It can get hot in Washington in the summer time, with lots of 90=degree days. Monday was the first 90-degree day here this year.- (Linda Davidson/The Washington Post)

Certain days of both cultural and meteorological significance, come but once a year, and in Washington, Monday was one of them. It was the first day of 2019 on which the temperature rose to 90 degrees.

That 90-degree reading was made at 3:01 p.m. at Reagan National Airport, where the National Weather Service makes the official readings for the nation’s capital.

It apparently stayed at 90 for about an hour, lest it be thought that this was a momentary phenomenon, a mere quirk of nature, rather than a plausible candidate for the day that started summer.

The reading, especially one that comes so late in the spring, carries cultural significance. It enables old Washington hands, veterans of many a torrid season, to smile knowingly.

Washington traditionally fills with newcomers around this time, and the reading enables veterans of city summers to assure the new arrivals that they haven’t seen anything yet.

Monday, according to the Capital Weather Gang, is close to the average time for the appearance of this harbinger, this portent, this foretaste of the days that are to come.

Washington can be a heat-and-humidity capital, with many 90-degree days and worse. Monday was the first 90-degree day here this year. (Linda Davidson/THE WASHINGTON POST)

On average, the CWG says, the first 90-degree day makes its appearance in Washington on May 17. Monday was the 20th, so it seems unlikely that anyone would quibble about whether the was any sort of freakish event.

For many, summer starts on the day of the solstice, June 21. But given that spring starts in March with the equinox, we are, it must be conceded, about one-third of the way there.

Others consider the true start of summer, the cultural start, to come on Memorial Day weekend.

Memorial Day, it may be noted, falls only a week from Monday.

In addition, signs of summer also show up on the campuses of schools and colleges.

Many institutions of higher education have held their commencement exercises.

It is time for students to pack up and go off for the summer.

On Monday, in Foggy Bottom, the home of George Washington University, parents helped their offspring load up cars and SUVs and pickup trucks.

Elsewhere in the District, in another indicator of the arrival of summer, it was time for the start of summer classes.

At Howard University, summer session No. 1 began Monday.

At American University, it was time for the start of the six-week summer session.

At George Washington, summer classes also began. A course on Democracy in Film, Literature and Museums was scheduled for its first meeting.

On the reading list was The Handmaid’s Tale.

The National Highway Traffic Safety Administration tweeted out summer messages.

It reminded motorists that safe cars save summers.