June 20, 2012 -- Hot enough for you? With a fearsome heat wave moving from the Midwest to the Eastern Seaboard, we hardly need a reminder that summer is upon us.
But we have one anyhow. The summer solstice -- the astronomical beginning of summer in the Northern Hemisphere -- takes place at 7:09 p.m. ET on June 20. That makes this the longest day of the year north of the equator. From now until December, the days gradually get shorter, though not immediately cooler.
We are already told the U.S. has had the warmest spring since record-keeping began in the 19th century. Today there are heat warnings for 13 states, with highs in the upper 90s in New York, Boston and Washington, D.C., and heat indexes higher than 100 for cities that include Philadelphia and Raleigh.
"You're talking about almost 15 degrees above normal," said Kristin Kline, a National Weather Service meteorologist in Mount Holly, N.J.
It is mere coincidence that this is happening on the day of the solstice. Generally, the sun's heat, trapped by the atmosphere, has a lagging effect, which is why August in the U.S. is usually hotter than April, even though the days are the same length.
A quick reminder of what's happening: Earth, turning on its axis as it circles the sun, is tilted at an angle of 23.5 degrees relative to its orbit. Whatever the season, the axis points the same way, with Polaris, the North Star, hovering over the North Pole.
This is the day that the axis, as seen from the north, points as much toward the sun as it will all year, and appears at its highest in northern skies. So Chicago and New York, for instance, get more than 15 hours of sunlight today, compared with 9.1 hours on the winter solstice Dec. 21. And everything north of the Arctic Circle will get 24 hours of daylight today -- compared with round-the-clock darkness six months from now.
Public health officials tried to remind people, as always on such days, to stay in air-conditioned buildings if possible, drink plenty of water and avoid exertion, as ozone builds up in the air. New York City's schools remained open for 1.1 million students, though only 64 percent of their classrooms are reported to have air conditioning.
Utilities and transit systems are also under stress. With demand peaking, equipment is more likely to break down in the heat.
"While extreme temperatures can affect our equipment and infrastructure, we will do everything possible to avoid service disruptions," said Joseph Lhota, CEO of New York's Metropolitan Transportation Authority.
Forecasters say the current heat wave will break by the weekend. There's a cold front moving eastward, currently stretching from Michigan to the central plains. Beyond that, you can at least take comfort that with summer here, fall can't be far behind.
The Associated Press contributed to this story. Additional information from ABC's Max Golembo.