When is spring coming?

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Let’s find out what spring is, and when it really comes.

The most popular and simple definition of spring is the calendar. In the northern hemisphere, the months of spring are March, April and May, and in the southern hemisphere—September, October and November. That is, spring is defined as the transitional months between the coldest (winter) and warmest (summer) parts of the year.

Basically, there’s nothing to add here. Except that this is the most useless definition for outdoor enthusiasts, because it doesn’t tell you anything about the actual weather. For example, in March in northern Finland, you can hardly ride a road bike comfortably, but in Spain, it’s a great time, as long as it’s not too hot.

Waiting until spring for bicycling doesn’t have to be up north, either. Photo: Dmitrii Vaccinium / Unsplash

Astronomical spring is like calendar spring, but a little better

Since astronomical factors primarily affect the change of seasons, it is more accurate to determine the arrival of spring by astronomy. The astronomical year, like the calendar year, is divided into 4 parts. For example, in the northern hemisphere it is defined as:
Summer. June 22 — September 22. In summer, the day is longer than the night, but the length of the day begins to decrease.

Fall. September 23 — December 21. The night is already longer than the day, the length of the day is still decreasing.

Winter. December 22 — March 21. The night is longer than the day, but the length of the day is already increasing.

Spring. March 22 to June 21. The day is longer than the night, the length of the day continues to grow.

It turns out that astronomical spring begins 3 weeks later than the calendar spring. So don’t be surprised if you are still skiing in snowdrifts at the beginning of March!

When the snow melts

In northern regions, where the winter is frosty, spring is often referred to as a rise in temperature above 0°C. In Sweden, for example, the meteorological term for spring is: «The first day of a period of 7 consecutive days with an average daily temperature above 0°C». It is in this weather that the snow melts actively and nature awakens.

It turns out that the date of spring changes every year, telling us that the warmth has arrived. It is only important to remember about 2 weather events that can spoil the arrival of spring:

• Freezing temperatures, when nighttime temperatures dip below freezing, while daytime temperatures are positive. This can lead to ice formation.
• A return of cold weather, when even average daily temperatures become negative again, due to the arrival of cold air. If snow has fallen, it will not melt for several days.

In regions with high and stable air temperatures (i. e. the tropics), this approach is not relevant. There, the change of seasons is more often linked to the calendar, or to the arrival of monsoon season.

Nature awakening

Do we need air temperature at all? Let’s track spring by natural signs only! Different flowers and trees bloom at different temperatures and based on different rainfall patterns, and these are very reliable indicators. Also, observing nature is calming, and distracts you from the hustle and bustle of daily life.

It’s easy to find out about the blooms of your favorite plants on the Internet. For example, the iNaturalist service, where users share photos of wildlife, has charts with different plants’ blooming seasons. And for most parts of the world, there are local services (here is an example for the UK).

Calendars based on observation of wildlife are called ecological calendars. By the way, some of them include not 4 but 6 seasons! In this case, spring is divided into two parts—early, when the first flowers bloom and there is often still snow, and late, when it gets really warm. Probably for outdoor enthusiasts, this is the ideal boundary between winter and summer activities.

Knowing the average dates of the seasons can help you plan your schedule in advance. And with a weather forecast, you can plan specific outings. Have fun in the new season!

Text: Eugenio Monti, a meteorologist and a climatologist.

Cover foto: Igal Ness / Unsplash

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