December 2021 Summary – Tied for Tenth Warmest on Record

Blue Hill Observatory December 2021 Summary:

December was very warm and much drier, less snowy and cloudier than average. The 24-hour adjusted mean temperature for the month of 36.2 deg F was 6.1 degrees warmer than the 1891-2020 130-year average for December, and it was 3.4 degrees cooler than the 1991-2020 30-year average. The month tied as the tenth warmest December on record, and it was the warmest since the monthly record was set in 2015. Much of the warmth occurred in the first half of the month with eight days exceeding 50F during the first seventeen days. The average maximum temperature of 43.0 deg F was 5.3 degrees warmer than the 130-year mean. The average minimum temperature of 29.7 deg F was 6.5 degrees warmer than the 130-year mean, and this mean was the sixth warmest on record for December. The highest temperature observed all month was 62F on the 11th, and the lowest temperature during December was 15F on the 20th. The total precipitation was 2.63 inches, which was nearly two inches less than the 130-year average and nearly three inches less than the 1991-2020 30-year mean. The greatest precipitation in 24 hours was 0.91 inches on the 18th-19th, which included rain, sleet, snow and freezing rain. There was one thunderstorm day during December on the 2nd, which raised the annual total to 26, or four more than average. Snowfall for the month totaled just 1.7 inches, which was about ten inches below average. Most of this snow, 1.2 inches, fell in less than 24 hours on the 8th-9th. The mean wind speed for the month was 12.1 mph, which was the third lowest on record for December, and the prevailing wind direction was from the west-northwest. The highest wind gust was 59 mph from the WNW during rain on the 6th. December sunshine was much lower than average with a total of 96.5 hours of bright sunshine, or 35 percent of possible, which was 11 percent below the long-term average for December.

BHO Warmest December 24-hour Adjusted Mean Temperature, deg F (1885-2021):

1) 42.9 in 2015
2) 38.1 in 2006
3) 37.5 in 2011
4) 37.3 in 2001
5) 37.2 in 1891
6) 36.9 in 1998
7) 36.7 in 1990
8) 36.6 in 1953
9) 36.3 in 1996
10) 36.2 in 2012
   36.2 in 2021

BHO Warmest December Average Minimum Temperature, deg F (1885-2021):

1) 36.9 in 2015
2) 31.2 in 2006
3) 30.8 in 2001
4) 30.5 in 1996
5) 30.1 in 2012
6) 29.7 in 2021
7) 29.6 in 2011

BHO Lowest December Mean Wind Speed, mph (1885-2021):

1) 11.8 in 2015
2) 12.0 in 2017
3) 12.1 in 2021
4) 12.2 in 2014
5) 12.3 in 2013

Mike Iacono
Chief Scientist
Blue Hill Observatory