September 2015 Summary – Warmest and Fifth Sunniest on Record

Blue Hill Observatory
September 2015 Summary:

The month was notable for being very warm and mostly dry with much more sun than average. The 24-hour mean temperature for the month of 67.2 deg F was 6.0 degrees warmer than the 120-year average for September, and five degrees warmer than the 1981-2010 30-year average, making September the warmest on record by more than one degree (see list below). The average maximum temperature for September of 77.8 deg F was also a new record for the month. The average minimum of 58.5 deg F was the second warmest on record. Also, the mean temperature for August and September combined was also the warmest in 130 years. The maximum temperature for the month was 97F on the 8th during a three day heat wave, which was not only a record for that date, surpassing 93F in 2007, it was the second warmest temperature ever measured in September and the warmest so late in the season. The coolest temperature was 46F on the 27th.  There were multiple daily temperature records, including four daily record maximum temperatures (on the 7th, 8th, 9th and 17th) and three daily record high minimum temperatures. The three consecutive days in the 90s on the 7th-9th provided only the second heat wave ever to be observed in September (the other occurring in 1895), and this was the hottest three consecutive September days since records began in 1885. There have been eight 90-degree days in 2015. The month ended slightly drier than average with a precipitation total of 3.88 inches, which was just 0.06 inches less than the 120-year average and 0.18 inches less than the 1981-2010 mean. However, much of  the month was very dry with only 1.35 inches falling through the afternoon of the 29th. Heavy rainfall late on the 29th and on the 30th brought a total of 2.53 inches in about 24 hours. There were two thunderstorm days during the month, on the 3rd and 30th, which is average for September, and the total for the year is up to 16 thunderstorm days, which is five fewer than average. The mean wind speed was 10.7 mph, which tied for the third lowest on record for September, and the prevailing direction was from the south-southwest. A peak gust of 47 mph from the south was observed during the heavy rainfall on the 30th. September sunshine was very high with 70 percent of the possible bright sunshine (or 251.9 hours) being observed, which is 14 percent more than the long-term average for the month.  September was the fifth sunniest on record. The total sunshine for August and September was the highest in the past 130 years  with 72 percent of the possible bright sunshine being measured over those two months.

BHO Warmest Septembers (24-hour adjusted mean), deg F (1885-2015):

1) 67.2 in 2015
2) 66.1 in 1961
3) 65.5 in 2005
4) 65.4 in 1999
65.4 in 2010

BHO Warmest Septembers (NWS mean), deg F (1885-2015):

1) 68.2 in 2015
2) 66.9 in 1961
3) 66.5 in 1930
4) 66.4 in 1999
5) 66.2 in 2002
66.2 in 2005

BHO Warmest September Average Maximum Temperature, deg F (1885-2015):

1) 77.8 in 2015
2) 76.6 in 1930
3) 75.7 in 2007
4) 75.6 in 1959
5) 75.5 in 1961

BHO Warmest September Average Minimum Temperature, deg F (1885-2015):

1) 58.8 in 1988
2) 58.5 in 2015
3) 58.2 in 1961
58.2 in 1999
5) 57.8 in 2005

BHO Warmest August-September, deg F (1885-2015):

1) 69.8 in 2015
2) 68.9 in 2005
3) 68.6 in 2002
4) 68.2 in 2010
5) 67.8 in 1959

BHO Slowest September Mean Wind Speed, mph (1885-2015):

1) 10.5 in 2014
2) 10.6 in 2013
3) 10.7 in 2011
10.7 in 2015
5) 10.9 in 1917
10.9 in 2003

BHO Sunniest Septembers, hours (1885-2015):

1) 267.7 in 1983 [74%] 2) 266.9 in 1941 [74%] 3) 265.9 in 2007 [74%] 4) 255.3 in 1948 [71%] 5) 251.9 in 2015 [70%]
6) 251.0 in 1967 [70%] 7) 246.8 in 1952 [69%] 246.8 in 1978 [69%]

BHO Sunniest Aug-Sep, hours (1885-2015):

1) 555.1 in 2015 [72%]
2) 553.0 in 1941 [72%] 3) 536.0 in 2007 [69%] 4) 527.3 in 2014 [68%] 5) 520.2 in 1932 [67%] 6) 515.3 in 1921 [67%] 7) 514.3 in 1968 [67%]