WASHINGTON — When does fall begin? Well, it depends on which fall you are talking about. There is “meteorological” fall and “astronomical” fall, but the two are very different concepts with different start dates.
Meteorologists and climatologists commonly look at seasons as three-month blocks for ease of calculating average temperatures and precipitation amounts for a given location. For instance, meteorological fall runs from Sept. 1 through Nov. 30. Using the same standard, Dec. 1 is when meteorological winter commences; meteorological spring arrives on March 1; and meteorological summer gets underway on June 1.
There are also “astronomical” seasons, which are what 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 also 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).
Fall will begin on the autumnal equinox, which occurs on Sept. 23 this year.
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 we call “Leap Years.” That’s why the astronomical seasons begin on approximate dates.
According to NOAA, it’s easier to measure seasonal temperatures over a specific three-month period.