When does winter start? Is it the first day that snow falls from the sky? The first day when you can no longer avoid digging your puffer jacket out from storage? The last school day before Christmas vacation?
The question is far from settled, but don’t tell that to an astronomer or a meteorologist. They’ll each tell you firmly that they know the official truth — and they will disagree.
Astronomers concern themselves with the relative positions of celestial bodies, and they like to define seasons based on the Earth’s trajectory around the sun. The Earth is tilted by 23.5 degrees from its rotational axis relative to its path around the sun, and this is what allows for seasonal change in the first place, so the logic of defining seasons relative to this is sound.
Ask an astronomer, and they’ll tell you that winter begins on the day of the winter solstice — the moment when the sun’s light is as scant as it will be all year. In the northern hemisphere, that moment will occur on December 21, 2016 at 5:44 a.m. EST. Of course, in the southern hemisphere, this same moment is the summer solstice, marking the longest days of sunlight.
Meteorologists aren’t exactly into this definition. It’s not that they disagree on the particulars of Earth’s rotation around the sun, and the light regime that results from it, it’s just that they’re more focused on our imperfect Gregorian calendar than our planet’s position in space. In the astrological method, annual solstices and equinoxes do not always fall on the same calendar date. Rather than having winter begin on December 21 some years, and December 22 others, weather-people plainly divide 12 calendar months by four and define the winter season as comprising all of December, January, and February.
But there’s something missing from both of these definitions, and that’s local context. Your personal feeling of the start of winter has to do with the place where you live, and there’s no good reason to prioritize global convention over the regional experience, unless you’re collecting weather statistics or tracking celestial movement.
How you might define a local winter is subjective and variable, but you might start with the idea that “the dead of winter” might be at its center. When is the dead of winter? Perhaps it is the shortest day of the year — the winter solstice. By this definition, winter would begin about a month and a half before solstice, about a week into November.
But for most people and most places, the notion of winter has more to do with temperature than light, and the coldest time of year tends to come after the solstice. The National Oceanic and Atmospheric Administration has helpfully mapped the average coldest day of the year across the United States, and it’s striking in its variation. Some parts of the west see their coldest day by mid-December, whereas spots close to the Great Lakes as well as high-elevation spots further west don’t see their “dead of winter” until mid-February. A local winter defined by temperature regime in the United States could therefore begin as early as the first of November or as late as the first of January.
In the end, defining seasons is less important than how we interact with them. Gardeners must plant according to temperature and light, for the crop of winter kale cares little about the calendar date.
If you put the official definition of winter to a deathmatch between astronomers and meteorologists, though, smart money would be on the skywatchers. Calendar dates exist to the extent that we observe and adhere to them. There will be no December 1st after humans meet their inevitable end, but solstices and equinoxes will continue for as long as Earth continues its journey around the sun, and long after that on planets and solar systems and galaxies beyond our own.
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