September 2024 Summary – Warmer and Drier Than Average

Blue Hill Observatory September 2024 Summary:

September temperature was above average with less rainfall than expected and near average sunshine. The approximated 24-hour mean temperature for the month of 63.8 deg F was 2.3 degrees warmer than the 1891-2020 130-year average for September, and it was 0.7 degrees warmer than the 1991-2020 30-year average. The average maximum temperature of 73.5 deg F was 0.8 degrees warmer than the 30-year normal, and the average minimum temperature of 56.2 deg F was 0.6 degrees warmer than the 30-year normal. September was close to average for much of the month with no daily records. The highest temperature for September reached 85F on the 13th, and the lowest temperature was 49F on the 3rd. The range of minimum temperatures during September was relatively low, and despite their mean being close to average, the monthly minimum of 49F was the highest on record for September. The mean water vapor pressure, which is an absolute measure of water vapor in the air, was 15.7 mb, which corresponds to a mean dew point of 57F. The long-term mean water vapor pressure for September is 14.8 mb, which equals a mean dew point of 55F. The total precipitation was 1.83 inches, which was 2.23 inches less than the 30-year normal. The greatest amount in 24 hours was 1.13 inches on the 21st, which was most of the storm total of 1.26 inches observed during light rain and light drizzle on the 20th-22nd. This storm ended a period of 29 days from August 21st to September 18th during which only 0.04 inches of rain was observed. This period rivaled the longest dry spells on record at the Observatory. There were no thunderstorm days during September. Although the mean barometric pressure for the month was near average, the lowest monthly sea-level pressure of 29.80 inches tied for the second highest on record. The mean wind speed was 9.4 mph, which is a new record low average wind speed for September that was also the fourth lowest for any month of the year. The prevailing wind direction was from the northeast. The highest wind gust was only 38 mph from the northeast during the light rain on the 20th. September sunshine was near average with 199.7 hours of bright sunshine, or 56 percent of possible, which was one percent less than the long-term average for the month.

BHO Highest September Monthly Minimum Temperature, deg F (1885-2024):

1) 49 in 2024
2) 48 in 1934
3) 47 in 2018
4) 46 in 1921
   46 in 1994
   46 in 1999
   46 in 2002
   46 in 2003
   46 in 2008
   46 in 2010
   46 in 2015
   46 in 2023

BHO Highest September Monthly Minimum Sea-Level Pressure, mb (1885-2024):

1) 29.82 in 2004
2) 29.80 in 2016
   29.80 in 2024
4) 29.78 in 1909
   29.78 in 1943
6) 29.77 in 1926
7) 29.76 in 1959

BHO Lowest Monthly Mean Wind Speed, mph (1885-2024):

1) 9.2 in June 2023
   9.2 in August 2024
3) 9.3 in July 2023
4) 9.4 in September 2024
5) 9.5 in August 1996
   9.5 in August 2008
   9.5 in August 2021
8) 9.6 in July 2019
   9.6 in October 2022

BHO Lowest September Mean Wind Speed, mph (1885-2024):

1) 9.4 in 2024
2) 10.1 in 2018
3) 10.2 in 2023
4) 10.3 in 2022
5) 10.5 in 2014
6) 10.6 in 2013

Mike Iacono
Chief Scientist
Blue Hill Observatory