August finished in Washington on Friday with a flourish, as the temperature reached at least 90 degrees for the 50th time this year, and a month that started with a full moon also ended with one.

It is uncommon for two full moons to appear in a single calendar month. It occurs about once in 2.7 years, and the second full moon has become known as a blue moon.

The relative rarity of the event is widely believed to be the origin of the expression “once in a blue moon,” which suggests unlikelihood.

Much about Friday’s temperature was unlikely. The mercury at Reagan National Airport reached 97 , a relatively uncommon figure that is 13 degrees above normal for the date and only 3 degrees shy of the record.

In addition, as The Washington Post’s Capital Weather Gang noted, Friday was the 28th day this year with a temperature 95 degrees or above, which was uncommon enough to tie a record.

Friday also raised August’s average temperature, as measured at National, to about 81, a figure seldom reached since records began in 1871.

Only three times has Washington’s average temperature in August been above 81.