December 2024 Summary – Warmer and Wetter Than Average

Sunrise on the summit of Great Blue Hill on the morning of 21 December 2024 following a nine-inch snowfall the previous day. Photo by Blue Hill observer Joshua Polen.

Blue Hill Observatory December 2024 Summary:

December temperature was above average with more rainfall and much more sunshine than expected. The approximated 24-hour mean temperature for the month of 32.9 deg F was 2.8 degrees warmer than the 1891-2020 130-year average for December, and it was just 0.1 degrees warmer than the 1991-2020 30-year average. The average maximum temperature of 40.4 deg F was 0.2 degrees warmer than the 30-year normal, and the average minimum temperature of 25.5 deg F was 0.6 degrees warmer than the 30-year normal. After a cool start during the first week, December brought large swings in temperature, though there were no daily records. The highest temperature was 62F during a warm rain and wind storm on the 11th, and the lowest temperature was 4F on the 23rd, which was also the coldest temperature for the year. The mean water vapor pressure, which is an absolute measure of water vapor in the air, was 5.2 mb, which corresponds to a mean dew point of 29F, which was above average for the month. December brought relief from a summer to fall drought, and the month was the first with above average precipitation since June. The total precipitation for the month was 7.58 inches, which was 2.19 inches more than the 30-year normal. The greatest amount in 24 hours was 3.43 inches on the 11th-12th, and the calendar day total of 3.38 inches on the 11th was the third wettest day ever observed in December. Another significant storm brought 1.17 inches of liquid equivalent and the largest snowfall of the month of 9.4 inches on the 20th. The snowfall total for the month was 12.6 inches, which equalled the 30-year normal. There was one thunderstorm day during December during the heavy rain on the 11th, which was one more than average, and the annual total was 20, which was two fewer than average. A large area of high pressure moved into New England after the storm on the 11th, and the sea-level pressure at the Observatory peaked at 30.92 inches on the 14th, which was the highest pressure for the year. This reading was the highest sea-level pressure in December since 31.04 inches, the monthly record, was observed in 1949. It was also the highest pressure for any month since the station record sea-level pressure of 31.08 inches was measured on 13 February 1981. The mean wind speed was 11.9 mph, which was the third lowest on record for the month, and the prevailing wind direction was from the west. The highest wind gust was 65 mph from the south-southwest during the heavy rain on the 11th. Despite the high rainfall, December sunshine was well above average with 154.4 hours of bright sunshine, or 56 percent of possible, which was ten percent more than the long-term average for the month.

BHO Wettest December Calendar Days, inches (1886-2024):

1) 3.94 on 12 Dec 1992
2) 3.64 on 26 Dec 1969
3) 3.38 on 11 Dec 2024
4) 3.23 on 9 Dec 2014
5) 3.03 on 17 Dec 1888
6) 2.93 on 17 Dec 1973
7) 2.87 on 17 Dec 2000
8) 2.86 on 5 Dec 1993

BHO Highest December Sea-Level Pressure, mb (1885-2024):

1) 31.04 on 25 Dec 1949
2) 30.98 on 1 Dec 1887
3) 30.95 on 31 Dec 1889
4) 30.92 on 14 Dec 2024
5) 30.88 on 14 Dec 1893
6) 30.87 on 21 Dec 1942
30.87 on 23 Dec 1971

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

1) 11.2 in 2023
2) 11.8 in 2015
3) 11.9 in 2024
4) 12.0 in 2017
5) 12.1 in 2021
6) 12.2 in 2014
7) 12.3 in 2013

Mike Iacono
Chief Scientist
Blue Hill Observatory