How do Meteorological and Astronomical Seasons Differ?

The Position of the Earth during the year    Source: NASA

People sometimes hear different definitions of the seasons when talking or reading about the weather.  Some scientists refer to “meteorological” summer, while others strictly refer to “astronomical” summer.  They are two different concepts.

Meteorologists and climatologists commonly look at seasons in three month blocks for ease of calculating average temperatures and precipitation amounts for a given location.  For instance, “meteorological” summer runs from June 1 through August 31 – so today is the unofficial halfway point of the summer.  A column I wrote earlier in the week discussed how much wetter than average the first half of the summer has been in the Nation’s Capital.  I was going by the definition of “meteorological” summer.  Using the same standard, September 1 is when “meteorological” fall begins; December 1 is when “meteorological” winter commences; and “meteorological” spring arrives on March 1.

There are also “astronomical” seasons.  Those are what many people are more familiar with.  The “astronomical” seasons are dependent on the position of the Earth in relation to the sun.  The summer and winter solstices occur when the sun’s path is farthest north or south of the Earth’s equator.  The summer solstice – or first day of “astronomical” summer – occurs on or about June 21 in the Northern Hemisphere.  That’s the first day of winter in the Southern Hemisphere.  The equinoxes occur when the sun passes directly over the Earth’s equator.  In the Northern Hemisphere the spring (vernal) equinox occurs around March 21 (which is the first day of “astronomical” spring) while the fall (autumnal) equinox occurs on or near September 22. 

Since it takes 365.24 days for the Earth to travel around the sun, an extra day is added to the calendar every four years in what are called “Leap Years.”  That’s why the astronomical seasons begin on approximate dates.  According to NOAA (which keeps climate records for the United States), it’s easier to measure seasonal temperatures over an exact three month period.