Welcome to brain stuff from how Stuff Works. Hey, they're brain stuff, Lauren Vogal bomb here. If you live in Earth's middle latitudes, you're accustomed to experiencing four traditional seasons winter, spring, summer,
and autumn, also known as fall. That means that existence in the two bands of latitude stretching around the planet from thirty to sixty degrees both north and south of the traffics, offers a lot more variety weatherwise than on the equator, where there's basically a hot, dry season and a hot rainy season. It's a similar story for the upper latitudes, whose residents get a cold winter with long dark nights and a slightly less cold summer with longer daylight.
To be technical about it, there are actually two different ways of defining the seasons, the astronomical definition and the meteorological definition. The astronomical definition is based upon change in the length of days that's caused by the relative tilt of the Earth's axis as it revolves around the sun.
So in that system, winter is the time between the winter's solstice, the shortest period of daylight of the year, which occurs around December twenty two, and the vernal or spring equinox when day and night are roughly equal, which occurs around March twenty one. Spring then lasts from the vernal equinox to the summer solstice, which is the longest daylight period of the year and happens around June two.
Summer runs from then until the autumnal equinox on September twenty three, another day when light and darkness are equal. Then autumn continues until the winter solstice. The meteorological method, on the other hand, has been around since at least the mid nineteen hundreds and is a lot simpler. The year is divided into four seasons by calendar months, so that winter is December January February, spring is March April, May, summer is June July August, and autumn is September October November.
Study conducted by climate researcher Kevin Trenberth found that the simpler meteorological definition more closely agreed with observable weather in the continental regions of the Northern Hemisphere, while the trickier astronomical definition fit reality better over the oceans in the southern Hemisphere. Nevertheless, the astronomical definition of the seasons continues to be the one generally used in the United States. Trenwerth told us after my article there were a few
places that tried to change, but it petered out. However, climate change, largely driven by human activity, has been messing with the traditional concept of four seasons. Scientists have discovered that as the planet warms up, the tropics have been expanding by point one to point two degrees of latitude every decade. That means places that once had four seasons are shifting to having just two. But even in regions that still experienced four seasons, weather and temperature patterns have
been altered. Across the United States. The shift from cold winter weather to warm spring temperatures happens earlier now than it did in the past, and the period of winter weather is shorter and generally milder. In the first few months of seventeen, Some eastern US cities were even beset by startling summerlike conditions in February, with many reaching or
surpassing their all time temperature records. Climate change induced seasonal creep a coupled with the overall warming trend, might make you wonder whether the concept of four seasons may eventually become obsolete. Trend Birth, who is currently a scientist at the National Center for Atmospheric Research and who was a co recipient of the two thousand seven Nobel Peace Prize for his work in climate studies, doesn't see that happening, but he says the definition of the seasons may change.
Will still have winter, spring, summer in autumn in the middle latitudes, just the timing and duration will be different, he said. A key point, though, is the idea of four seasons, and the way I think of it is the two extreme seasons summer and winter, and to transition seasons. One could redefine the latter to be shorter. In some sense, the summer is becoming longer. The summer is not just temperature, but also the character of the weather, more convective thunderstorms, etcetera,
versus winters, more extratropical storms, cold frinds, etcetera. There's one analysis that suggests summers are now thirteen days longer and winters are twenty days shorter than they used to be. Today's episode was written by Patrick J. Keiger and produced by Tristan McNeil. For more on this and lots of other atmospheric topics, visit our home planet, how stuff Works dot com