By now it has grown clear to most of us that summer, with its many days of great heat, is about to end. But if a celestial sign was required, on Saturday in Washington it may have been the hour and minute at which the sun rose.

That moment was, as listed by the National Weather Service, 6:30 a.m.

Saturday morning was the last time this summer that the sun would rise here so early. On Sunday, the Weather Service said, sunrise would occur at 6:31 a.m.

That early sunrise, almost early enough on some weekends to greet homebound revelers, represents one of the astronomical features of summertime. But of course it would scarcely be summertime without the meteorological features, such as those 90-degree temperatures.

So far in 2019, we have had 51 days of 90 degrees or above. But Saturday was nothing like that. (Nor, for that matter, was Friday). In Washington, the high reading on Saturday as recorded at Reagan National Airport was 81 degrees, five below normal.

Yes, we have had cooler readings this summer, including, in fact, Friday’s 79. In fact, in July, that broiling month of searing heat, there came a day (July 23) when the mercury reached only 77. It seemed hard to believe, a sign of nature’s capriciousness, perhaps.

But Saturday’s 81-degree reading, so late in August, inevitably suggests not a meteorological oddity, but rather an obvious sign that the seasons are changing.

“The cool air came as a surprise,” said April Brown. “It’s a nice treat,” she said. “With sleeves. of course.”

Another vivid terrestrial sign of that change could be seen in the streets of such neighborhoods as Foggy Bottom. Classes for the 2019-2020 academic year at George Washington University begin on Monday.

Students and sometimes their family members moved carts of belongings through the streets. From them lamps projected at rakish angles, their bases embedded in the other appurtenances and items needed to convert dormitory rooms into homes-away-from home.

In College Park, the first day of classes at the University of Maryland is also Monday.

Georgetown University lags by only two days, with classes set to start on Wednesday.

Public school systems have also been preparing to welcome students. The first day of school in the Fairfax County system is on Monday. Suddenly, in one of the most salient signs of the end of summer, the streets and roads of the large Washington suburb will fill with the familiar yellow of school buses.

On Twitter someone expressed the mixture of feelings experienced by many on a day like Saturday in particular. The tweet read: “I’m many years removed from school and yet I still get the end of summer blues mixed with feelings of being overwhelmed this time of August. Is this PTSD or just sadness that summer is over and life returns to normal?”