Every year we look forward to the first warm rays of sunshine, the chirping of birds, and the first blossoms sprouting from the ground. But when exactly does spring actually begin? The answer to this seemingly simple question is surprisingly varied, because there are in fact several definitions for the start of spring. ## Meteorological Spring: Starting on 1 March In meteorology, spring begins every year on **1 March** and lasts until **31 May**. This designation may seem arbitrary at first glance, but it has a very practical background. Meteorologists divide the year into four seasons of three full calendar months each: - **Spring:** March, April, May - **Summer:** June, July, August - **Autumn:** September, October, November - **Winter:** December, January, February

The meteorological seasons are based on whole calendar months because this considerably simplifies the statistical analysis of climate data. Monthly averages can thus be calculated cleanly and compared across decades. For climate research and the compilation of climate statistics, this uniform division is indispensable.

## Why Not the Calendar Start of Spring? Many people are mainly familiar with the **calendar (astronomical) start of spring**, which falls on the spring equinox. This usually takes place on **20 or 21 March**. At this point, the sun stands directly above the equator, and day and night are approximately equal in length. For meteorology, however, this date is less suitable, for the following reasons: - The astronomical start of spring varies by one to two days each year. - It falls in the middle of a calendar month, which complicates data analysis. - Monthly statistics would have to be laboriously split up. ## Phenological Spring: When Nature Decides Besides the meteorological and the astronomical spring, there is a third definition: the **phenological spring**. This is based on the actual developmental stages of plants and animals. It begins when certain indicator plants start to bloom. The phenological spring is divided into three phases: - **Early spring:** Blooming of snowdrops and hazelnut - **First spring:** Blooming of forsythia and cherry - **Full spring:** Blooming of the apple tree and lilac

The phenological spring in Central Europe has shifted noticeably earlier over the past few decades. Many plants today bloom on average about two weeks earlier than they did 50 years ago. This is a direct consequence of global warming and a striking piece of evidence of how climate change affects nature.

## Why Does 1 March Often Not Yet Feel Like Spring? A fair question! At the beginning of March, there is often still snow in Switzerland and in many parts of Central Europe, and temperatures can be quite wintry. The meteorological start of spring therefore does not mean that it immediately becomes springlike warm. Rather, 1 March marks the **beginning of a transitional phase**. The days become noticeably longer, the solar radiation increases, and the average temperatures rise significantly over the course of March. Statistically, the months of March, April, and May are in fact the months with the greatest temperature increase over the course of the year in Central Europe. ## The Significance for Climate Research The uniform division into meteorological seasons is of central importance for climate research. It makes it possible to systematically compare temperatures, precipitation amounts, and other weather data over long periods. This allows, for example, questions such as these to be answered: - Was the meteorological spring of 2026 warmer than average? - How has the amount of precipitation in spring changed over the past 100 years? - Are there trends in the number of frost days in March? Without the clear assignment to whole calendar months, such analyses would be considerably more laborious and prone to error.

There are three different definitions for the start of spring: The **meteorological spring** begins on 1 March and was defined this way to allow climate data to be analysed simply and uniformly. The **astronomical spring** starts around 20 or 21 March with the equinox. The **phenological spring** is based on nature and begins when certain plants start to bloom. For weather forecasting and climate research, it is above all the meteorological start of spring that is of great importance, since it defines clear and comparable periods.