Is It Winter Yet? Maybe, Maybe Not…

Snow frozen onto a tree in Germany.
Photo courtesy of Wikipedia

Tuesday, December 21, is the Winter Solstice and usually considered the first day of winter. However, meteorological winter is already here! So what’s the deal? Well, the definition of winter depends on whom you ask.

A meteorologist defines winter as the three coldest months of the year: December, January and February. An astronomer defines winter as the three months between the Winter Solstice (December 21 or 22) and the Spring or Vernal Equinox (March 20 or 21). What’s the difference?

The definition of meteorological winter is fairly straightforward. Climatologically-speaking, the three coldest months of the year are December, January and February in the Northern Hemisphere. Therefore, meteorological winter begins on December 1 and ends on February 28 (or 29). In the Southern Hemisphere, the seasons are reversed, meaning if you lived in Rio or Buenos Aires, summer would begin in December and winter would begin in June.

Astronomical winter begins on the Winter Solstice, the shortest day of the year. The Earth has seasons and variable daylight hours because its axis is tilted 23.5° relative to its orbit around the sun. Winter in the Northern Hemisphere occurs when the northern half of the planet is tilted away from the sun. This tilt causes sun’s energy to be weaker on the Earth’s surface because:

• The sun shines on the Earth’s surface at an oblique angle.
• The sun’s energy is spread out over a larger area, diluting its strength. 
• The sun’s rays travel through more atmosphere before they reach the surface. 
• Days are shorter so there is less time for the sun to heat the surface. 

Less energy from the sun on the Earth’s surface means colder temperatures in the atmosphere – so winter arrives. For more information, check out last year’s blog about the Winter Solstice: http://sciencemuseumofvirginia.blogspot.com/2009/12/winter-solstice.html

Here are some questions for you to ponder:

• If the Earth’s axis had no tilt, would we have seasons? 
• If the tilt was at a greater angle, what would our seasons be like? 
• Do you think other planets in the solar system have seasons too? 
• How did Earth’s axis get tilted in the first place?

Winter Solstice *

At 12:47 pm today, December 21, 2009, winter begins in the Northern Hemisphere. The first day of winter is called the Winter Solstice; likewise, the first day of summer is the Summer Solstice. The word solstice is derived from Latin and means “sun standing still.” On these two days of the year, the sun’s apparent position in the sky has reached its most southern or northern extreme. Today there will only be 9 hours and 33 minutes of daylight in Richmond, but tomorrow the number of daylight hours will once again begin to increase.
The solstices and the seasons occur because the Earth’s axis is tilted 23.5° relative to its plane of revolution around the sun (see illustration). In the Northern Hemisphere winter, the northern half of the globe is tilted away from the sun; therefore, the sun is low in the sky giving us shorter days and longer nights. At this oblique angle, the sun’s energy is spread over a larger area of the Earth’s surface and thus is weaker than if it was hitting the surface more directly. Also, the sun’s rays must travel through more atmosphere before they reach the Earth’s surface, and some of the solar energy is reflected back into space. In addition, there are less daylight hours to warm the Earth. With all these factors combined, is it any wonder that we have winter weather?
Contrary to what many believe, the Earth is not farther from the sun during the winter. Actually, the Earth is almost at its closest point to the sun at the time of the northern hemisphere’s Winter Solstice. This variation in the Earth’s distance from the sun is small and does not greatly affect the weather; however it does slightly modify the severity of the Northern Hemisphere winters and summers.
While we are experiencing winter, the southern hemisphere is experiencing summer. Our Winter Solstice is the southern hemisphere’s Summer Solstice. If you lived in Rio or Sydney, the winter months would be June through August and the summer months would be December through February.
Here’s some food for thought: what if the Earth’s axis was not tilted? Would we have seasons? What if the axis was tilted more than 23.5°? Or less than 23.5°? What would our seasons be like? How did the Earth get its tilt? All the planets in our solar system have some axial tilt, but they are all different. Why?

*Technical difficulties delayed the posting of this blog entry.