What to know about the summer solstice, the longest, brightest day of the year
Summer officially begins Friday, June 20, at 10:41 p.m. Eastern time. That's the precise moment of the summer solstice — making for the longest, brightest day of the year in the Northern Hemisphere.
Temperatures are toasty and the humidity is sweltering across much of the Lower 48. There's no denying it's summer.
On Friday, Washington, D.C., will enjoy 14 hours and 54 minutes of daylight. Dallas will get 14 hours and 19 minutes, and Seattle will get just shy of 16 hours.
Remember — the days will be longest farther north, with the fewest daylight hours in the Southern Hemisphere. There's more annual variation in day length toward the poles, and the most consistency at the equator.
The summer solstice has to do with Earth's tilt on its axis. Because of our 23.5-degree tilt, the amount — and intensity — of sunlight any given area receives varies throughout the year.
For half the year, the sun's most direct rays shine on the Northern Hemisphere; the other half of the year, the Southern Hemisphere.
Since our tilt is 23.5 degrees, that means the sun's direct rays fall between 23.5 degrees north and 23.5 degrees south latitude at all times throughout the year. Those lines represent the Tropics of Cancer and Capricorn. (Technically, the actual latitudes are 23.4394 degrees north and south.)
On the June solstice, the sun shines directly on the Tropic of Cancer in the Northern Hemisphere. That means that, poleward of that line, everyone in the Northern Hemisphere is experiencing their longest, brightest day. And if you stand on that line, you'll have no shadow at noon — the sun will be directly over you.
After Friday, the sun's most direct rays begin retreating southward again.
On the autumnal (fall) equinox, or Monday, Sept. 22, those most intense rays will cross into the Southern Hemisphere. Our days will get shorter and darker until the winter solstice on Dec. 21.
Even as the days get shorter, that doesn't mean it will start getting cooler. There's something called seasonal lag that prevents that from happening.
The atmosphere has a certain amount of thermal inertia, or resistance to changing temperature. Light hits the ground and is converted to heat, which is dispersed through the atmosphere. The diffusion of that heat takes a while.
The lag — or time between greatest amount of sunlight and warmest temperatures — is about a month. That's why July tends to be the hottest month for North America, Europe and Asia.
Technically speaking, every place on Earth gets about the same duration of sunlight every year. It would be exactly the same if we were a perfect sphere, but between mountains and Earth's equator, which bulges outward 27 miles, it's a bit imperfect.
What varies is the intensity of the daylight.
Take the north pole, for example. Even on the summer solstice, the sun is very low in the sky. The sunlight comes at a steep angle and is weak — which is why it's so cold at the poles.
Day length changes the most at the poles and changes the least at the equator. At the equator, every day is a little more than 12 hours long. In the Arctic and Antarctic circles, there's at least one day per year with 24 hours of sunlight, and another with 24 hours of darkness.
At the mid-latitudes, people experience a couple of hours of variation, but it's not that extreme.
In D.C., for instance, the summer solstice has 14 hours and 53 minutes of sunshine. The winter solstice has just 9 hours and 26 minutes.
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