Despite what your calendar says, today is the start of winter.
- Sun's rays on the winter solstice (Courtesy: NOAA)
Despite what your calendar says, winter begins today. The winter solstice is still weeks away, but to meteorologists, today is the official first day of winter. Meteorological winter is classified as December, January and February and is based on sensible weather rather than astronomy. It's also easier for record keeping.
Astronomical winter is what you see on your calendar and represents the winter solstice. This is when the sun's most direct rays are the farthest south of the equator for the year, which is over the Tropic of Capricorn in the southern hemisphere (23°27′S latitude). According to the Glossary of Meteorology issued by the American Meteorological Society, the winter solstice is defined as: "For either hemisphere, the solstice at which the sun is above the opposite hemisphere." The glossary also defines a solstice as: "Either of the two points on the sun's apparent annual path where it is displaced farthest, north or south, from the earth's equator, that is, a point of greatest deviation of the ecliptic from the celestial equator." This occurs at 12:30am on December 22nd this year (see image). Keep in mind that in the southern hemisphere it's the summer solstice as seasons are reversed.
Lastly, daylight reaches a minimum around our winter solstice with 9 hours and 26 minutes of daylight in D.C. on December 21st and 22nd. Compare that to the 5 hours and 27 minutes of daylight Anchorage, Alaska receives at that time.