Updated: September 08, 2020 12:23 PM
Created: September 08, 2020 11:11 AM
As of September 1st we officially hit the start of meteorological fall, while the astronomical start to fall (autumnal equinox) is still a few weeks away. You may have noticed that meteorologists and climatologists define seasons differently from “regular” (astronomical) spring, summer, fall, and winter. So, why do meteorological and astronomical seasons begin and end at different times? In short, it is due to the fact that the astronomical seasons are based on the position of Earth in relation to the sun, while the meteorological seasons are based on the annual temperature cycle.
The Astronomical Seasons
The natural rotation of Earth around the sun forms the basis for the astronomical calendar, in which we define seasons with two solstices and two equinoxes. Earth’s tilt and the sun’s alignment over the equator determine both the solstices and equinoxes. This is something humanity has done for thousands of years.
In the Northern Hemisphere, the summer solstice falls on or around June 21, the winter solstice on or around December 22, the vernal or spring equinox on or around March 21, and the autumnal equinox on or around September 22. These seasons are reversed but begin on the same dates in the Southern Hemisphere. It is during this time that the sun passes directly over the equator.
The exact date of the solstices and equinoxes to vary since it take the Earth a little more than 365 days to travel around the sun. Additionally, the elliptical shape of Earth’s orbit causes the lengths of the astronomical seasons to vary between 89 and 93 days. These variations in season length and season start would make it very difficult to consistently compare climatological statistics for a particular season from one year to the next. Thus, the meteorological seasons were created.
The Meteorological Seasons
Meteorologists and climatologists break the seasons down into four groups of three months based on the annual temperature cycle as well as our calendar. We consider winter as the coldest time of the year and summer as the warmest time of the year, with spring and fall being the transition seasons. That is what the meteorological seasons are based on. Meteorological spring is March, April, and May; meteorological summer is June, July, and August; meteorological fall is September, October, and November; and meteorological winter is December, January, and February.
Meteorological observations and forecasting led to the creation of these seasons, and they are more closely tied to our monthly calendars in comparison to the astronomical seasons are. The length of the meteorological seasons is also more consistent, ranging from 90 days for winter of a non-leap year to 92 days for spring and summer. By following the civil calendar and having less variation in season length and season start, it becomes much easier to calculate seasonal statistics from the monthly statistics, both of which are very useful for agriculture, commerce, and a variety of other purposes.
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