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?