2021 Summary – Tied for Fifth Warmest Year on Record

The year 2021 was exceptionally warm and ended in a tie as the fifth warmest on record. The 24-hour adjusted mean temperature of 50.9F was 3.1F warmer than the 130-year (1891-2020) long-term mean and 1.4F above the 30-year (1991-2020) normal. Every month was warmer than the long-term mean except for July. The annual average maximum temperature of 59.7F, which tied as the ninth warmest on record, was 2.6F warmer than the long-term mean. The annual average minimum temperature of 43.1F was 3.6F warmer than the long-term mean, and it tied as the fourth warmest on record.

The year started with generally warm temperatures in January and February, though the coldest period in those months occurred from January 28th through February 13th with several snow storms in this period. The coldest temperatures of the year included a high of 15F on January 29th and lows of 4F, 3F and 2F on January 29th, 30th and 31st. After a cold start to March, warmer temperatures arrived with a daily record high of 71F on March 11th, which was one of the earliest occurrences on record of a temperature above 70F so early in the season. April and May were slightly warm, though the latter month was notable for high temperatures of 87F on the 22nd and 23rd followed a week later by high temperatures of only 46F on the 29th and 30th during several days of soaking rain. These were the coldest high temperatures so late in the season since 46F was observed on June 2nd, 1946, and the 46F on May 30th was a record low daily maximum for the date. June started warm and stayed that way, ending as the warmest June on record. The month included two three-day heat-waves on the 6th-8th and 28th-30th, which had never before occurred in June at Blue Hill. The six 90-degree days tied the record for June previously set in 1997, and one of these days, June 29th, reached 95F, which was the warmest temperature all year. July was very cool due to excessive precipitation and cloudiness, and only one day in July reached 90F. After a cool and wet start to August, the month ended as the third warmest on record. A slightly warm September was followed by the sixth warmest October on record, which included a period of eight out of eleven days in the middle of the month that exceeded 70F. November was near average for temperature, and the first freeze of the season, 31F on November 4th, was about two weeks later than average. December finished the year tied as the tenth warmest on record with eight days reaching 50F or higher during the first half of the month and only one day with a minimum temperature colder than 20F.

Due to the period of relatively cold weather in February, Ponkapoag Pond was not considered free of ice until March 13th, about three days earlier than the long-term mean date, and Houghton’s Pond thawed on March 20th, it’s mean date. The first ripe blueberries were observed on the summit of Great Blue Hill on June 9th, which was twelve days earlier than the average date. Late in the year, Ponkapoag Pond froze over on December 27th and Houghton’s Pond had not yet frozen over by the end of December, so that pond will have its first freeze date in January, which will be one of the latest on record.

The first half of the year brought near normal precipitation, then about twice the average rainfall was observed from July to October, and the year ended with drier weather during the last two months. The annual total precipitation of 59.40 inches was about ten percent more than the 1991-2020 30-year average. Nearly twenty percent of the annual total occurred in July, which was the second wettest on record with 11.59 inches. There were several significant storms during the year, including the impacts of three tropical cyclones. A large storm in mid-April brought 2.52 inches of liquid equivalent precipitation on the 15th-16th that included 3.6 inches of snow on the 16th that was not only a daily record, but it was the sixth largest snowfall on record this late in the season. A drenching rainfall caused by a strong late season coastal storm brought 3.57 inches of rain on May 28th-31st and temperatures that remained in the 40’s for more than two days. Nearly ten inches of rain fell during the first twelve days of July, which included four calendar days with more than an inch of rain with one of these totaling 3.02 inches on the 9th. Two of these days set new daily records, including 1.49 inches on July 2nd, and 1.98 inches on the 3rd. Rainfall was so frequent during July that there were 24 days with measurable precipitation, which was a new record for July and for any month of the year. 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. Most of the rainfall in September occurred on the first two days of the month, when former Category 4 Hurricane Ida passed New England as a post-tropical storm bringing 4.00 inches of rain between mid-day on the 1st and sunrise on the 2nd along with a wind gust to 49 mph from the ENE early on the 2nd. This 4.00 inch storm total was also the greatest precipitation in 24 hours during 2021. October was also very wet, and most of the October rainfall occurred during three rainfall events, including 2.08 inches on the 3rd-5th, 2.38 inches during a powerful coastal storm on the 26th-27th that also brought damaging winds to eastern Massachusetts and a wind gust to 66 mph at Blue Hill, and 1.69 inches on October 30th-31st.

