The official start to winter this year may be Dec. 21 at 11:28 a.m. EST,but meteorologists consider the season underway once the calendar turns to December.
The date on your calendar when winter begins is referred to as astronomical winter,and it can vary each year between Dec. 21 and 22.
These astronomical seasons are based upon the position of the Earth relative to the sun.
As the Earth revolves around the sun,we experience each of the four seasons,in which there are two solstices and two equinoxes. These are based on the Earth's tilt and the sun's alignment over the equator. The solstices represent times when the Earth's equator is tilted farthest,north or south,away from the sun. The equinoxes occur when the equator is aligned directly with the sun.
The winter solstice occurs when the Northern Hemisphere is tilted farthest away from the sun. In the winter,the sun rises in the southeast and sets in the southwest,a much shorter path across the sky,which is why the length of daylight is significantly shorter,usually only 8 to 9 hours.
The equinoxes occur between the solstices,with the autumnal equinox (fall) happening Sept. 22 or 23 and the vernal equinox (spring) occurring March 20 or 21. The sun rises due east and sets due west at the time of the equinoxes,which gives an equal amount of day and night (12 hours of each).
Meteorological Seasons
Meteorologists break down the seasons into groups of three months,based on the annual temperature cycle around the globe. The common person thinks of summer as the hottest time of the year and winter as the coldest time of the year,while fall and spring are the transition seasons. This is exactly the basis of meteorological seasons.
Meteorological summer runs from June 1 to Aug. 31,meteorological fall from Sept. 1 to Nov. 30,meteorological winter from Dec. 1 to Feb. 28 and meteorological spring from March 1 to May 31.
These four meteorological seasons were developed for weather-observing and forecasting purposes since they are more closely tied to our monthly civil calendar than the astronomical seasons,which can start on a different date each year.
The meteorological seasons are always 90 to 92 days,depending whether or not it's a leap year. This breakdown makes it much easier to calculate seasonal statistics since every season starts on the first of the month,rather than the 20th to 22nd with the astronomical seasons.
As shown in the tweet below from Dr. Brian Brettschneider,a climatologist with the University of Alaska-Fairbanks,the coldest temperatures of the year more closely align with winter's meteorological definition than its astronomical definition for the majority of the United States and Canada.
Brettschneider also analyzed the average coldest 90-day periods for dozens of U.S. cities and found that only Honolulu,Hawaii,had its coolest 90-day stretch extend deep into March. The remainder of the 62 cities in the analysis clearly showed their coldest temperatures occurring generally from December,or even late November,into February or early March.
Brian Donegan is a digital meteorologist at www.weathernow24.com. Follow him on Facebook, Twitter and Instagram.
