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Speaker 1: Welcome to brain stuff from how Stuff Works. Hey, they're

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Speaker 1: brain stuff, Lauren Vogal bomb here. If you live in

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Speaker 1: Earth's middle latitudes, you're accustomed to experiencing four traditional seasons winter, spring, summer,

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Speaker 1: and autumn, also known as fall. That means that existence

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Speaker 1: in the two bands of latitude stretching around the planet

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Speaker 1: from thirty to sixty degrees both north and south of

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Speaker 1: the traffics, offers a lot more variety weatherwise than on

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Speaker 1: the equator, where there's basically a hot, dry season and

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Speaker 1: a hot rainy season. It's a similar story for the

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Speaker 1: upper latitudes, whose residents get a cold winter with long

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Speaker 1: dark nights and a slightly less cold summer with longer daylight.

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Speaker 1: To be technical about it, there are actually two different

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Speaker 1: ways of defining the seasons, the astronomical definition and the

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Speaker 1: meteorological definition. The astronomical definition is based upon change in

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Speaker 1: the length of days that's caused by the relative tilt

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Speaker 1: of the Earth's axis as it revolves around the sun.

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Speaker 1: So in that system, winter is the time between the

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Speaker 1: winter's solstice, the shortest period of daylight of the year,

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Speaker 1: which occurs around December twenty two, and the vernal or

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Speaker 1: spring equinox when day and night are roughly equal, which

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Speaker 1: occurs around March twenty one. Spring then lasts from the

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Speaker 1: vernal equinox to the summer solstice, which is the longest

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Speaker 1: daylight period of the year and happens around June two.

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Speaker 1: Summer runs from then until the autumnal equinox on September

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Speaker 1: twenty three, another day when light and darkness are equal.

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Speaker 1: Then autumn continues until the winter solstice. The meteorological method,

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Speaker 1: on the other hand, has been around since at least

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Speaker 1: the mid nineteen hundreds and is a lot simpler. The

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Speaker 1: year is divided into four seasons by calendar months, so

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Speaker 1: that winter is December January February, spring is March April, May,

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Speaker 1: summer is June July August, and autumn is September October November.

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Speaker 1: Study conducted by climate researcher Kevin Trenberth found that the

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Speaker 1: simpler meteorological definition more closely agreed with observable weather in

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Speaker 1: the continental regions of the Northern Hemisphere, while the trickier

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Speaker 1: astronomical definition fit reality better over the oceans in the

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Speaker 1: southern Hemisphere. Nevertheless, the astronomical definition of the seasons continues

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Speaker 1: to be the one generally used in the United States.

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Speaker 1: Trenwerth told us after my article there were a few

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Speaker 1: places that tried to change, but it petered out. However,

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Speaker 1: climate change, largely driven by human activity, has been messing

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Speaker 1: with the traditional concept of four seasons. Scientists have discovered

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Speaker 1: that as the planet warms up, the tropics have been

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Speaker 1: expanding by point one to point two degrees of latitude

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Speaker 1: every decade. That means places that once had four seasons

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Speaker 1: are shifting to having just two. But even in regions

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Speaker 1: that still experienced four seasons, weather and temperature patterns have

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Speaker 1: been altered. Across the United States. The shift from cold

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Speaker 1: winter weather to warm spring temperatures happens earlier now than

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Speaker 1: it did in the past, and the period of winter

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Speaker 1: weather is shorter and generally milder. In the first few

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Speaker 1: months of seventeen, Some eastern US cities were even beset

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Speaker 1: by startling summerlike conditions in February, with many reaching or

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Speaker 1: surpassing their all time temperature records. Climate change induced seasonal

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Speaker 1: creep a coupled with the overall warming trend, might make

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Speaker 1: you wonder whether the concept of four seasons may eventually

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Speaker 1: become obsolete. Trend Birth, who is currently a scientist at

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Speaker 1: the National Center for Atmospheric Research and who was a

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Speaker 1: co recipient of the two thousand seven Nobel Peace Prize

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Speaker 1: for his work in climate studies, doesn't see that happening,

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Speaker 1: but he says the definition of the seasons may change.

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Speaker 1: Will still have winter, spring, summer in autumn in the

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Speaker 1: middle latitudes, just the timing and duration will be different,

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Speaker 1: he said. A key point, though, is the idea of

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Speaker 1: four seasons, and the way I think of it is

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Speaker 1: the two extreme seasons summer and winter, and to transition seasons.

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Speaker 1: One could redefine the latter to be shorter. In some sense,

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Speaker 1: the summer is becoming longer. The summer is not just temperature,

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Speaker 1: but also the character of the weather, more convective thunderstorms, etcetera,

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Speaker 1: versus winters, more extratropical storms, cold frinds, etcetera. There's one

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Speaker 1: analysis that suggests summers are now thirteen days longer and

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Speaker 1: winters are twenty days shorter than they used to be.

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Speaker 1: Today's episode was written by Patrick J. Keiger and produced

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Speaker 1: by Tristan McNeil. For more on this and lots of

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Speaker 1: other atmospheric topics, visit our home planet, how stuff Works

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Speaker 1: dot com