According to the calendar, the First Day of Winter in the northern hemisphere isn't until December 21, yet for meteorologists and climatologists December 1 is the start of Meteorological Winter. Here's why.
The way that we typically track the seasons is by their astronomical definitions. No matter whether you are in the northern or southern hemisphere, Spring starts on the day of the Vernal Equinox, when our tilted Earth is just in the right spot in its orbit that the Sun appears to cross the equator, towards being higher in the daily sky. In the north this is in late March, while in the south, it's in late September.
The Summer Solstice is when the Sun reaches its peak height in the sky, in late June in the north and late December in the south. The Autumnal Equinox is the exact opposite of the Vernal Equinox, as the sun appears to cross the equator, and is headed towards being lower in our daily skies, and the Winter Solstice is when the Sun reaches its lowest height in the sky and the cycle repeats.
The only part that varies with this cycle is that the exact day - the 20th, 21st, 22nd or 23rd of the appropriate month - can differ from year to year.
WATCH BELOW: THE CHANGE IN OUR NORTHERN SEASONS AS EARTH ORBITS THE SUN
While that works for us in the astronomical sense, however, it does not necessarily work with our weather and climate.
To better account for how temperatures change throughout the year, atmospheric scientists set their 'seasonal calendar' a bit differently.
Meteorological seasons are in three-month blocks, just like astronomical seasons, but they start on the 1st of the month. Meteorological Spring begins on March 1, Summer on June 1, Autumn on September 1 and Winter on December 1.
DOES IT MAKE THAT MUCH OF A DIFFERENCE?
When it comes to keeping climate records, when you start and end a season can make a big difference, especially if you want consistency (and we do!).
Astronomical seasons can last anywhere from 88 to 94 days (depending on the year and what time-zone you live in). So, rather than having to account for that difference in length when you're comparing seasonal averages, each meteorological season has the same length as the previous year's, and they're much closer in length to each other throughout the year as well. Meteorological springs and summers are 92 days long, autumns are 91 days long and winters are 90 days long (or 91 every four years, due to the Leap Year). Reverse all of that for the southern hemisphere, but they're just as regular.
Also, by setting those as the dates for meteorological seasons, it allows the records to capture the weather that's most associated with that season. For example, for winter, you really want to be recording the coldest part of the year. The northern hemisphere is generally cooling down as the calendar ticks away towards the winter solstice in December, and it's starting to warm up (overall) again as the calendar approaches the spring equinox in March.
There's a lag period, of course - called "thermal lag" - as one season transitions into another. This differs based on where you are in the world (shorter towards the poles and longer near the equator), but in mid-latitudes, it typically lasts for between 25-35 days after the end of summer, and about 20-25 days following the end of winter. So, starting the season too early would cause problems with temperature records, but starting it a few weeks before the astronomical season begins works out quite well.
AN EVEN BETTER WAY?
We've recorded seasons this way for a very long time. Astronomical seasons have been in place since the days of ancient Rome, and meteorological seasons have been tracked since the late 1700s. There's a school of thought, however, that says we might have a better way of looking at the seasons, at least in the astronomical sense.
Rather than the equinoxes and solstices marking the transition between seasons, there's been some suggestion that perhaps having those dates marking the middle of the season would be better.
Countries around the world have (and some still do) mark the seasons in this way.
Midsummer's Day and Midsummer's Eve occur around the summer solstice, and simply by their names they denote a celebration of the middle of the season, rather than its start. Seasons in Japan are traditionally based on their lunar calendar, which mark winter as starting around November 8 and ending around February 4.
This makes a certain amount of sense, just based on the astronomical calendar, since it would better reflect the number of hours of daylight as it changes throughout the year. Therefore, winter wouldn't start on the shortest day of the year, but instead, that day would mark the exact middle - or deepest part - of the winter season. Similarly, the summer solstice - the longest day of the year - would be right in the middle of summer.
This doesn't take the actual weather into account, however, so this would be purely for astronomical reasons, and it wouldn't change the meteorological seasons. Those still need to take into account the 'thermal lag' that each hemisphere experiences as it transitions from one season to another.