By the calendar, the First Day of Winter in the northern hemisphere for 2024 is on the 21st of December, during the Winter Solstice. However, for meteorologists and climatologists, the 1st of December marks the start of the season. Here's the story behind Meteorological Winter.
In addition to days, weeks, and months, our calendar is also broken down into four seasons. North of the equator, we observe spring from March to June, summer from June to September, fall from September to December, and 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.
WINTER 2025: El Niño is a distant memory; will Canada feel winter's wrath?
The timing of these seasons isn't arbitrary, though. Long ago, our ancestors noticed a pattern in how the Sun travels across our skies. While it crosses from east to west each day, the exact path the Sun follows from horizon to horizon changes on a day-to-day basis.
In the northern hemisphere, for each day from late December through late June, the Sun's path is slightly higher in the sky than the day before. Within a day or so of the 20th of June, this stops and the Sun reaches its maximum height in the sky for the year. Then, the pattern reverses, and the Sun's path gets lower each day for the rest of the year, reaching a minimum around the 21st of December.
This day-by-day change is due to us observing the Sun's position from the surface of a tilted planet. As Earth orbits the Sun, it maintains a roughly 23.4 degree axial tilt, putting the bright star Polaris right above our North Pole in the sky.
Since it is defined by the Sun, this cycle describes our 'astronomical' seasons.
This 'solargraph' image captures the Sun's path across the sky, day by day, between June 21 and December 21, 2023, from atop Weather Network Headquarters. Credit: Bret Culp
Spring and Fall start at the Equinoxes. These moments in time occur when an observer standing on Earth's equator sees the Sun directly overhead at noon, lighting both the northern and southern hemispheres equally. The exact season beginning at that time depends on which hemisphere you are in and which direction the Sun is "crossing" the equator. For example, in March, the Sun appears to cross the equator travelling towards the north, so that is the beginning of spring for the northern hemisphere and fall for the southern hemisphere.
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. Again, which hemisphere you are in is the deciding factor for which season is beginning at that time. In late June, the Sun reaches its maximum height above the northern hemisphere, marking the start of northern Summer. At that same time, it reaches its minimum height in the sky for the southern hemisphere, denoting the start of southern Winter. In late December, it's the reverse: the Sun is at its maximum height in the southern hemisphere, marking the beginning of Summer there, while Winter starts in the northern hemisphere.
These satellite views of Earth show the start of the four seasons. From left to right, in the northern hemisphere, there is summer, fall, winter, and spring. Credit: NASA
RELATED: The science and celebration of the winter solstice
Using astronomical timing to define our seasons works fine. So, there's not necessarily any need to change that now.
However, the start and end dates of the seasons do not line up perfectly with our weather and climate. Also, they definitely do not mesh well with how we keep records of weather conditions throughout the year.
When keeping weather records, consistency is essential. Daily, weekly, monthly, and even yearly records can maintain this consistency, since a day is always 24 hours, a week is always 7 days, and except for on leap years, each month is always the same length and each year is always 365 days. This helps atmospheric scientists to easily make comparisons, find extremes, and track trends.
Comparing seasonal trends is important too. However, astronomical seasons are anything but consistent. Thus, they throw an added complication into the process.
Due to slight changes in the timing of Earth's orbit and rotation, the equinoxes and solstices change from year to year. This changes the exact day and time each season starts and ends. As a result, the lengths of the seasons can vary.
It's not that comparisons can't be made. These days, computers can easily tally all the weather records between the precise start and end times of any astronomical season. However, attempting to compare seasonal weather can still result in a whole host of adjustments and corrections being made to account for those changes in length.
Besides, when Societas Meteorologica Palatina established the first meteorological network in 1780, record-keeping and calculations were all done by hand -- a practice that continued until the latter half of the 20th century. So, making seasonal comparisons and calculating trends would have been far more cumbersome and time-consuming.
To better align the seasons with how meteorological records were kept, the meteorological seasons were created.
Each meteorological season is still three months long. Unlike astronomical seasons, though, they align precisely with our calendar months, and they start and end on the exact same dates every year.
Meteorological spring begins on the 1st of March, meteorological summer starts on the 1st of June, meteorological fall begins on the 1st of September, and meteorological winter starts on the 1st of December.
Meteorological seasons are useful for more than just bringing consistency to seasonal comparisons. As it turns out, they are also well-timed to capture the most 'representative' weather of each season.
This graph plots average daily temperatures between October and April for seven cities across Canada. (Data from Environment and Climate Change Canada)
As the graph above shows, while some of these Canadian cities experience much colder winters than others, nearly all follow a similar temperature trend throughout the season.
With the meteorological and astronomical seasons plotted on the graph, meteorological winter captures the coldest part of that temperature trend far better than its astronomical counterpart.