Spring has sprung – or has it?

By Helen Dacre                                 Department of Meteorology homepage

What is spring? “It is the sun shining on the rain and the rain falling on the sunshine…” (The Secret Garden)

Spring is one of the four temperate seasons, the transition period between winter and summer. In spring, the axis of the Earth is tilted toward the Sun and the length of daylight rapidly increases for the relevant hemisphere. The hemisphere begins to warm significantly causing new plant growth to ‘spring forth’, giving the season its name. But when does spring officially start? The answer to this question largely depends upon your definition of spring.

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Astronomical seasons are defined by the relative position of the Earth’s orbit in relation to the Sun.  Spring therefore starts on the day of the vernal equinox (Latin for ‘spring’ and ‘equal night’, respectively). At this point in the Earth’s orbit, the centre of the sun is visible for exactly 12 hours – this usually occurs on 20/21 March. It marks the start of 6 months of uninterrupted daylight at the North Pole and vice-versa at the South Pole.

Meteorological seasons however, are based on the annual temperature cycle as well as being designed to coincide with the calendar months. By convention, each season is made up of three months, making it easier for meteorological statistics. By the meteorological definition spring starts on 1 March and lasts until 31 May. Meteorological seasons therefore run approximately 3 weeks ahead of those of the astronomical seasons.

Alternatively, the start of spring may not be defined by fixed calendar dates. The phenological definition of spring relates to biological indicators such as the blossoming of a particular plant, or the activities of animals. For example, in my garden I use the date of the first flowering daffodil to mark the start of spring. This should happen any day now – a little later than expected, thanks to recent cold weather.

The seasonal timing of biological events, such as the date of the first flowering daffodil, is of great interest to gardeners and climate scientists alike. In recent decades ‘season creep’ has been observed in the UK. Season creep refers to the fact that some phenological signs of spring, such as flowering, leafing of plants, egg laying in birds and emergence of insects, is occurring earlier in the year. Whilst there are regional differences in the observed changes, reflecting different species, climate and soil types, changes in spring phenology are a strong indication of the impact of climate change. The response of individual species and even whole ecosystems (e.g. pollinating insects emerging out of sync with flowering plants) are as yet unknown and provide an interesting avenue for scientific research.

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