Summer arrives at 7:59 this evening. (Misled by a typo on a reference book we use, we mistakenly said on the print weather page last week that the solstice arrived on the 19th. Just a reminder that humans produce this stuff.)

We have visitors from Sweden with us this week. For them, tonight is the night to celebrate Midsummer's Night and dance around a pole with candles in their hair. Or something. That's MID-summer's Night. Not BEGIN-summer's Night.

They conceded to us that they, too have been confused about why summer BEGINS on the date called MID summer, when the sun is in the sky longer than any other. And where they're from, it never does get truly dark at night at this time of year. 

We were visiting them one summer many years ago in Sweden, and we all went to dinner at the home of some friends. We drank. We ate. We talked alot. And each time I looked out the window it was dimmer, but still light. My wife and I, accustomed to summers living around a latitude of 40 degrees or so, figured it was, maybe, 9 p.m.

When we finally looked at the clock we were astonished to discover it was past midnight! We were dining at around 57 or 58 degrees north latitude, the equivalent of Churchill, Manitoba, on Hudson's Bay. The sun was down - we weren't above the Arctic Circle. But it was still twilight. I could have read a book without a flashlight.

Anyway, the idea that summer BEGINS around June 20 or 21 is a recent notion. Many of our ancestors did indeed see this as MID-summer. They used "cross-quarter days" - the midway points between the equinoxes and the solstices - to mark the beginning and end of the seasons. For example, by the Celts' reckoning, our summer began somewhere between the 4th and 10th of May, on a day they called Beltane. And it will end between the 3rd and 10th of August, on Lughnasadh.

Some also find it curious that the hottest days in Maryland - the warmest long-term daily average temperatures - are in mid-July, a month after the solstice. If the days are longest, and the sun is highest, and its rays most direct and intense around the 20th of June, why does this month not also produce our highest average temperatures?

The answer lies in the oceans. Literally. It just takes a few weeks for the increased solar energy falling on the Northern Hemisphere to heat the oceans to their maximum seasonal highs. That, in turn, delays heating of the atmosphere. It's like heating a pot of water on the stove. It takes a while on HI to get the water to boil. By the time temps reach their peak, the incoming solar energy is waning, and by late July, the oceans and air finally begin to cool again.

Anyway, enjoy the long day today. From here on until Dec. 21, the days only get shorter. And don't forget to get a look at the solstice moon tonight. It rises at 10:24 p.m. in Baltimore. It's just past full, but still a gorgeous sight in the east.