Observatory Reaches 99F on Third Day of Heatwave

After reaching a high temperature of 91F on June 22nd and 94F on June 23rd, 2025, the heatwave reached its third day on Tuesday, June 24th when the temperature soared to a high of 99F at the Observatory on the summit of Great Blue Hill. This reading not only surpassed the record for the date of 91F set in 1914, 1976, and 2013, it tied the warmest temperature ever observed during the month of June at Blue Hill. The only previous measurement of 99F in June occurred on June 4th, 1919 in the midst of a three-day heatwave on June 3rd-5th that year.

The last time the temperature reach 99F at Blue Hill was on July 24th, 2022. Although the maximum temperature has reached 99F at the Observatory fifteen times in its history since 1885, the temperature has reached 100F or more only four times. The last occurrence of 100F was on July 21st, 1977 with another 100F on August 9th, 1949. The station record temperature is 101F, which occurred on August 10th, 1949 and on August 2nd, 1975.

The minimum temperature on June 24th 2025 was 76F, which was also a new record for the date, surpassing the previous record of 70F set in 1909. This reading was the second highest daily minimum temperature ever measured at the Observatory during June, surpassed only by a reading of 78F on June 6th, 1925.

This late June heatwave ended on its fourth day with a high temperature of 93F on June 25th, which tied the record for that date previously set in 1943, 1980, and 2005. The minimum temperature on June 25th was 76F, which surpassed the previous record high minimum for the date of 71F set in 1909 and matched the second warmest daily minimum temperature ever recorded in June that was observed on the previous day.

BHO Highest June Daily Maximum Temperature, deg F (1885-2025):

1) 99 on June 4, 1919
   99 on June 24, 2025
3) 98 on June 3, 1919
   98 on June 10, 2008
5) 96 on June 26, 1952
6) 95 on June 6, 1925
   95 on June 5, 1930
   95 on June 28, 1991
   95 on June 29, 2021

BHO Highest June Daily Minimum Temperature, deg F (1885-2025):

1) 78 on June 6, 1925
2) 76 on June 24, 2025
   76 on June 25, 2025
4) 75 on June 4, 1919
5) 74 on June 2, 1918
   74 on June 5, 1925
   74 on June 21, 1923
   74 on June 21, 2012

Mike Iacono
Chief Scientist
Blue Hill Observatory