"By the calendar, the First Day of Winter in the northern hemisphere is on December 21. However, for meteorologists and climatologists, the 1st of December marks the start of the season. Here's the story behind Meteorological Winter.

Our calendar year is broken down into four seasons. In the northern hemisphere, we observe Winter from December to March. The exact day on which each season starts and ends varies from year to year. However, it's typically somewhere around the 20th of the month, give or take a day or two. There's a specific reason for this. Long ago, our ancestors noticed patterns in the motion of the Sun across our skies. Summer and Winter begin at the Solstices. These occur at the exact moment when the Sun appears to reach its maximum and minimum height in the sky, respectively.

However, these dates do not necessarily line up with our seasonal weather and climate. Also, they definitely don't mesh well with how we keep records of weather conditions throughout the year. To align the seasons better with how meteorological records were kept, meteorological seasons were created.

Each meteorological season is still three months long. Unlike astronomical seasons, though, they start and end on the exact same dates from year to year. Also, they align precisely with our calendar months.

With the meteorological and astronomical seasons plotted on a graph, meteorological winter captures the coldest part of that temperature trend far better than its astronomical counterpart." Scott Sutherland