September 2025 Summary – Warmer and Sunnier Than Average

Blue Hill Observatory September 2025 Summary: 

September 2025 was warmer than average with near normal precipitation and much more sunshine than average. The approximated 24-hour mean temperature for the month of 64.3 deg F was 2.8 degrees warmer than the 1891-2020 130-year average for September, and it was 1.2 degrees warmer than the 1991-2020 30-year average. The average maximum temperature of 74.1 deg F was 1.4 degrees warmer than the 30-year normal, and the average minimum temperature of 56.8 deg F was 1.2 degrees warmer than the 30-year normal. The first three weeks were relatively cool, which turned to warmer weather during the last nine days. The highest temperature for the month was 84F on the 6th, and the lowest temperature was 49F on the 8th. September mean water vapor pressure, which is an absolute measure of water vapor in the air, was 16.4 mb, which corresponded to a mean dew point of about 58F. This amount was above the long-term mean water vapor pressure for September of 14.8 mb, which equals a mean dew point of about 55F. The total precipitation was 4.49 inches, which was 0.43 inches more than the 30-year normal. The greatest amount in 24-hours was 2.02 inches on the 6th-7th, which was nearly one-half of the month’s rainfall and occurred in part during a thunderstorm on the afternoon of the 6th. There were three thunderstorm days during the month, which was one more than average, and the total for the year through September increased to 21, which was close to average. The mean wind speed was 9.4 mph, which tied as the lowest on record for the month and tied as the fourth lowest for any month of the year, and the prevailing wind direction was from the south-southwest. The highest wind gust was 55 mph from the west during the thunderstorm on the 6th. September sunshine was well above average with 236.5 hours of bright sunshine, or 66 percent of possible, which was ten percent more than the long-term average for the month.

BHO Lowest Monthly Mean Wind Speed, mph (1885-2025):

1) 9.2 in June 2023
   9.2 in August 2024
3) 9.3 in July 2023
4) 9.4 in September 2024
   9.4 in September 2025
6) 9.5 in August 1996
   9.5 in August 2008
   9.5 in August 2021
   9.5 in August 2025
10) 9.6 in July 2019
   9.6 in October 2022

BHO Lowest September Mean Wind Speed, mph (1885-2025):

1) 9.4 in 2024
   9.4 in 2025
3) 10.1 in 2018
4) 10.2 in 2023
5) 10.3 in 2022
6) 10.5 in 2014
7) 10.6 in 2013

Mike Iacono
Chief Scientist
Blue Hill Observatory