How Do Meteorological Seasons Differ From Astronomical Seasons?

Just as the diurnal sunrise-to-sunset cycle marks the passing of each day, Earth’s seasons—spring, summer, fall (autumn), and winter—mark the passing of a year. And similarly to how the time of day can be tracked using a clock or the Sun's position in the sky, the seasons can be marked in a number of ways, including by the Earth-Sun relationship (astronomical) or the weather (meteorological). 

Not as familiar with the meteorological seasons? You're not alone. While their origins are largely unknown, some scientists believe they've existed since the late-18th century days of the Palatine Meteorological Society. A Twitter search reveals they didn't gain mainstream popularity until the 2010s. Most people have been confused about which set of seasons to mark on their calendars ever since.

Meteorological Seasons

While the meteorological seasons may be new to some people in name, in theory, they’re how most of us envision the seasons. That is, they’re based on the changes we observe in nature, namely the annual rising and plunging of air temperatures. Dividing the year into three-month periods of similar temperatures results in the four meteorological seasons.

For those of us living in the Northern Hemisphere, meteorological summer, the warmest season, corresponds to the three hottest months: June, July, and August.

Similarly, meteorological winter, the coolest season, corresponds to the three coldest months: December, January, and February.

Spring and fall are the transition seasons between these two. Spring, the bridge between cooler and warmer weather, runs from March 1 to May 31. And fall, the season during which warmer temperatures fade to cooler temperatures, runs from September 1 to November 30.

Astronomical Seasons

Unlike the meteorological seasons, the astronomical seasons have existed for millennia, and perhaps even date back to the erection of Stonehenge in 2500 BC. And because our ancient ancestors observed them throughout the whole of history, the tradition has stuck with us to this day. As their name suggests, the astronomical seasons are based on planetary goings-on, namely the axial tilt of Earth, and how this 23.5 degree tilt dictates how our planet heats up as it orbits the Sun within the course of a year.

For people who live in the Northern Hemisphere, the summer season is the span of months, beginning with the summer solstice, when the Northern Hemisphere is tilted its innermost towards the Sun, thereby receiving the Sun's most direct light; this corresponds to the calendar dates of late-June to late-September. (In reality, the tilt begins to gradually lean away from the Sun after the summer solstice, but because air temperature lags behind changes in solar irradiance, the earth continues to warm up.)

Similarly, astronomical winter, which begins with the winter solstice, occurs when the Earth's axis is tilted its farthest away from the Sun, thereby receiving the Sun's indirect light. It occurs from late-December to late-March.

Astronomical spring and fall occur when Earth's tilt is neutral. If Earth’s axis shifts from leaning away from the Sun to a neutral tilt, the spring or vernal equinox occurs; if it shifts from leaning towards the Sun to a neutral tilt, the fall or autumnal equinox takes place.

Because Earth takes 365 days to orbit the Sun in some years, and 366 days in others, the solstices and equinoxes fall on slightly different days from one year to the next. The spring equinox takes place around Mar. 20; the summer solstice occurs between June 20 to 21; the fall equinox, between Sept. 22 to 23; and the winter solstice between Dec. 21 to 22.

So … When Does Each Season Actually Start?

Weather scientists and weather enthusiasts tend to observe both sets of seasons. They prefer the meteorological seasons because their static dates allow for a "cleaner" comparison of seasonal weather and climate data. They also celebrate the astronomical seasons to honor tradition. The rest of the world typically observes the astronomical seasons only. 

Of course, the true question is, which should you use? That is, which of the two most closely aligns with the average surface temperatures we actually experience?

According to a study in the Bulletin of the American Meteorological Society, that answer depends on which hemisphere (Northern or Southern) you live in, and whether you’re a coastal or a continental dweller. For Northern Hemispherians, most of whom are land-locked, the meteorological seasons win out. For those living south of the equator, where oceans have a larger influence on weather and climate, the astronomical seasons more closely define temperatures.  

Could Climate Change Blur Season Start Dates?

Add Earth’s warming climate into the conversation, and neither the astronomical nor the meteorological seasons fit very well. A study in Geophysical Research Letters finds that between 1952 and 2011, seasons in the Northern Hemisphere have shifted in length; winter waned from 76 to 73 days, spring shrank from 124 to 115 days, and autumn fell from 87 to 82 days. Summer, however, soared from 78 to 95 days.

This same study also warns that if greenhouse-gas-triggered atmospheric warming continues at its current rate, summers could last nearly six months by the year 2100, while winters might wither away to a mere 2 months. At that point, our seasons may begin to resemble those of locations near the Equator: Either wet or dry.