Average Weather in September in Juneau Alaska, United States
In Juneau, the month of September is characterized by falling daily high temperatures, with daily highs decreasing by 7°F, from 55°F to 48°F over the course of the month, and rarely exceeding 62°F or dropping below 44°F.
Daily low temperatures decrease by 5°F, from 45°F to 39°F, rarely falling below 31°F or exceeding 49°F.
For reference, on July 31, the hottest day of the year, temperatures in Juneau typically range from 48°F to 61°F, while on December 31, the coldest day of the year, they range from 20°F to 28°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 Juneau experiences gradually increasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy increasing from 74% to 79%.
The clearest day of the month is September 1, with clear, mostly clear, or partly cloudy conditions 26% of the time.
For reference, on October 8, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 81%, while on May 15, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 32%.
Cloud Cover Categories in September
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Juneau, the chance of a wet day over the course of September is increasing, starting the month at 64% and ending it at 70%.
For reference, the year's highest daily chance of a wet day is 74% on October 10, and its lowest chance is 42% on May 27.
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 Juneau is very rapidly increasing, starting the month at 11.2 inches, when it rarely exceeds 16.3 inches or falls below 5.9 inches, and ending the month at 14.9 inches, when it rarely exceeds 19.6 inches or falls below 10.5 inches.
Average Monthly Rainfall in September
Over the course of September in Juneau, the length of the day is very rapidly decreasing. From the start to the end of the month, the length of the day decreases by 2 hours, 26 minutes, implying an average daily decrease of 5 minutes, 2 seconds, and weekly decrease of 35 minutes, 12 seconds.
The shortest day of the month is September 30, with 11 hours, 32 minutes of daylight and the longest day is September 1, with 13 hours, 58 minutes of daylight.
Hours of Daylight and Twilight in September
The earliest sunrise of the month in Juneau is 5:57 AM on September 1 and the latest sunrise is 1 hour, 3 minutes later at 7:00 AM on September 30.
The latest sunset is 7:55 PM on September 1 and the earliest sunset is 1 hour, 23 minutes earlier at 6:32 PM on September 30.
Daylight saving time is observed in Juneau during 2017, 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 3:51 AM and sets 18 hours, 17 minutes later, at 10:07 PM, while on December 21, the shortest day of the year, it rises at 8:44 AM and sets 6 hours, 22 minutes later, at 3:07 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 Juneau is essentially constant during September, remaining around 0% throughout.
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 Juneau is increasing during September, increasing from 5.7 miles per hour to 7.1 miles per hour over the course of the month.
For reference, on January 1, the windiest day of the year, the daily average wind speed is 10.5 miles per hour, while on July 4, the calmest day of the year, the daily average wind speed is 4.6 miles per hour.
Average Wind Speed in September
Wind Direction in September
Juneau 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 Juneau is gradually decreasing during September, falling by 3°F, from 54°F to 51°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 Juneau typically lasts for 4.4 months (135 days), from around May 14 to around September 26, rarely starting before April 27 or after May 31, and rarely ending before September 6 or after October 16.
During September in Juneau, the chance that a given day is within the growing season is very rapidly decreasing falling from 95% to 40% 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 Juneau is essentially constant during September, remaining within 10°F of 337°F throughout.
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 Juneau is decreasing during September, falling by 1.2 kWh, from 2.8 kWh to 1.6 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 Juneau are 58.302 deg latitude, -134.420 deg longitude, and 1,047 ft elevation.
The topography within 2 miles of Juneau contains extreme variations in elevation, with a maximum elevation change of 3,504 feet and an average elevation above sea level of 895 feet. Within 10 miles also contains extreme variations in elevation (5,453 feet). Within 50 miles also contains extreme variations in elevation (8,409 feet).
The area within 2 miles of Juneau is covered by trees (38%), water (26%), and shrubs (23%), within 10 miles by trees (30%) and water (23%), and within 50 miles by trees (28%) and water (24%).
This report illustrates the typical weather in Juneau 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 2 weather stations near enough to contribute to our estimation of the temperature and dew point in Juneau.
For each station, the records are corrected for the elevation difference between that station and Juneau 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 Juneau is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Juneau and a given station.
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.