Spring already?

The question posed in the blog title as to whether it is already spring can be answered in different ways. No, if you look at the calendar, the calendar or astronomical start of spring is not until March 20. For us meteorologists, on the other hand, the meteorological start of spring is always on March 1. With yes also with regard to the stage of development of the vegetation (so-called phenological spring beginning with the long since completed flowering of the hazel). Below is more information on the individual beginnings of spring.

Astronomical or calendar start of spring: March 20

For most people, spring begins when the calendar says it does. The calendar start of spring is defined by the position of the sun. As the earth is inclined to the sun (approximately 23.45°), the point at which the sun shines vertically on the earth at midday moves from south to north and vice versa over the course of the year as the earth orbits the sun. At the astronomical beginning of spring, the sun is perpendicular to the equator and then returns to our half of the globe. This day is also known as the equinox. The sun is also perpendicular to the equator at the beginning of fall, which is known as the fall equinox. After the spring equinox, the sun's path runs north of the celestial equator again and the earth's northern hemisphere tilts more and more towards the sun (see Fig. 1).

The exact date of the equinox is constantly shifting because the Earth's orbital period around the sun is a few hours longer than 365 days. Leap years every four years compensate for this, but a certain degree of inaccuracy remains. The calendar date for the beginning of spring is either March 19, 20 or 21; this year it is March 20 at exactly 04:06 Central European Time. Spring lasts until June 21 at 22:50, when the sun is perpendicular to the Tropic of Cancer (summer solstice).

The start of spring in the calendar also determines the date of Easter: Easter Sunday is always the first Sunday after the first full moon in the calendar spring. This year, the first full moon of the calendar spring is on Monday, March 25, so Easter Sunday is on March 31.

Meteorological start of spring: March 1

For reasons of simplicity and comparability, the meteorological start of spring was set to March 1 before the computer age in the 20th century, as whole months are easier to evaluate statistically. Basically, fixed periods of time are required for the comparability of weather data, which are not given with the calendar seasons. For example, each meteorological season lasts exactly three months, i.e. meteorological spring lasts until May 31. However, if you look at the weather patterns, the meteorological seasons often reflect the climatic situation of the seasons better than the astronomical ones.

Phenological start of spring: early spring from the end of January this year

This is the beginning of spring in nature. In phenology, the periodically recurring growth and development phenomena of plants over the course of the year are considered and divided into phases. For this purpose, the onset times of characteristic growth stages of typical plants (so-called phenological indicator plants) are observed and recorded in a phenological calendar. These include, for example, the beginning of flowering or leaf unfolding, the ripening of fruit or leaf discoloration and leaf fall. The phenological calendar divides the year into a total of 10 seasons (see Fig. 2).

According to this definition, spring can be divided into three phases: Early spring, first spring and full spring. The key phase for early spring is the start of hazel blossom, for first spring the start of forsythia blossom and for full spring the start of apple tree blossom. In the long term, early spring in northern Switzerland begins on average around mid-February, early spring towards the end of March and full spring around the end of April/beginning of May.

This year, the hazel blossom and thus early spring began very early in the lowlands and in the deep Alpine valleys, in many places as early as the second half of January (see Fig. 3). This means that, by definition, spring in nature practically began in the middle of the calendar or meteorological winter! The reason for the very early hazel blossom was the above-average mild weather in the second half of January.

Due to the record-breaking mild February, the vegetation continued to develop rapidly, and in some regions the forsythia are already in bloom, indicating the first signs of spring (see Fig. 4). Forsythia do not normally start to flower until the end of March, so the vegetation is around three weeks ahead. This poses a number of risks, in particular late frosts could cause damage (see here).

So what is the right start to spring?

This question cannot be answered in this way, as each definition has its raison d'être. However, the highly variable phenological start of spring best reflects the actual weather conditions and is therefore certainly the most consistent with human perception. After all, it hasn't just felt like spring for a long time...