Why meteorologists start spring earlier than astronomers

The days are getting longer, temperatures are rising and the first plants are blooming: spring has arrived. Or is it still winter? There is no agreement about this. A few facts for anyone who has lost perspective.

The beginning of spring is a matter of perspective: for astronomers, spring (usually) starts on March 20, for meteorologists it has been spring since March 1. And for biologists it is not only different every year, but also different per location.

No, not for a few years. Astronomically speaking, spring begins with a day and night equinox – exactly when the sun is vertically above the equator. This year the astronomical start of spring is on March 20 at 4:06 am.

In the Central European time zone, the astronomical beginning of spring usually falls on March 20. But that wasn’t always the case – and it won’t always be that way. The time depends, among other things, on the distance to the last leap year and the time zone. The astronomical beginning of spring could also be on March 19 or 21 instead of March 20.

Until 1916, spring always started on March 21. From the 1980s onwards, March 21st occurred only once every four years. The last time the astronomical beginning of spring fell on March 21 was in 2011; the next time this will not be the case until 2102. Because in the year 2100 a leap year will be skipped due to a special leap year rule, which shifts the astronomical start of spring on the calendar to March 21.

Until then, spring increasingly starts on March 19. Next time in 2048.

Out of pure pragmatism. March 1 was declared the meteorological start of spring by the World Meteorological Organization (WMO). This keeps statistics on climate changes. For comparisons and statistics it is useful if all four seasons last the same length. That is why the WMO has set all seasons to exactly three months. The start of a season is at the beginning of each month.

The biologists did not show as much pragmatism as the WMO in defining the seasons. They define them by the stage of development of the plants, the so-called phenology. The phenological calendar has ten seasons that are not tied to fixed dates. The start of a new season is determined by the flowers, fruit ripening or leaf color of certain plant species. So it depends on the weather, but also on the location and altitude.

The phenological calendar has three phases for spring: early spring, first spring and full spring. The first is heralded when the snowdrops, crocuses and primroses bloom. The following first spring is characterized by the flowering of violets, wood anemones and forsythia. Full spring is characterized by the flowering of apple trees and lilacs and ends with the flowering of raspberries. (cst/sda)