May 2023 Summary – Sunniest on Record

Blue Hill Observatory May 2023 Summary: 

May 2023 was warmer and drier than average and set a monthly record for sunshine. The approximated 24-hour mean temperature for the month of 57.7 deg F was 2.2 degrees warmer than the 1891-2020 130-year average for May, and it was 1.0 degrees warmer than the 1991-2020 30-year average. The average maximum temperature of 69.4 deg F was 2.6 degrees warmer than the 30-year normal, and the average minimum temperature of 47.0 deg F was 1.2 degrees cooler than the 30-year normal. After a cool start to the month, seven days reached 80F or higher, including the highest temperature of 86F on the 28th. A brief cool spell in the middle of the month brought temperatures down to the middle 30’s, including the coldest temperature of 34F on the 18th, which was the second latest temperature below 35F on record at the Observatory. The total precipitation was 2.55 inches, which was 1.27 inches less than the 30-year normal. Most of that rainfall, 2.06 inches, fell in less than 24 hours during the largest storm of the month on the 20th-21st. This amount included 1.09 inches during moderate to heavy rain in the last two hours of the 20th and a calendar day total of 1.97 inches on the 20th, which surpassed the previous record for the date of 1.65 inches in 1969. Most of the rest of the May precipitation, 0.41 inches, occurred during the cool and rainy start to the month on the 1st-5th. Only ten days during May had measurable precipitation. One thunderstorm day was recorded during the month on the morning of the 2nd. There was no snowfall during May, and the final snowfall total for the 2022-2023 season was 34.9 inches, which was nearly 35 inches below normal. The minimum sea-level pressure for the month was 29.07 inches on the 1st, and this was also the lowest sea-level pressure of the year to date. The mean wind speed was 11.3 mph and the prevailing wind direction was from the south-southwest. The highest wind gust was 44 mph from the south-southeast on the 1st. May bright sunshine was exceptionally high due to the largely dry weather with 329.9 hours, or 75 percent of possible, which was twenty-three percent more than the long-term average for the month. May was the sunniest on record, and in terms of total hours it was the fourth sunniest on record for any month of the year. 

BHO Latest Minimum Temperatures Below 35 deg F (1885-2023):

1) 34 on 19 May 2002
2) 34 on 18 May 2023
3) 34 on 17 May 1956
   34 on 17 May 1957
5) 30 on 16 May 1936
   33 on 16 May 1917
7) 33 on 14 May 1895
   34 on 14 May 1928

BHO Highest May Bright Sunshine, hours (1886-2023):

1) 329.9 in 2023 [75%]
2) 318.9 in 1964 [73%]
3) 304.4 in 1977 [70%]
4) 295.8 in 1955 [68%]
5) 288.4 in 1991 [66%]

BHO Highest Monthly Bright Sunshine, hours (1886-2023):

1) 336.1 in June 1971 [76%]
2) 335.5 in June 1908 [76%]
3) 331.6 in July 1952 [75%]
4) 329.9 in May 2023 [75%]
5) 327.4 in June 1953 [74%]
6) 326.4 in July 1966 [73%]
7) 322.6 in June 1949 [73%]
322.6 in July 2022 [73%]

Blue Hill Observatory Spring (Mar-May) 2023 Summary: 

Spring 2023 was much warmer, drier, and sunnier than average. The approximated 24-hour mean temperature for the season of 48.3 deg F was 3.4 degrees warmer than the 1891-2020 130-year average for spring, and it was 1.9 degrees warmer than the 1991-2020 30-year average. The season was the seventh warmest spring on record. The average maximum temperature of 58.5 deg F was 2.8 degrees warmer than the 30-year normal, which tied for the ninth warmest on record. The average minimum temperature of 39.7 deg F was 2.0 degrees warmer than the 30-year normal, which tied for the seventh warmest on record. The largest storm of the season was on March 13th-15th, which brought 3.73 inches of liquid precipitation and 7.2 inches of snowfall to the Observatory. This storm total snowfall was the single largest for any event during the winter of 2022-2023. Other notable spring storms brought 1.94 inches of cold rain from April 30th to May 1st, and 2.06 inches of rain on May 20th-21st. Spring precipitation totaled 12.10 inches, which was 2.00 inches below the 30-year normal. About one-half of that amount was measured in March as the season became increasingly dry. Spring snowfall was 10.6 inches, which was 7.2 inches below average, and all of this amount fell in March, The mean wind speed for spring was 11.7 mph, which was the lowest on record, and the prevailing wind direction was from the west-northwest. The highest wind gust was 51 mph from the north-northeast during heavy rain on March 14th. Spring bright sunshine was much above average with 704.4 hours, or 60 percent of possible, which was ten percent more than the long-term average for the season, and in terms of total hours Spring 2023 was the fourth sunniest on record. 

BHO Warmest Spring (Mar-May) Approximated 24-Hour Mean Temperature, deg F (1885-2023):

1) 51.3 in 2010
2) 50.9 in 2012
3) 49.4 in 1991
4) 49.0 in 1977
5) 48.4 in 1998 
   48.4 in 2022
7) 48.3 in 2023
8) 48.2 in 1976

BHO Warmest Spring (Mar-May) Average Maximum Temperature, deg F (1885-2023):

1) 61.4 in 2010
2) 61.1 in 2012
3) 60.5 in 1991
4) 60.3 in 1977
5) 59.6 in 1945
6) 59.0 in 1976
   59.0 in 2022
8) 58.8 in 1903
9) 58.5 in 1921
   58.5 in 2021
   58.5 in 2023

BHO Warmest Spring (Mar-May) Average Minimum Temperature, deg F (1885-2023):

1) 42.6 in 2012
2) 42.4 in 2010
3) 40.5 in 1945
4) 40.4 in 1998
5) 39.9 in 1973
6) 39.8 in 1991
7) 39.7 in 1979
   39.7 in 2023
9) 39.6 in 2022

BHO Highest Spring (Mar-May) Bright Sunshine, hours (1886-2023):

1) 750.6 in 1977 [63%]
2) 715.4 in 1964 [60%]
3) 709.6 in 1963 [60%]
4) 704.4 in 2023 [60%]
5) 694.2 in 1911 [59%]
6) 693.8 in 1962 [59%]
7) 691.7 in 2021 [58%]

BHO Lowest Spring (Mar-May) Mean Wind Speed, mph (1885-2023):

1) 11.7 in 2023
2) 12.0 in 2012
3) 12.3 in 2016
4) 12.4 in 2022
5) 12.6 in 1995
    12.6 in 2018

Mike Iacono
Chief Scientist
Blue Hill Observatory