SPRING. The vernal equinox which takes place this Wednesday, March 20, 2024, marks the start of spring. Will the weather be good? We take stock of the weather that awaits us in the coming weeks.

[Updated March 20, 2024 at 12:45 p.m.] The spring equinox marks the beginning of milder temperatures, the first buds, the first rays of sunlight, the greening of the trees and longer days in the Northern Hemisphere and, conversely, the start of low temperatures in the southern hemisphere. The start of spring is generally marked by milder temperatures “often over 15 degrees” in the north of France.

We take stock of the major weather trends that await us this spring. Explanations around the phenomenon, relationship with spring, time and date… Here are all the keys to understanding the equinox.

After a start to the month marked by heavy rain and gray weather, the month of March saw temperatures above normal and the return of the sun since the equinox. But let’s not get too excited: spring weather can sometimes be more unpredictable. A cold snap, with up to minus 10 degrees in certain departments, is announced this weekend by Météo France:

“Minimum temperatures are on average 5°C and maximums 12°C” during the month of March, announces the Weather channel.

The disturbances of March will not ease in April with a difficult return of the anticyclone in a lasting manner. This unstable weather will also be present in the southern half of the country. “We must expect 23% of the time to be threatened by generally heavy rain. For this month of April, minimum temperatures are on average 8°C and maximums 16°C,” announces the Weather channel.

The month of May should be marked by 18 sunny days but expect 29% of the time to be threatened by strong disturbances. For this month of May, “minimum temperatures are on average 11°C and maximums 19°C,” announces the Weather channel.

Although weather trends are difficult to predict more than three months out, the start of summer 2024 looks dry with only an estimate of 3 to 8 rainy days on average in France in June.

Astronomically speaking, spring begins at the time of the vernal equinox, which can occur between March 19 and 21. It lasts until the summer solstice. Which means that spring lasts about three months. The date of the equinox is calculated for each year by astronomers and mathematicians. This involves predicting the exact moment when the plane of the equator and that of the Earth’s trajectory coincide. A calculation made necessary by the discrepancy between our calendar, our time system and the movements of the stars. First, the Earth’s orbit is not perfectly circular, which means that depending on its position in this orbit, the Earth can be more or less close to the sun (between 147 million kilometers minimum and 152 million kilometers maximum). This inevitably makes the duration of each season very irregular and therefore the date of spring variable.

Another explanation: It doesn’t take exactly 365 days for the Earth to go around the sun. In this respect our Gregorian calendar, established in the 16th century, is far too simplistic. It takes exactly 365.2422 days (365 days, 5 hours and 46 minutes) for us to have traveled completely around the star! We are therefore obliged to add a February 29 from time to time (during leap years) to partially (and only partially) correct this shift. A one-off addition that artificially pushes back the date of spring by one day during leap years. Which explains why astronomers “advanced” it to March 20 last year. And that the situation presents itself again this year.

When Caesar established the Julian calendar in 45 BCE, the spring equinox was set at March 25, based on imprecise observations at the time. But the absence of February 29 in this ancient calendar ended up moving this date to March 11 in the 16th century… It was not until the establishment of the Gregorian calendar (the one we use today) , in 1582, that a date approaching March 21 was chosen.

The date of “common” spring should not be confused with that of meteorological spring, which begins each year on March 1, allowing meteorologists to make their seasonal calculations on the scale of entire months. In meteorology, we consider that spring begins on March 1 and ends on May 31: in this discipline, we characterize spring as a period of warming temperatures (in our latitudes) and an increase in the duration of the day. It is also one of the most difficult seasons to analyze in this discipline. However, it is the equinox that continues to mark the advent of spring in people’s minds. This is evidenced by the demonstrations organized around March 20 in France, starting with the Printemps du cinéma.

The word equinox comes from the Latin “æquinoctium” (“equal night”) because the most visible phenomenon for man is that the length of the day becomes identical to that of the night. The reason for this phenomenon? The equinox corresponds to the time of year when the Sun crosses the equatorial plane of the earth. The star is then at the zenith of the Equator, which allows day and night to share time equally. And this in both hemispheres, southern and northern. For our part, the days are getting longer and we are halfway between the short days of December and the long days of June. During the equinox, the axis of rotation of the Earth on itself and the axis of rotation of the Earth around the Sun therefore correspond exactly. On our planet, during the equinox, our star appears right in the east at dawn only to disappear right in the west.

This phenomenon is further linked to geometry. The Earth’s axis of rotation is naturally inclined by 23.4° relative to the plane of its orbit. In other words, our planet “leans” relative to the plane on which it revolves around the Sun (see diagram below). The star therefore illuminates it differently depending on the time of the year. This phenomenon explains why the days lengthen or shorten between summer and winter. This also gives rise to the seasons, due to the heating or cooling of air masses and oceans depending on the time spent each day under the rays of the star. The distance between the sun and the Earth, however, has no direct link with temperature. Know for example that the Earth reaches the closest point to the sun (perihelion) on January 3, that is to say in the heart of our winter.

The equinox occurs twice a year: between March 19 and 21 (spring or vernal equinox) and between September 22 and 23 (autumn equinox). In spring the duration of sunshine increases at the equinox to reach 16 hours at the end of June, during the summer solstice. Conversely, the autumnal equinox begins a period of daylight reduction that drops to just over 8 hours at the winter solstice, around December 21. During the equinox, however, there is no jealousy: day and night are supposed to last 12 hours sharp each. These data nevertheless vary slightly since the shape of the Earth is not perfectly regular and the atmosphere slightly deflects the sun’s rays. So, in Paris, on March 20, last year, the sun rose at 6:52 a.m. and set at 7:03 p.m. The length of the day was therefore very slightly longer than 12 hours. It is also at the time of the equinoxes that the length of the day increases/decreases the fastest in our latitudes.

Spring equinox and pagan festival are closely linked. Thus, various celebrations of the spring equinox around March 21 existed in very ancient times, some of which still survive today. Among them are bonfires that symbolize freedom from the darkness of winter; or even cakes offered to a deity; when it is not a straw mannequin that we burn or put in water, as if to “destroy” winter.