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Meteorological winter starts on Friday, Dec. 1, but winter as we know it begins on the solstice — this year on Dec. 21 around 7:30 p.m. in Seattle. Why the difference in when winter begins?

Astronomical Seasons

The , the shortest day of the year in the Northern Hemisphere, is based upon astronomical seasons that people have used to mark time for thousands of years. For the Southern Hemisphere, Dec. 21 is the summer solstice and longest day of the year.

Astronomical seasons are based upon the position of Earth as the planet rotates around the sun. Earth’s 23.5-degree tilt and the sun’s alignment over the equator determine both the winter and summer solstices and equinoxes. The equinoxes mark the time when the sun is centered above the equator, around March 21 for the spring equinox and around Sept. 21 for the autumnal equinox.

Meteorological Seasons

Many think of winter as the coldest time of the year and summer as the warmest, with spring and fall being the transition seasons. This is the basis for meteorological seasons. Meteorologists and climatologists break the seasons into groups of three months based on the annual temperature cycle combined with the calendar.

The coldest months of the year are December, January and February, and hence determine meteorological winter.

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In addition, the length of meteorological seasons is also more consistent by following the calendar. The calendar makes it much easier to calculate seasonal statistics from monthly weather data versus astronomical seasons — very useful for commerce, agriculture and a variety of other needs.

And speaking of the calendar, as December begins so does typical Pacific Northwest winter weather. Right on cue, rain and blustery conditions will resume in Western Washington, and snow begins piling up in the mountains, with snow levels generally below highway pass levels. So buckle up. After this period of unusually cool, dry, locally foggy, and sunny November weather, winter conditions return.

Back to astronomical versus meteorological seasons! In summary, astronomical seasons are based on the position of Earth in relation to the sun while meteorological seasons are based upon the annual temperature cycle and the calendar. And now you are ready for that trivia question!

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