three things you should know about autumn

If the meteorological seasons are based on atmospheric criteria and always fall on the same date (September 1 for autumn in the northern hemisphere), the calendar seasons based on astronomical criteria are somewhat more fluctuating. This year, autumn arrived on friday september 23. In 2021 it was September 22, in 2023 it will be September 23 again.

From a calendar point of view, autumn begins on the day of the autumnal equinox, when day and night are exactly the same length. The next day, the nights begin to last longer than the days until the spring equinox, from which the opposite phenomenon will begin. Meanwhile, we will have passed through the winter solstice (December 21), the date of the shortest day, while the summer solstice (June 21) corresponds to the longest day.

However, there are two flaws in this system. The first: the Earth’s axis of rotation is not perfectly circular and describes a cone every 26,000 years, with the addition of an oscillation phenomenon, which influences the duration of the seasons (between 90 and 93 days). The second imperfection concerns our calendar. The Earth makes its revolution around the Sun not in 365 days but in 365 days 5 hours 48 minutes and 45 seconds. To align with the astronomical year, our calendar provides for 365-day years and some 366-day years (leap years, like 2020). The combination of these imperfections means that the seasons do not always coincide on the same date.

A season of contrasts

Like spring, autumn is a season of contrasts: it transitions between the hot season and its summer heat and the cold season and its snowfall. This transition can be gradual or sudden depending on the year. We can live a beautiful season and we will talk about an Indian summer, but the rains can become more numerous and violent and the temperatures drop sharply.

On average, the normal temperature in France (national thermal indicator calculated from 30 metropolitan stations) is 13.1°C, according to Météo-France. This normal is rather around 14.4 °C in Bordeaux, 14.2 °C in La Rochelle, 15.4 °C in Biarritz or 13.4 °C in Dordogne. Still, since 1900, the fall of 2006 was by far the hottest, with an average temperature of 15.5°C, or 2.4 degrees above normal. It is closely followed by autumn 2014 (average 15.4 °C, +2.3 degrees). The fall of 1912 turned out to be the coldest with an average temperature of 10.1°C, or 3 degrees below normal. And among the ten warmest autumns, nine have occurred since the beginning of the 21st century (2006, 2014, 2011, 2018, 2020, 2009, 2019, 2005 and 2013).

Approximately 268 mm of precipitation falls on the Hexagon on average each autumn. Since 1959, the fall of 1960 was the wettest with more than 414 mm and 1978 the driest with less than 81 mm.

Why do trees lose their leaves?

If this year the trees shed their leaves particularly early due to the drought, to preserve their water resources, normally, who says autumn, says dead leaves scattered on the ground. In question: the cessation of photosynthesis, this complex chain reaction that transforms light energy into chemical energy, from which plants feed.

The green color of the leaves is due to chlorophyll, one of the pigments that absorb light in the process of photosynthesis, a process that allows trees to feed on the sugars contained in water, carbon dioxide and sunlight. . In autumn, with the days getting shorter, photosynthesis slows down and “chlorophylls degrade, gradually giving way to the other pigments that make up the leaves: yellow to red carotenoids, even reddish anthocyanins”, explains Alain Badoc. curator of the botanical garden of Talence.