The largest snowstorm of the year brought 14.3 inches on February 1st-2nd, including a daily record of 12.7 inches on February 1st. This was followed by a snowfall of 11.6 inches on February 7th and another of 9.9 inches on the 18th-20th. February was the snowiest month of the year with a total of 39.1 inches, which made it the seventh snowiest February on record. The greatest snow depth on the ground during the year was 18 inches on February 20th. The total snowfall for the calendar year was 52.7 inches, which was about ten inches less than the long-term average.

The mean station pressure during 2021 at the elevation of the mercury barometer at the Observatory was 29.30 inches, which was 0.01 inches higher than the long-term annual mean of 29.29 inches. The maximum sea-level pressure during the year was 30.69 inches on December 15th. The lowest sea-level pressure during 2021 was 29.25 inches on January 16th during the passage of a warm rain storm that also caused a peak wind gust of 48 mph from the east-southeast. This reading was the second highest annual minimum sea-level pressure on record at Blue Hill.

Water vapor pressure, which is derived from the dew point and is an absolute measure of the amount of moisture in the air in terms of the pressure that water vapor contributes to the total surface pressure, averaged 10.9 mb (0.322 inches) during 2021. This value tied as the third highest on record and continued a trend of higher vapor pressures in recent decades, which is consistent with the increasing trend in temperature. Another way to interpret this value is to state that water vapor on average comprised a little over one percent of the total annual mean station pressure during the year.

The annual mean wind speed of 11.5 mph was the lowest on record. This average continued the several decade long trend of decreasing mean wind speeds on Blue Hill. Only one wind gust exceeded 70 mph during 2021, which was a gust to 72 mph from the south-southeast on November 11th. Other strong gusts during the year included 69 mph WNW on March 1st and 66 mph NNE on October 27th.

Bright sunshine for the year was 53 percent of possible, which was one percent more than the long-term average. The least sunny months, February and December had, respectively, 37 and 35 percent of the possible bright sunshine. The sunniest months of the year were March and November, which had, respectively, 68 and 67 percent of the possible sunshine, and November was the sunniest on record.

BHO Warmest Annual 24-Hour Adjusted Mean Temperature, deg F (1885-2021):

1) 51.7 in 2012
2) 51.3 in 2020
3) 51.2 in 2016
4) 51.0 in 2010
5) 50.9 in 1999
   50.9 in 2021
7) 50.8 in 1998
8) 50.6 in 2006
9) 50.5 in 1953
10) 50.4 in 2011

BHO Warmest Annual Maximum Temperature, deg F (1885-2021):

1) 60.8 in 2012
2) 60.6 in 2016
3) 60.5 in 1949
4) 60.4 in 2020
5) 60.2 in 1999
6) 60.1 in 1953
   60.1 in 2010
8) 59.9 in 1991
9) 59.7 in 1990
   59.7 in 2021

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

1) 44.0 in 2012
2) 43.4 in 2020
3) 43.2 in 1998
4) 43.1 in 2006
   43.1 in 2021
6) 43.0 in 2010
7) 42.8 in 2016

BHO Highest Annual Minimum Sea-Level Pressure, inches (1885-2021):

1) 29.27 in 1921
2) 29.25 in 2021
3) 29.23 in 1905
4) 29.21 in 1904
5) 29.20 in 1890

BHO Highest Annual Mean Water Vapor Pressure, mb (1932-2021):

1) 11.0 in 2012
   11.0 in 2018
3) 10.9 in 2011
   10.9 in 2021
5) 10.8 in 2020
6) 10.7 in 1973

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

1) 11.5 in 2021
2) 11.8 in 2012
3) 12.0 in 2020
4) 12.1 in 2018
   12.1 in 2019
6) 12.2 in 2013
   12.2 in 2015
   12.2 in 2017

Mike Iacono
Chief Scientist
Blue Hill Observatory