It’s Not (Astronomical) Winter Until December 21-22

The Weather Authority
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Photo: Ron Cochrane

Despite all of the holiday songs and decorations, it’s not actually winter yet (though for meteorologists, it technically is).

Confused? That is because “winter” can span two different time frames that don’t exactly coincide with each other.

Astronomical Winter

Diagram of Winter Solstice. Due to the tilt of the earth’s axis, the Northern Hemisphere points away from the sun’s direct rays. (Source: NASA)

When people hear the word “winter,” often they are thinking of astronomical winter.

Here in the Northern Hemisphere, astronomical winter occurs when the northern half of the earth is tilted as far from the sun as possible, resulting in less daylight for higher latitudes as well as less direct solar radiation, thereby less heat from the sun. In fact, locations above the Arctic Circle experience as much as 67 days of straight darkness (known as “polar night”) during the months spanning from November to January!

Keep in mind that the earth is constantly spinning on its axis (which is tilted 23.5 degrees from the geographical “north pole”). This is what causes our days and nights. At the same time, though, the earth is also orbiting around the sun.

During astronomical winter, the earth is ironically at its closest approach to the sun in its orbit (called perihelion), but because the Northern Hemisphere is tilted away from the sun, the resulting sunlight is much less intense compared to non-winter months. Instead, the intense sunlight is focused upon 23 degrees latitude south of the equator, which is known as the Tropic of Capricorn. The date in which the sun is directly overhead the Tropic of Capricorn at noon is called the Winter Solstice, and it is the shortest “day” of the year with respect to the number of hours of daylight.  Winter Solstice happens each year around December 21st or 22nd. In 2015, Winter Solstice will occur in the Tennessee Valley on December 21st at 10:49 p.m.

Astronomical winter continues until the March, when the Vernal Equinox occurs.

Meteorological Winter

Complex mathematics are used to calculate the official Winter Solstice, resulting in a variation of dates as well as times for locations across the globe. This variation occurs year after year, so trying to keep a consistent record of temperature and precipitation extremes for comparison, winter after winter, becomes extremely difficult, if not impossible.

For this reason, meteorologists “simplified” the math and define winter as the period between December 1 and February 28.

Bottom line: The reason we have seasons here on earth is due to the earth’s 23.5 degree tilt. Our weather would be MUCH different without it!

Nevertheless, I hope you and your family have a safe and warm winter season!

– Christina Edwards

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Facebook: Christina Edwards, WHNT
Twitter: @ChristinaWHNTwx

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