July 2021 Summary – Second Wettest on Record

Blue Hill Observatory July 2021 Summary:

July was very cool and very wet with much less sunshine than average, and July set a new record for the greatest number of days with measurable rainfall in any month. The 24-hour adjusted mean temperature for the month of 67.9 deg F was 2.1 degrees cooler than the 1891-2020 130-year average for July, and it was 3.8 degrees cooler than the 1991-2020 30-year average. The contrast between the July mean temperature and the warmest June on record that preceded it was so extreme that July was 2.1 degrees cooler than the 70.0 degrees observed in June, which was the largest negative difference in July-June mean temperature on record. This was also only the second time since 1885 that July was cooler than June with the other negative difference occurring in 2001. The average maximum temperature of 76.3 deg F was 3.9 degrees cooler than the 130-year mean, and this mean was the tenth coldest average maximum for July on record and the lowest since 2000. The average minimum temperature of 61.3 deg F was just 0.4 degrees cooler than the 130-year mean. The highest temperature observed all month was 90F on the 16th, which was the only 90-degree day all month, and the lowest temperature during July was 52F on the 31st. July total precipitation was extreme at 11.59 inches, which was 7.97 inches more than the long-term 130-year average and more than eight inches greater than the 1991-2020 30-year mean. The July total rainfall was the second highest on record for July and the greatest amount in a single month since 12.11 inches was observed in June 2013. Nearly ten inches of the rainfall fell during the first twelve days of the month, 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 the 2nd, which surpassed 1.42 inches in 1986, and 1.98 inches on the 3rd, which surpassed 1.36 inches on that date in 1985. The greatest precipitation in 24 hours was 3.09 inches on the 9th-10th. The greatest rainfall in a single hour was 0.94 inches early in the morning of the 2nd. Rainfall was so frequent that there were 24 days with measurable precipitation, which was a new record for July and for any month of the year. There were ten thunderstorm days during July, which was five more than average, and the annual total through July has climbed to 19. This was the first month with ten thunderstorm days since ten were observed during June 1998. A thunderstorm during the evening of the 7th was severe with one-eighth inch diameter hail, frequent lightning and a peak wind gust to 63 mph from the NW, which was the highest wind gust for the month The mean wind speed for the month was 9.7 mph, which was the second lowest on record for July, and the prevailing wind direction was from the northeast. With all the precipitation, July sunshine was well below average with 192.9 hours of bright sunshine, which was 43 percent of the possible sunshine and 15 percent less than the 130-year mean. This amount was the seventh lowest sunshine on record for July at the Observatory and the lowest since 1967.

BHO Smallest July-June Mean Temperature Difference, deg F (1885-2021):

1) -2.1 / 70.0 / 67.9 in 2021 (July-June / June / July)
2) -0.9 / 68.5 / 67.6 in 2001
3) +0.7 / 64.8 / 65.5 in 1895
4) +1.0 / 65.8 / 66.8 in 1923
   +1.0 / 68.3 / 69.3 in 1930

BHO Coldest July Average Maximum Temperature, deg F (1885-2021):

1) 73.2 in 1914
2) 73.9 in 1891
3) 74.0 in 1902
4) 74.3 in 1889
5) 75.0 in 1895
6) 75.3 in 1888
7) 75.8 in 1906
8) 76.1 in 1915
9) 76.2 in 2000
10) 76.3 in 2021

BHO Greatest July Precipitation, inches (1885-2021):

1) 11.67 in 1938
2) 11.59 in 2021
3) 10.68 in 1988
4) 10.43 in 1921
5) 9.52 in 1915
6) 8.47 in 1889
7) 7.62 in 2009
8) 7.55 in 1916

BHO Greatest July Number of Days with Measurable Precipitation (1886-2021):

1) 24 in 2021
2) 20 in 1906
3) 19 in 1916
4) 18 in 1938
   18 in 1988

BHO Greatest Monthly Number of Days with Measurable Precipitation (1886-2021):

1) 24 in July 2021
2) 22 in December 1972
3) 21 in May 1888
   21 in May 2019
5) 20 in July 1906
   20 in April 1912
   20 in October 1913
   20 in June 1916
   20 in April 1929
   20 in January 1950
   20 in May 2005
   20 in April 2019
   20 in October 2019

BHO Lowest July Sunshine, hours (1885-2021):

1) 165.5 in 1916  [37%]
2) 183.0 in 1938  [41%]
3) 189.2 in 1958  [43%]
4) 189.5 in 1967  [43%]
5) 189.9 in 1927  [43%]
6) 192.0 in 1921  [43%]
7) 192.9 in 2021  [43%]
8) 195.7 in 1897  [44%]

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

1) 9.6 in 2019
2) 9.7 in 2021
3) 9.8 in 2012
   9.8 in 2020
5) 10.0 in 2017
6) 10.1 in 1906
   10.1 in 2013

Mike Iacono
Chief Scientist
Blue Hill Observatory