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Happy Winter! Solstice is Wednesday

20 Dec 2016, 9:58 am

The Winter Solstice arrives Wednesday at 5:44 a.m. EST. For the Northern Hemisphere, the Winter Solstice is the shortest day of the year. But what is the solstice exactly? NASA describes it as having to do with some imaginary lines on our planet. These lines are important, because they help people navigate and measure time. The equator is an imaginary line drawn right around Earth’s middle, like a belt. It divides Earth into the Northern Hemisphere and the Southern Hemisphere. Another imaginary line drawn straight through Earth connecting the North Pole to the South Pole is Earth’s axis of rotation. This line is tilted 23.5° from Earth’s orbital path around the Sun. This tilt is the cause of Earth’s seasons.

You may have noticed two special lines of latitude on a globe of the world: One in the Northern Hemisphere called the Tropic of Cancer at +23.5° latitude and one in the Southern Hemisphere called the Tropic of Capricorn at − 23.5° latitude. These are the latitudes where the Sun is directly overhead at noon once a year. In the Northern hemisphere, on the Tropic of Cancer, that is the Summer Solstice, usually June 21. In the Southern Hemisphere, on the Tropic of Capricorn, that is the Winter Solstice, usually December 21. These solstice days are the days with the most (for Summer) or fewest (for Winter) hours of sunlight during the whole year. Two other significant lines of latitude are the Arctic Circle (around the North Pole) and the Antarctic Circle (around the South Pole). These circles are as far from the poles as the Tropic of Cancer and the Tropic of Capricorn are from the equator. On the Arctic Circle, the Sun does not set at all on the Summer Solstice. On that one day, the Sun traces a complete circle just above the horizon as the Earth rotates. On the Antarctic Circle, the Sun does not set at all on the Winter Solstice.

 

On the Winter Solstice, the polar North receives no energy from the Sun. In contrast, the amount of incoming solar energy the Earth receives on June 21, Summer Solstice, is 30 percent higher at the North Pole than at the Equator. The image below shows the amount of sunlight that is reflected from the Earth as measured by the CERES instrument on NASA’s Terra satellite. In the image taken on Winter Solstice 2004, the far North is dark blue, indicating that no sunlight is being reflected back into space. The most sunlight is being reflected out of the Southern Hemisphere, where December 22 marked the longest day of the year.

The polar North reflects less light during its summer (June) than the polar South reflects during its summer (December). The reason behind this is land cover. Antarctica in the south is nearly entirely covered with bright white snow, which reflects light well. The northern oceans are also capped with sea ice, but the area covered with sea ice during the summer has declined in recent years. Snow cover has also decreased on land. According to a study published in Science on October 28, 2005, snow in the Arctic melted on average two and a half days sooner per decade between 1961 and 2004. Open water and snow-free land both absorb more sunlight than reflective ice, and they warm the atmosphere as they release the absorbed heat. Heat in the Arctic increased by about 3 Watts per square meter per decade between 1961 and 2004, the same amount of heating that climate change models predict after several decades of greenhouse gas warming. The Arctic is now warmer than at any time in the past 400 years, and the warming trend is likely to continue. Warmer temperatures and thawing soil will allow shrubs and trees to move farther north, and they absorb even more energy than the tundra that currently covers the North.

Welcome to winter Northern Hemisphere!

Posted by NOAA Science On a Sphere on Monday, December 22, 2014

(Images courtesy of NASA)

For WeatherNation: Meteorologist Mace Michaels

One response to “Happy Winter! Solstice is Wednesday

  1. You might want to explain the rotational and orbital mechanics that account for the days around the winter solstice still getting shorter at sunup and longer at sundown until the first of January.

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