Astronomical Winter vs Meteorological Winter
2016-12-17 15:56:13.000 – Adam Gill, Weather Observer/IT Specialist
Meteorological winter begins on December 1st while the astronomical winter does not begin until the 21st. There are a few reasons as to why meteorologist uses different dates for the beginning and ending of seasons as compared to the calendar.
The main reason we have seasons start almost a month before astronomical seasons is because of temperature. Average temperatures are colder in December, January, and February rather than January, February, and March. Up on the summit, the average temperature in December is 10.1°F where in March the average temperature is 12.8°F. Winter supposed to be the coldest season, it would make sense to use the three coldest months as winter. Most locations across the US will have their three coldest months typically being December, January, and February on average.
On a year to year basis this could vary due to variations in weather. For instance last year, December was 24.4°F and March was 17.5°F so the coldest months did fall into the astronomical winter.
In December, we have the Winter Solstice which is where the days are the shortest and the sun is at the lowest angle in the sky. This spreads the solar radiation over a larger area similarly to shining a flashlight directly overhead then tilting the angle and the light spreads out over a larger area. The nights are really long so that helps cool off the surface with plenty of radiational cooling. Add some snow cover and the low angle of the sun will cause some incoming solar radiation to reflect off of the surface and back into space. The deeper the snow the more incoming radiation will be reflected thus keeping the temperature even colder! This results in vastly different December temperatures from year to year depending on the snow cover in a region. Thus the lack of snow cover over New England last year contributed to the warm month while this year we have so far been below average!
In March you have the Spring Equinox so the days are much longer and the sun angle is higher. In late February the sun will reach a high enough angle that the solar radiation will penetrate deeper and deeper into snow and usually the snow will begin to melt from below. Temperatures also don’t need to be all that warm either to melt snow because you have a higher concentration of solar radiation reaching the surface of the Earth. By March there is quite a bit of warming and a greater chance that warm air from a snow free area moving into a given location so typically March will be warmer than December.
Adam Gill, Weather Observer/IT Specialist