Sometimes the weather can be confusing. You get a cold week at the beginning of December, followed by a warm one that melts any snow that has fallen.
But did you know that people can’t agree on when winter starts?
One group says it begins on December 1 and ends on the last day of February.
Meteorologists use fixed three-month periods for each season because it makes year-on-year comparisons easier.
Others, however, claim winter starts on December 21, a time known as the winter solstice, which is the shortest day of the year. If you follow this line of thought, the season then runs until the vernal, or spring, equinox, which in 2026 falls on Friday, March 20.
The natural rotation of Earth around the sun forms the basis of the astronomical calendar, in which seasons are defined by two solstices and two equinoxes. Earth’s tilt and the sun’s alignment over the equator determine both the solstices and the equinoxes.
The astronomical calendar is how humans have organized their lives and civilizations for thousands of years. Long before the invention of the mechanical clock, our ancestors looked to the skies to understand their time and place on the planet.
By observing the sun’s changing path, people could predict when harsh weather was due to arrive, when it was a good time to hunt certain animals, which foods were in season, and so on.
Because Earth travels around the sun in 365.24 days, an extra day is needed every four years, creating what we call a leap year.
This also causes the exact dates of the solstices and equinoxes to vary from year to year. Additionally, the elliptical shape of Earth’s orbit around the sun means the lengths of the astronomical seasons can vary, lasting anywhere from about 89 to 93 days.
To make it easier to consistently compare climatological statistics for a particular season from one year to the next, meteorological seasons were born.
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We all have a sense of what each season is like. The cold and dark of winter ends in spring, a time of colorful rebirth. Temperatures climb through summer, the brightest and hottest time of year, before giving way to autumn, with its crisp air and falling leaves. Then the cycle repeats — or at least it’s supposed to.
Last year, scientists published a study arguing that human-driven climate disruption is altering Earth’s annual rhythms so significantly that the characteristics and everyday experience of the seasons are becoming increasingly disconnected from their traditional definitions.
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To make sense of these emerging patterns, the researchers propose changing the way we look at the calendar and outline a few new concepts that they believe have recently grown in prominence:
- Emergent seasons – entirely new seasonal patterns that didn’t previously exist in a given region.
- Extinct seasons – traditional seasons that have effectively disappeared or become unrecognizable.
- Arrhythmic seasons – disruptions to the expected timing and duration of seasonal cycles.
- Syncopated seasons – irregular fluctuations in the intensity or character of seasons.
