Summer fall spring winter: Season Definition: When Do They Start?

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Every season actually begins twice—here’s why

Every three months, a new season begins—twice. 

Spring creeps in like a lion on March 1, and then again a few weeks later with the equinox. Likewise, some people welcome the arrival of summer on June 1, while others celebrate it a few weeks later with the solstice. So who is right about when the seasons begin and end?

It depends on why you’re asking. Seasons are defined in two ways: astronomical seasons, which are based on Earth’s position as it rotates around the sun, and meteorological seasons, which are based on annual temperature cycles. Both divide the year into spring, summer, fall, and winter—yet with slightly different start and end dates for each. Here’s what they mean and how to tell them apart.

What are astronomical seasons?

People have always looked to the skies to determine the season. Ancient Rome was the first to officially mark those seasons with the introduction of the Julian calendar. Back then, the seasons began on different days than the modern era because of discrepancies with the Gregorian calendar used primarily today. Now, the start of each astronomical season is marked by either an equinox or a solstice.

Equinoxes are when Earth’s day is split almost in half. They occur every six months in the spring and fall, when Earth’s orbit and its axial tilt combine so that the sun sits directly above the Equator. On an equinox, roughly half the planet is light while the other half is dark. As the new season progresses, the sun’s position continues to change—and, depending which hemisphere you live in, the days will get progressively lighter or darker until the arrival of the solstice.

Solstices mark the brightest and darkest days of the year. They are also driven by Earth’s tilt and mark the beginning of astronomical summer and winter. When the Northern Hemisphere is tilted toward the sun, it is brighter and feels like summer—while, at the same time, the Southern Hemisphere is tilted away from the sun, plunging it into a dark winter.

But this method of measuring the seasons presents some challenges. The solar year is approximately 365.2422 Earth days long, making it impossible for any calendar to perfectly sync with Earth’s rotation around the sun. As a result, astronomical seasons start on slightly different days and times each year—making it difficult to keep the climate statistics that are used in agriculture, commerce, and more. That’s why weather forecasters and climatologists turned to meteorological seasons instead.

What are meteorological seasons?

Since at least the 18th century, scientists have sought better methods of predicting growing seasons and other weather phenomena. Over time, that gave rise to the concept of meteorological seasons, which are more closely aligned with both annual temperatures and the civil calendar.

Meteorological seasons are far simpler than astronomical seasons. They divide the calendar year into four seasons that each last exactly three months and are based on the annual temperature cycle. Winter takes place during the coldest three months of the year, summer in the hottest three months, and spring and fall mark the remaining transition months.

In the Northern Hemisphere, that means the start date for each season is March 1 (spring), June 1 (summer), September 1 (fall), and December 1 (winter). In the Southern Hemisphere, the seasons are reversed; spring begins in September, summer in December, fall in March, and winter in June.

The consistency of meteorological seasons allows meteorologists to make the complex statistical calculations necessary to make predictions and compare seasons to one another. “Dealing with whole-month chunks of data rather than fractions of months was more economical and made more sense,” climatologist Derek Arndt told the Washington Post in 2014. “We organize our lives more around months than astronomical seasons, so our information follows suit.”

So when is the first day of every season? It isn’t the first of the month or the position of the sun—it’s both.

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Meteorological Versus Astronomical Seasons | News

You may have noticed that meteorologists and climatologists define seasons differently from “regular” or astronomical spring, summer, fall, and winter. So, why do meteorological and astronomical seasons begin and end on different dates? In short, it’s because the astronomical seasons are based on the position of Earth in relation to the sun, whereas the meteorological seasons are based on the annual temperature cycle.

The Astronomical Seasons

People have used observable periodic natural phenomena to mark time for thousands of years. The natural rotation of Earth around the sun forms the basis for the astronomical calendar, in which we define seasons with two solstices and two equinoxes. Earth’s tilt and the sun’s alignment over the equator determine both the solstices and equinoxes.

The equinoxes mark the times when the sun passes directly above the equator. In the Northern Hemisphere, the summer solstice falls on or around June 21, the winter solstice on or around December 22, the vernal or spring equinox on or around March 21, and the autumnal equinox on or around September 22. These seasons are reversed but begin on the same dates in the Southern Hemisphere.

Because Earth actually travels around the sun in 365.24 days, an extra day is needed every fourth year, creating what we know as Leap Year. This also causes the exact date of the solstices and equinoxes to vary. Additionally, the elliptical shape of Earth’s orbit around the sun causes the lengths of the astronomical seasons to vary between 89 and 93 days. These variations in season length and start date would make it very difficult to consistently compare climatological statistics for a particular season from one year to the next. Thus, the meteorological seasons were born.

The Meteorological Seasons

Meteorologists and climatologists break the seasons down into groupings of three months based on the annual temperature cycle as well as our calendar. We generally think of winter as the coldest time of the year and summer as the warmest time of the year, with spring and fall being the transition seasons, and that is what the meteorological seasons are based on. Meteorological spring in the Northern Hemisphere includes March, April, and May; meteorological summer includes June, July, and August; meteorological fall includes September, October, and November; and meteorological winter includes December, January, and February.

Meteorological observing and forecasting led to the creation of these seasons, and they are more closely tied to our monthly civil calendar than the astronomical seasons are. The length of the meteorological seasons is also more consistent, ranging from 90 days for winter of a non-leap year to 92 days for spring and summer. By following the civil calendar and having less variation in season length and start, it becomes much easier to calculate seasonal statistics from the monthly statistics, both of which are very useful for agriculture, commerce, and a variety of other purposes.

Learn more about the monthly, seasonal, and yearly global and U.S. climates in NCEI’s State of the Climate reports. Get the latest temperature, precipitation, drought, and hazards outlooks from NOAA’s Climate Prediction Center.

Film Spring, summer, autumn, winter… and spring again (South Korea, Germany, 2003) watch online — Afisha-Kino

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