Meteorological Seasons vs. Astronomical Seasons

In our investigation of the development of Leap Day, we learned the seasons would drift on our calendars if the intercalary date didn’t exist. One could argue that seasons – the result of axial tilt – are the most important planetary aspect of human existence. For millennia, in many places, people have planned lives, communities, and civilizations around the changing weather. Times of planting, harvest, and barrenness drive the rhythms of life on Earth.

During the project, we have explored and marked the passing of the seasons, from the Winter Solstice to the Vernal Equinox to the Summer Solstice to the Autumnal Equinox. We celebrate these occasions as days, but each is technically a moment. The solstices and the equinoxes are the moment the aforementioned tilt of the planet reaches a certain milestone. In summer and winter, the moment arrives as the point of maximal tilt (either toward the sun or away, based on the hemisphere and specific season). The equinoxes transpire when the Earth is “straight up” in relation to the sun. The solstices also happen to feature the longest and shortest amounts of sunlight. The equinoxes mark dates when day and night are close to even, though, due to the shape of the planet and optical physics, the equilux – the date on which day and night are the same – differs by a few days, depending on location. These moments demarcate our calendar into fourths. Because these seasons happen thanks to celestial interactions, we call these quarters the astronomical seasons.

The astronomical seasons are clean: before the solstice or equinox is one season, after is another.

The four main tilt points; clockwise from upper left are March Equinox, June Solstice, December Solstice, and September Equinox - photos by Japan Meteorological Agency

The act of breaking the calendar into quarters is named “four-season reckoning.” The astronomical method is perhaps the most popular, but it is not the only approach.

Another practice employed by many scientists has been dubbed the meteorological seasons. The word for the study of Earth’s atmosphere – most commonly applied to the weather and climate – comes from the Greek meteōrologia, which translates to “discussion of high things.” In the meteorological method of defining seasons, the tilt of the Earth is not considered (even though it does affect meteorological seasons). Instead, all that matters is temperature. The quarter of the year with the warmest weather becomes summer and the coldest winter. Between the two are spring and fall.

In this scenario, summer and winter begin a few weeks before their respective solstices. In the Northern Hemisphere, summer lasts from June through August, while December, January, and February comprise winter. Meteorological spring consists of March, April, and May. Autumn gets September, October, and November.

Average high and low temps in Ohio - graphic by weatherspark; highlights by TMAC

Why use one method over the other?

Unless humans mess up the calendar – which we have done a few times – astronomical seasons are the metronomes of our celestial clocks. They will happen at the same point each year. As you can see from the chart of Ohio temperatures above, though, the solstices do not necessarily reflect the hottest and coldest days of the year. For many life sciences, temperature matters strictly more than the length of days. Derek Arndt, of the National Climatic Data Center, told the Washington Post, “dealing with whole-month chunks of data rather than fractions of months was more economical [to scientists] and made more sense – and still does, in many ways. We organize our lives more around months than astronomical seasons, so our information follows suit.” Further, humans often apply temperatures to their concepts of the seasons, so it might make sense to label the hottest quarter as summer. Still, temperature, climate, and, by extension, meteorological seasons can and will change. Will the hottest months continue to be June, July, and August in the Northern Hemisphere? Probably, but it’s not a given.

All in all, the distinctions are arbitrary. Nothing magical occurs when we move to March 1, from meteorological winter to spring. The same can be said at the Vernal Equinox. Which makes the most sense to you? Is early March a spring month? Does winter begin on 1 December? Or are you a traditionalist who waits for the Earth to move through her tilting? March might fall within the bounds of the shoulder season of spring when it comes to temperature, but I often internalize the early weeks to feel more like winter than spring. Give me the equinoxes and solstices!

Further Reading and Exploration

Meteorological and Astronomical Seasons – National Oceanic and Atmospheric Administration

The difference between meteorological and astronomical seasons – Royal Meteorological Association

Astronomy vs meteorology: When does summer start? – BBC

Meteorological vs. astronomical seasons: Which is more useful? – The Washington Post

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