Spring has two official start dates, depending on your priorities. For meteorologists, spring already sprung on 1 March, according to their neat, evenly spaced seasons, formalised in the 1900s. But if you plot the seasons in line with our planetary activity, as humans have done for thousands of years, the “astronomical” seasons show spring starting at the vernal equinox, which falls this year on 20 March. Just a few days to go …
The equinoxes (spring and autumn) lie halfway between the shortest and longest days of the year. At these points, fleetingly, day and night are of roughly even lengths all over the planet – closer to conditions in Africa, where our species began life, and where seasonal swings in daylight hours are less dramatic, especially closer to the equator.
These conditions may well best suit the human circadian rhythm – the daily cycle that tells the body when to sleep, wake, eat and carry out various other biological processes. Stuart Peirson, professor of circadian neuroscience at Oxford University, says: “We do all our laboratory experiments in 12 hours of light and 12 hours of dark. It’s a very balanced, neutral middle ground. We know that longer nights and longer days can influence the circadian system.”
The key thing with circadian rhythms, he says, is that even if you live in a cave, your body “will still operate on a 24-hour cycle because we have an inbuilt biological clock that can tell the time”. The clock runs slightly longer than 24 hours, varying slightly from person to person, so the light in our environment adjusts the clock to the correct environmental time, to stop it drifting. “That’s what we call entrainment,” he says. If we’re well entrained, we’re more likely to sleep well at night, and feel good when the sun’s up.
Good light is why the onset of spring feels so pivotal - it certainly isn’t the temperature. “You’re more likely to get snow in March than you are in December,” says the Met Office meteorologist Aidan McGivern. “It’s just a lot brighter.” There is a sudden jump in the hours of sunshine we get, which is dictated by both daylight hours and weather conditions. “December is the dullest month, with an average 41 hours of sunlight in the UK,” says McGivern. “In January, it’s 47 hours, February jumps to 70 hours and then there’s quite a leap into March where we get 102 hours of sunshine. April sees another big leap to 148 hours on average.” Who doesn’t have a cherished memory of sunbathing weather at Easter?
Surprisingly, perhaps, May is the sunniest month of the year, even though the days are longest in June. Overall, says McGivern, “spring is the most settled time of the year. In June you get what is known as ‘a return of the westerlies’. Atlantic weather fronts make a return after being blocked through the spring.” In other words, areas of low pressure and rain-bearing fronts arrive from the Atlantic.
It is no illusion that the lengthening of the days speeds up as we approach spring. The wall opposite my kitchen window is in shadow over winter; in March, a stripe of sunlight appears at the top and within weeks the entire wall is golden. “Days get longer at a quicker rate at this time of year. And that rate of change peaks during the spring equinox,” says McGivern. “And then it slows down around the solstices, like an S-shaped curve. At the moment, daylight is increasing in length by just over four minutes a day, and that is quite noticeable. After seeing so much gloom for a few months, it suddenly becomes brighter much more quickly.”
It’s also no illusion that the sky glows bluer as we leave winter behind. To understand this, you need to be clear about why the sky looks blue in the first place, so here is McGivern’s handy recap: “The sunlight that comes into our atmosphere is made up of all the different colours of the spectrum, but the colours have different wavelengths.” Red light has longer wavelengths, while blue and violet light have the shortest wavelengths. “The oxygen, nitrogen and other molecules that the atmosphere is made up of scatter the light as it bounces off them, and the shorter the wavelength of light, the more efficiently it is scattered. And so you get more scattering of blue and violet light than you do of red light.” The colours with longer wavelengths don’t linger in the sky, but land on the Earth’s surface instead.
So the sky is glowing with beautiful blue light, but when the sun sets, says McGivern, “then it’s closer to the horizon.
happy kids
