Why Winter Starts For Meteorologists Weeks Earlier Than What's on Your Calendar

The date on your calendar when winter begins is referred to as astronomical winter, and it can vary each year between Dec. 21 and 22. On rare occasions, it can be as early as Dec. 20 or as late as Dec. 23.

Astronomical seasons are based upon the position of the Earth relative to the sun.

The winter solstice occurs when the Northern Hemisphere is tilted farthest away from the sun. In the winter, the sun rises in the southeast and sets in the southwest, a much shorter path across the sky, which is why the length of daylight is significantly shorter, usually only 8 to 9 hours.

The equinoxes occur between the solstices, with the autumnal equinox (fall) happening Sept. 22 or 23 and the vernal equinox (spring) occurring March 20 or 21. The sun rises due east and sets due west at the time of the equinoxes, which gives an equal amount of day and night (12 hours of each).

Meteorologists and climatologists, however, break down the seasons a little differently for forecasting and tracking historical weather records.

Meteorological spring runs from March 1 to May 31, meteorological summer runs from June 1 to Aug. 31, meteorological fall from Sept. 1 to Nov. 30 and meteorological winter from Dec. 1 to Feb. 28.

The meteorological seasons are always 90 to 92 days, depending on whether or not it's a leap year. This breakdown makes it much easier to calculate seasonal statistics since every season starts on the first of the month, rather than the 20th to 22nd with the astronomical seasons.

Winter's meteorological and climatological definition of Dec. 1 to Feb. 28 also more closely aligns with the coldest temperatures of the year for much of North America when compared to astronomical winter, as illustrated by the tweet below from climatologist Dr. Brian Brettschneider.

Brettschneider also analyzed the average coldest 90-day periods for dozens of U.S. cities and found that only Honolulu, Hawaii, had its coolest 90-day stretch extend deep into March.

The remainder of the 62 cities in the analysis clearly showed their coldest temperatures occurring generally from December, or even late November, into February or early March.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.