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Weather Almanac for Summer Solstice
A MID-SUMMER DREAM
Earth Date: June 21: Location: 23 degrees, 27 minutes, North, the Tropic of Cancer. Today is the Summer Solstice, the longest daylight duration of the year and, of course, the shortest night (if there is one at all) over the Northern Hemisphere.
For many, this date signifies the official start of the summer season, although some cultures -- more attuned to the sun's celestial position -- recognize this date as midsummer, the central date in that quarter of the year with the most potential sunlight.
The word solstice derives from the Latin and literally means sun standing still. (Sol meaning sun and sistere meaning to stand still.) "How can the sun stand still?" you ask. Well, it doesn't actually stand still in the sky, it continues its unrelenting daily march westward across the sky, but for a moment, its north/south movement is halted. On this day, the annual swing of the sun's position reaches its northernmost point and, like a pendulum at the apex of its swing, halts momentarily and then reverses direction.
If you have been watching the elevation of the noonday sun increase daily since the equinox (actually since the winter solstice last December) or the northward advance of the points on the horizon of sunrise and sunset, you will have noted the change in position slowing over the past few days (in sharp contrast to the rapid changes occurring around the equinox). In the days following the solstice, the position of the noon elevation will be a little lower, and the positions of the sunrise and sunset a little further south. But during these few days surrounding the solstice, the changes may be almost imperceptible to the eye. This gives the impression that the sun is standing still in the sky.
You can check this for yourself if you are able to see the point on the horizon where the sun rises or sets each day. Sunrise reaches its furthest northeast location on the day of the solstice, moving further and further south each day thereafter until at the winter solstice when the drift stops and the solar trip back north begins. (Or similarly, you can watch the western sunset location.) By watching the sunrise/sunset locations change over the next six months or more, you will also notice that the day-to-day changes are greatest around the equinoxes and become very slow around the two solstices -- someone said a better name might be the "stallstice."
Sometime today, the sun will be directly over the Tropic of Cancer. This is as far north as you can be on the Earth and see the sun directly overhead at noon local solar time. For Northern Hemisphere latitudes between the Tropic of Cancer and the Equator, the sun is directly overhead at noon twice per year, once before the solstice as the sun begins moving northward of the equator after the Spring Equinox, and again as it moves southward from the Tropic of Cancer after the solstice. (This movement is mirrored in the Southern Hemisphere between the September and March equinoxes with an approximate six-month time lag.)
Only Hawaii of the 50 US States is located south of the Tropic of Cancer although several American territories, commonwealths and protectorates are also located in the tropics. The closest continental US city to the Tropic of Cancer is Key West, Florida at one-degree north.
Most of the continental United States, all of Canada, Alaska and Europe and much of Asia benefit during the summer months from substantially longer day lengths, countering those long winter nights. The further north one goes, the longer the arc of solar travel between sunrise and sunset, and thus the longer the day length. And for regions north of the Arctic Circle (at a latitude of 66.5 degrees), there will be at least one 24-hour "day" during which the sun will not set as the solar arc lengthens to a complete circle.
At the latitude of the Arctic Circle, solstice day brings the only 24-hour "day" of total daylight. But as we move north toward the Pole, the number of 24-hour periods between sunrise and sunset increases. The ultimate long-day occurs at the North Pole where the sun rises with the Spring Equinox and finally sets again 189 "days" later with the Autumnal Equinox.
[Because the Earth's orbit is an ellipse rather than a circle and orbital speed varies along the path, the seasons are not exactly one-quarter year (91.31 days) long. At the North Pole, daylight lasts 189 "days" and the night 179 "days" -- the time between the Spring and Autumnal Equinoxes. For the South Pole the lengths of daylight and night are reversed. Note that Summer, that astronomical period between the Summer Solstice and Autumnal Equinox, in the Northern Hemisphere is approximately 93 days 15 hours, the longest of the four northern seasons.]
Actually, areas slightly south of the Arctic Circle may experience a midnight sun. The latitude where the sun will just dip below the horizon has been calculated assuming no atmospheric influences on the solar light rays, but the bending of the sun's rays by the air as they traverse the atmosphere can make the solar disk appear to be at or above the horizon when it is actually positioned just below it.
Even at those northern latitudes that do not experience a midnight sun, the nights can be quite bright -- a continual twilight lasting all night -- because the sun does not sink far below the horizon during midsummer nights. The solar path in these regions resembles a swimmer's shallow dive below the horizon more than a diver's plunge into deep darkness, which is characteristic of equatorial sunsets.
The region with twilight nights can be found as far as 18 degrees south of the Arctic Circle at midsummer. This region encompasses areas north of 49 degrees North which includes all of Alaska, most of Canada (particularly west of Lake Superior) and major European cities such as London, Stockholm, Copenhagen, Berlin, Warsaw and Moscow.
Such nighttime brightness can reduce the favourable opportunities for stargazing during a season where air temperatures are generally comfortable for the activity. Similarly, the bright skies can mask faint auroral displays and may even be confused with them.
For those of us living in latitudes north of 45 degrees, it is natural to consider the Summer Solstice as the start of summer weather as well because the three warmest months of the year are usually June, July and August. The lag in air temperature behind the solar intensity peak is due to the use of much of the Spring solar energy to melt snow and ice, heat land surfaces and warm waters from their winter chill. In continental areas such as the Canadian Prairie Provinces and American Plains States, annual temperature cycle peaks from late July to early August, whereas in the Pacific Northwest, the warmest stretch of days annually occurs from late August to early September. That extra month is required to heat the cold Pacific waters offshore.
The Summer Solstice has long held special significance to many cultures, particularly for nature- and sun-based religions and in agrarian communities. To them, Midsummer, as the day is called, held special magic.
For example, it is believed that on this day, if a young girl gathered yarrow flowers from the grave of a young man and placed them under the pillow of the man she desired, then during the night he would dream her dreams. Sharing their dreams presumably would initiate a courtship between the two. This belief was the basis for William Shakespeare's classic play: A Midsummer Night's Dream.
The joy of the midsummer night, of course, is tempered by the realization that tomorrow the steady decline in day length would begin, culminating with the darkest day at the Winter Solstice. But for now, life is easy, perhaps carefree, a time for celebrations.
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