Winter 2010-2011: Warmer Than Normal? or Colder Than Normal?



Question: Was Winter 2010-2011 colder-than-normal or warmer-than-normal?

Answer depends on how you define "Winter."


There are two ways to define the winter season. One way is to define it based on its astronomical endpoints: the winter solstice and the spring equinox. The other way is to define it in more general terms, as simply a three-month period of time when temperatures tend to be coldest.  

When we use the astronomical endpoints to define winter it is called, not surprisingly, astronomical winter. For Winter 
2010-2011, the first full day of the winter season was December 22--which was the first day when both the daily high and low were most likely to occur after the solstice. The last full day of winter was March 20--which was the last day when both the daily high and low were most likely to occur before the equinox.

The time of the winter solstice was 538 pm CST, December 21, 2010. The spring equinox occurred at 621 pm CDT, March 20, 2011.

Astronomical winter included 89 days.

When we use the more general definition for winter it is called, meteorological winter. This is the definition typically used by climatologists and includes the full months of December, January and February. The Climate Prediction Center also defines winter using the meteorological definition.

Meteorological winter included 90 days. 

Virtually the entire month of December 2010 was unusually cold, with the monthly average more than 6 degrees below normal. Much of the cold, however, occurred before the winter solstice. In addition, the most extreme negative departures for the December-through-February period occurred prior to the winter solstice, on December 13 and 14--when the departures from normal were -24 and -23 degrees, respectively. Thus, much of the most abnormally cold weather for the winter was heavily "front-loaded' during the month of December--and some of the most bitter cold actually occurred before the winter solstice even arrived! 

Due to the front-loaded cold weather during the first several weeks of December, the average temperature for Nashville during meteorological winter ended up 2.2 degrees below normal.

A graph showing the frequency of colder-than-normal and warmer-than-normal days at Nashville for meteorological winter is shown below:


The normal daily temperature for the astronomical winter of 2010-2011 was 41.0. The actual average turned out to be 41.9 degrees, or 0.9 degrees above normal. This occurred largely because of the unusually warm temperatures that were "back-loaded" to the last 3 or 4 weeks of the season, and included the very warm weather observed during the first 20 days of March. 

A graph showing the frequency of colder-than-normal and warmer-than-normal days at Nashville for astronomical winter is shown below:



 As you can see, the astronomical winter was almost a perfect balance between warmer-than-normal and colder-than-normal temperatures. In addition, extremely warm and extremely cold weather occurred with almost the exact same frequency. The
most extreme temperatures, however, during astronomical winter were actually on the warm side. While there were no days when 
daily normals were more than 22 degrees below, there were two days when  temperatures were more than 22 degrees above normal (on December 31st and February 28th).

So, if someone asks you if this last winter was colder-than-normal or warmer-than-normal, you can honestly answer, both! (depending, of course, on your definition of "winter")










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