The spring index is determined by means of the first ten phenological spring phases of the year and updated at the end of each May. It draws on the observations documented in the corresponding year at the nearly 80 stations in the phenological measurement network, using datasets of adequate length.

The graph and data file can be «downloaded» from the «Spring Index» application.

Calculation of the spring index

The following ten phenological phases are used to characterise the phenological spring as a whole; they occur between January and May:

• Hazel bush flowering
• Coltsfoot flowering
• Wood anemone flowering
• Cherry tree flowering
• Leaf unfolding in horse chestnut
• Leaf unfolding in hazel bush
• Needle appearance in larch
• Dandelion flowering
• Lady’s smock flowering
• Leaf unfolding in beech

The deviation from the average occurrence date is determined by means of a main component analysis. This method is practical for structuring, simplifying and visualising complex datasets. In addition, it can be used to filter spatial and chronological dependencies. The result of the first main component is then converted back into number of days’ deviation from the average.