Most people consider the first day of winter to be the Winter Solstice, which this year occurs December 21 at 4:48 p.m. EST.

Winter begins then because, astronomically-speaking, the sun will be directly overhead of the Tropic of Capricorn - the southern-most point the sun will be directly overhead during the year.

As the earth orbits around the sun, at different times of the year the sun will be situated directly overhead at midday. The days the sun is straight up at noon over the Tropic of Cancer and Capricorn (23.5 degrees N and S latitude, respectively) are the summer and winter solstices, respectively. The Vernal (Spring) Equinox is the day the sun is again highest in the sky at noon over the equator as the apex progresses northward.

Many refer to Astronomical Winter generally as December 21 or 22, as the first "official" day of winter. Most meteorologists will argue that winter has been well under way by then, and they have been enjoying at least three weeks-- if not longer-- of "winter" already.

Meteorologists, however, observe seasons over different time periods. Meteorological winter begins December 1, spring begins March 1, summer begins June 1 and fall begins September 1.

There are a couple of very important reasons why this is the case. The most important is for climate record-keeping. Climatologists require set time periods to calculate averages and do seasonal comparisons over the years. Astronomical dates will fall on different days depending on the year and keeping seasonal climate records based on those dates would be confusing and inaccurate.

A second reason is that weather-wise, it makes more sense around the globe. For example, much of the northern hemisphere is entrenched in winter weather by the first day in December. Cold winds blow, snowstorms are crossing the country, and most areas north of the mid-latitudes have experienced their first freezes. December 21 is the first day of astronomical winter this year, but most regions usually have nearly a month of winter weather by the time it "officially" begins.

In spring, mild surges of air from the south are becoming a regular occurrence and severe weather threats begin to kick in by March 1. The heat of summer has been experienced in most areas across the country by June 1 and the heat of summer is waning by September 1.
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Photo: Snow on a street in Park Slope, Brooklyn, N.Y. (Anna Kucsma, Wikimedia Commons)

When Does Winter Begin?