ASK TOM: Why is spring called “meteorological spring” when, in fact, it’s just based on the human calendar?

Dear Tom,

Why is spring called “meteorological spring” when, in fact, it’s just based on the human calendar? The “real” spring begins with the vernal equinox.

— Barbara Nicholson, Glenview

 

Dear Barbara,

 

The seasons are popularly defined by the occurrences of the equinoxes in spring and fall (about March 21 and Sept. 21, respectively) and summer and winter solstices (about June 21 and Dec. 21). The begin/end dates of those “astronomical seasons” vary from year to year because the dates of the equinoxes and solstices range from the 20th to 22nd in their respective months. But, valid comparisons of weather variables (like average temperatures) require the calculations always be made over identical intervals. “Meteorological seasons” like meteorological spring, which is always March 1 to May 31, establishes that uniformity.