitude, and 59 ft elevation.
The topography within 2 miles of Portland contains only modest variations in elevation, with a maximum elevation change of 161 feet and an average elevation above sea level of 23 feet. Within 10 miles contains only modest variations in elevation (492 feet). Within 50 miles contains significant variations in elevation (2,339 feet).
The area within 2 miles of Portland is covered by water (49%) and artificial surfaces (45%), within 10 miles by water (41%) and trees (29%), and within 50 miles by trees (48%) and water (45%).
This report illustrates the typical weather in Portland, 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 3 weather stations near enough to contribute to our estimation of the temperature and dew point in Portland.
For each station, the records are corrected for the elevation difference between that station and Portland 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 Portland is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Portland and a given station.
The stations contributing to this reconstruction are: Portland International Jetport (93%, 4.5 kilometers, southwest); Naval Air Station Brunswick (3.9%, 37 kilometers, northeast); and Auburn/Lewiston Municipal Airport (3.1%, 43 kilometers, north).
All 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 © 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.