Meteorological Winter Begins Tuesday

Tuesday not only begins the month of December, but it also starts the beginning of Meteorological Winter!  Astronomical Winter doesn't begin until December 21st.

But, why the difference?  Astronomical seasons are based on the positions of the earth in relation to the sun.  That's why the days change from year to year.  Meteorological seasons are based on the annual temperature cycle.  You see, meteorologists and climatologists break down the seasons in three month groupings: Meteorological Summer: June, July & August.  Meteorological Fall: September, October & November.  Meteorological Winter: December, January & February.  Meteorological Spring: March, April & May.  We typically think of winter as the coldest time of the year, while summer is the warmest.  Based on monthly average temperatures, the coldest three month period falls during the months of December, January and February - hence Meteorological Winter.  The same holds for Meteorological Summer.  Fall and spring are considered the transition seasons and are broken up accordingly.

Having a consistent three month period not only benefits meteorologists and climatologists from an observing and forecasting standpoint, but it's also easier to develop seasonal statistics from the monthly statistics.  It would be much more difficult to figure out monthly and seasonal statistics if the days on which the seasons started changed frequently.  Even if it were by one day.