The beginning of spring is different depending on the definition: There is the astronomical or calendrical spring that occurred yesterday, the meteorological spring that always begins on March 1, and the phenological spring that depends on the development of the plants. But which of the three spring beginnings is the right one now?
The question of when spring begins can be answered differently. In principle, a distinction is made between three spring beginnings: The astronomical or calendrical, the meteorological and the phenological beginning of spring. Below you will find more information about the individual spring beginnings.
Astronomical or calendrical beginning of spring: Sun is perpendicular above the equator.
For most people, spring begins when the calendar shows it. The calendrical beginning of spring is defined by the position of the sun. Since the earth is inclined to the sun (about 23.45°), the point at which the sun shines perpendicularly on the earth at noon moves from south to north and vice versa during the course of the year as the earth circles the sun. At the astronomical beginning of spring, the sun is thereby perpendicular above the equator and then returns to our half of the globe. The sun passes through the so-called Aries point, its path intersects the celestial equator. This day is also called the equinox or equinox. At the beginning of autumn, the sun is also perpendicular above the equator; this is called the autumnal equinox. After the spring equinox, the sun's path again runs north of the celestial equator, and the earth's northern hemisphere tilts more and more toward the sun (see Figure 1).
Fig. 1: Sun position and seasons; Source: MeteoNews
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 compensate for this, but a certain inaccuracy remains. So the calendrical beginning of spring falls either on March 19, 20 or 21, this year it was yesterday at 22:24. Spring lasts this year until June 21 at 4:57 p.m., at which time the sun is perpendicular above the Tropic of Cancer (summer solstice).
At the beginning of spring and autumn, day and night are therefore of equal length, as mentioned above - but only theoretically. Because not the center of the sun is decisive for the rising and setting times, but the edge of the sun disk, the day was already a few minutes longer than the night on yesterday, March 20. In purely mathematical terms, the equinox was already on March 17.
By the way, the calendrical beginning of spring also determines the date of Easter: Easter Sunday is always on the first Sunday after the first full moon in the calendrical spring.
Meteorological start of spring: For statistical reasons always on March 1
For reasons of simplicity and comparability, the meteorological beginning of spring was set to March 1 before the computer age in the 20th century, since whole months can be better evaluated statistically. Basically, meteorology needs fixed periods of time for comparability, which are not given with the calendrical seasons. Thus, each meteorological season lasts exactly three months, i.e. the meteorological spring lasts until May 31. However, if we look at the weather sequence, the meteorological seasons often reflect the climatic situation of the seasons better than the astronomical ones.
Phenological start of spring: Depending on the state of development of nature
This is the beginning of spring in nature. In phenology, the periodically recurring growth and development phenomena of plants in the course of the year are considered and divided into phases. For this purpose, the onset times of characteristic phenomena, the characteristic growth stages, are observed and recorded in a phenological calendar. These include, for example, the beginning of flowering or leaf unfolding, the ripening of fruits or leaf discoloration and leaf fall. The phenological calendar divides the year into a total of 10 seasons and is based on characteristic development stages of typical plants (so-called phenological indicator plants, see Figure 2).
Fig. 2: The phenological calendar of Switzerland with a total of 10 seasons; Source: MeteoNews
According to this definition, spring can be divided into three phases: Early, First, and Full Spring. The leading phase for early spring is, among others, the beginning of flowering of the hazel, for first spring the beginning of flowering of the forsythia and for full spring the beginning of flowering of the apple trees. In the long term, early spring in northern Switzerland begins on average around mid-February, first spring around the end of March and full spring around the end of April.
The onset of hazel flowering and thus of early spring was very early this year in the lowlands and in the deep Alpine valleys, and in many places already in the first half of January (see Figure 3). Thus, according to definition, spring in nature has already begun in the middle of the calendrical or meteorological winter! The reason for the extraordinarily early hazel blossom was the very mild end of the year and the far above average temperature in the first half of January.
Fig. 3: Flowering hazels in mid-January in Sarganserland; Source: Roger Perret
The second phenological phase of spring (first spring), which is indicated, among other things, by the flowering of the forsythia, has also already begun (cf. Figure 4).
Fig. 4: Currently blooming forsythia bush in Sarganserland (in the background the snow-covered Churfirsten); Source: Roger Perret
Thus, the onset of first spring has also occurred earlier than average, with a lead of about one week to ten days. Due to the increasingly milder winters on average as a result of climate change, the onset of phenological spring is shifting further and further forward. This also applies to all subsequent phases.
So which is the correct start of spring?
This question cannot be answered in this way, as each definition has its raison d'être. However, the very variable phenological beginning of spring best reflects the actual weather pattern and thus certainly best corresponds to human perception.