It is almost certain to get hotter. But for now, with five months of 2013 ended, no day all year has been hotter than Friday.

It was 91 degrees at Reagan National Airport. The mercury had reached that level once before, on April 10. But Friday took Washington’s thermal title on an unofficial tiebreaker. Its average temperature at the official measuring spot was 81, four degrees above April 10’s.

For weeks, April 10 remained the year’s only day of 90 degrees or above. Then came Wednesday’s 90, and Thursday’s. Friday was this week’s third day of 90 or above, the fourth this year, and the third in May.

With three successive 90-degree days, the weather met some standards for a heat wave, Washington’s first, and thus longest, of the year. So far.