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As everyone knows, during autumn the length of the day gradually decreases, reaching its minimum at the time of the solstice. This day, which usually falls on December 21, marks the onset of winter in many cultures in the Northern Hemisphere. The opposite happens after six months: the longest day is usually June 21, the day of the summer solstice in the northern hemisphere.
Astrological winter
The concept of a solar solstice is based on A. Astrological definition of the seasons. This depends on two characteristics of our planet: its inclination of about 23 degrees with respect to the sun and its orbit around it. This tilt, associated with the position of the Earth in its orbit around the sun, determines what will be the alignment of the sun above the equator, which in turn determines the date of the solstices (June 21, December 21) and the equinox (March 21, September 21).
For example, the winter solstice occurs when the sun is at its lowest in the sky. The tilt of the Earth also causes the northern hemisphere to get less energy from the sun. All this causes shorter, cooler days. In 2020, in MontrealThis will happen on December 21 at 5:02 AM.
However, the date of the winter solstice is not always the same. Solstice occurred on December 22, 2011.
Over time, we can even notice the discrepancy between the ephemeris and the civil calendar. In fact, it takes just over a year for the Earth to orbit the sun: 365.42 days to be exact. To prevent the gap from building up, our calendar provides an additional day (February 29) every four years.
Additionally, the Earth’s slope is not completely constant. Over a period of 26,000 years, the axis actually moves a few degrees. It’s little, but it’s enough for Over the centuries, The date of the winter solstice changes. For example, in the Roman era, it was held at the end of November.
Meteorological winter
These differences annoy meteorologists who, for their statistics comparing temperatures from year to year, want a specific date. So they proposed a new definition of seasons based on the annual temperature cycle and the civil calendar. Thus, the “meteorological winter” corresponds to the three coldest months of the year, that is, December, January and February. In other words, for meteorologists, we’ve been in the winter since December 1st.
It should be noted that the months when the Northern Hemisphere is less exposed to the sun’s heat are rather November, December and January. This transformation It is explained by the fact that the Earth and the water bodies of the planet have stored solar energy during the previous months.
Climatic winter
We could finally end up with a third category: climatic winter. This is because both the meteorological winter and the astronomical winter pose a problem for researchers studying climate change.
This is how some climate scientists do it and it has been suggested The definition of winter is based on “breaking points,” which may vary depending on where you are on the planet. for example, Which is the moment when the temperature drops below freezing point and causes snow to accumulate. Or the beginning of the monsoon season in India (June). This would make it possible, according to proponents of this idea, to document the evolution of climate change spillovers over time – particularly along the length and intensity of winter. It can also be beneficial to some sectors of the economy that have to plan their activities for the winter. For example, the city cannot use the meteorological definition of winter only if snow starts early in November – or, conversely, if the outdoor skating rinks are forced to open later and later in the season.
But this third definition has not yet taken root: at present, we find a lot of literature where “climatic winter” and “meteorological winter” are synonymous.
Rule
Celestial mechanics adequately explain the changes of the seasons, but the temperature stats are more in line with what residents of cold and snowy regions experience.