August 2021 Summary – Third Warmest on Record

Blue Hill Observatory August 2021 Summary:

August was very warm and wetter than average. The 24-hour adjusted mean temperature for the month of 72.6 deg F was 4.3 degrees warmer than the 1891-2020 130-year average for August, and it was 2.3 degrees warmer than the 1991-2020 30-year average. The month tied with 2012 as the third warmest August on record. The average maximum temperature of 81.6 deg F was 3.4 degrees warmer than the 130-year mean. The average minimum temperature of 65.4 deg F was 4.9 degrees warmer than the 130-year mean, and the average minimum was the second warmest on record. The highest temperature observed all month was 92F on the 13th, and the lowest temperature during August was 58F on the 1st. The monthly minimum temperature of 58F tied for the second highest ever observed in August. The three 90-degree days during the month bring the annual total up to ten, which is four more than the 130-year mean and one more than the 1991-2020 30-year average. The mean water vapor pressure during the month was 21.9 mb, which reflected the high level of moisture in the air during August and was the second highest on record for the month. August total precipitation was 5.37 inches, which was more than an inch more than the 130-year average and about an inch and a half more than the 1991-2020 30-year mean. Two tropical cyclones contributed to the August rainfall. On August 19th, former Tropical Storm Fred passed to the west of the Observatory as a post-tropical system and brought 0.62 inches of rain and a peak gust to 41 mph from the south. Three days later on August 22nd, former Category 1 Hurricane Henri made landfall near Westerly, Rhode Island. This storm initially brought 0.78 inches of rain during the morning to early afternoon of the 22nd and a wind gust to 47 mph from the ESE in the early afternoon, before moving toward the west. On the following day, the remnant low crossed the area again from west to east and brought an additional 1.17 inches of rain with minimal wind during the evening of the 23rd. The greatest precipitation in 24 hours was 2.38 inches, nearly half of the monthly total, on the 4th-5th. The greatest rainfall in a single hour was 0.47 inches during a thunderstorm on the evening of the 23rd as post-tropical cyclone Henri crossed the area. There were two thunderstorm days during August, which was two fewer than average, and the annual total through August is 21, which is three more than average through the first eight months of the year. The mean wind speed for the month was 9.5 mph, which tied for the lowest on record for August and for any month, and the prevailing wind direction was from the south-southwest. The highest wind gust was 47 mph from the ESE during the passage of Henri on the 22nd. August sunshine was well below average with 210.2 hours of bright sunshine, which was 51 percent of the possible sunshine and eight percent less than the 130-year mean. This amount was the lowest sunshine observed in August since 1986.

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

1) 74.1 in 2016
2) 73.4 in 2018
3) 72.6 in 2012
   72.6 in 2021
5) 72.5 in 1988
6) 72.4 in 2015
7) 72.3 in 2005
   72.3 in 2020

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

1) 66.4 in 2018
2) 65.4 in 2021
3) 65.0 in 2016
4) 64.9 in 2001
5) 64.6 in 2012

BHO Highest August Monthly Minimum Temperature, deg F (1885-2021):

1) 59 in 2018
2) 58 in 1939
   58 in 2015
   58 in 2021
5) 57 in 1901
   57 in 2001

BHO Highest August Average Water Vapor Pressure, mb (1932-2021):

1) 22.5 in 2018
2) 21.9 in 2021
3) 21.6 in 1937
4) 21.5 in 1988
5) 21.0 in 1955
   21.0 in 2009

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

1) 9.5 in 1996
   9.5 in 2008
   9.5 in 2021
4) 9.8 in 2019
5) 10.0 in 1899
   10.0 in 2009

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

1) 9.5 in August 1996
   9.5 in August 2008
   9.5 in August 2021
4) 9.6 in July 2019
5) 9.7 in July 2021

Blue Hill Observatory Summer (June-August) 2021 Summary:

Summer 2021 was generally warm and much wetter than average. The 24-hour adjusted mean temperature for the season of 70.2 deg F was 2.7 degrees warmer than the 1891-2020 130-year average for June-August, and it was 1.0 degrees warmer than the 1991-2020 30-year average. Warm weather in June, which was the warmest on record, and August, which tied for the third warmest, was interrupted by a cool and very wet July. The season tied with 2005 as the ninth warmest summer on record. The average maximum temperature of 79.6 deg F was 1.9 degrees warmer than the 130-year mean, but the average minimum temperature of 62.4 deg F was 3.1 degrees warmer than the 130-year mean and tied for the third warmest on record. The highest temperature observed during the summer was 95F on June 29th, and the lowest temperature was 50F on June 1st. The mean water vapor pressure, which represents the amount of moisture in the form of water vapor in the air, was 19.9 mb, and this amount was the highest ever observed during the summer months. June to August precipitation totaled 19.68 inches, with most of that falling during the second wettest July, which was about eight inches more than the long-term average, and summer 2021 was the seventh wettest on record. Three tropical systems, former Hurricane Elsa in July and former Tropical Storm Fred and Hurricane Henri in August, contributed 5.55 inches of rain to the summer total. Two of these cyclones, Elsa and Henri made landfall in New England before making their closest approach to the Observatory. This was the first year since 2004 that multiple tropical cyclones made landfall in New England. There were 18 thunderstorm days during summer, with ten of these occurring in July, and the summer total was four more than average. The mean wind speed was 10.0 mph, which tied for the lowest on record during June-August, and the prevailing wind direction was from the south-southwest. The peak wind gust during summer was 63 mph from the NW during a thunderstorm, which also produced small hail, on July 7th. Summer sunshine was below average with 52 percent of the possible bright sunshine, which was five percent less than the 130-year summer average of 57 percent.

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

1) 71.6 in 2016
2) 71.3 in 2010
3) 71.1 in 2020
4) 70.9 in 2018
5) 70.8 in 1949
6) 70.6 in 1999
7) 70.4 in 1952
8) 70.3 in 2019
9) 70.2 in 2005
   70.2 in 2021

BHO Warmest Summer (Jun-Aug) Average Minimum Temperature, deg F (1885-2021):

1) 63.1 in 2020
2) 62.5 in 2010
3) 62.4 in 2018
   62.4 in 2021
5) 62.3 in 2016

BHO Highest Summer (Jun-Aug) Mean Water Vapor Pressure, mb (1932-2021):

1) 19.9 in 2021
2) 19.6 in 1973
3) 19.5 in 2018
4) 19.4 in 2013
   19.4 in 2020

BHO Highest Summer (Jun-Aug) Precipitation, inches (1886-2021):

1) 25.22 in 1998
2) 24.61 in 1955
3) 24.09 in 1938
4) 21.56 in 2006
5) 21.02 in 1922
6) 20.23 in 1982
7) 19.68 in 2021
8) 18.71 in 1989

BHO Lowest Summer (Jun-Aug) Mean Wind Speed, mph (1885-2021):

1) 10.0 in 2019
   10.0 in 2021
3) 10.1 in 2020
4) 10.2 in 2012
5) 103 in 2008
   10.3 in 2009

Mike Iacono
Chief Scientist
Blue Hill Observatory