Let’s start by coming to an agreement: seasons, as we know them, are complete fabrications. They’re an attempt by our brains to neatly package broad, variable trends in weather conditions. Any attempt to define distinct boundaries is a futile effort.
So now that we agree that this is all a waste of time, let me tell you why the way we traditionally demarcate seasons is wrong. Traditionally, seasons are based on the length of the day. As you learned in elementary school science class, the earth’s axis is tilted with respect to the plane of our orbit. The days get shorter and colder when we are titled away from the sun (in the northern hemisphere, that happens to be when we’re closer to the sun).
The shortest day of the year is the day of the winter solstice (this year, the winter solstice occurs at 0448 UTC on December 22, or just before midnight on the 21st in the Eastern time zone). For the next six months, the days get progressively longer. In June, the summer solstice occurs and the days start getting shorter again.
The winter solstice in particular has long been an event of spiritual meaning. Ancient cultures celebrated the return of the sun. Many Christmas traditions (and perhaps the date itself) come from Roman and other pagan celebrations.
“But what does this have to do with weather?” you ask. Absolutely nothing, which is my point. The change in insolation (incoming solar radiation) is the driving force behind the weather seasons we experience, but it’s well out of phase with the astronomical boundaries. The shortest day may occur in December, but the coldest days tend to be in January or February. Similarly, the hottest days are in late July and August, even though the longest day is in June.
Since the late eighteenth century, meteorologists have used the first of December as the start of winter (the first of March is the beginning of spring, the first of June is the beginning of summer, and the first of September is the beginning of fall). This is considerably more convenient for calculations and the fuzziness doesn’t matter because seasons are a lie anyway.
Over the years, I’ve come to favor the meteorological definition of the seasons more and more. At least it’s based on the weather. I’ll leave the solstices and equinoxes to the astronomers.