ABILENE, Texas (KTAB/KRBC) —Today marks the start of meteorological fall – but I wouldn’t break out the cable knit sweaters and hot pumpkin spiced lattes yet. Now I know what you’re thinking, isn’t the first day of fall September 22nd? You would be correct, as September 22nd marks the beginning of astronomical fall. But why are there two different start dates for the same season?
The Astronomical Seasons
Astronomical seasons have been used to mark time for thousands of years. The astronomical calendar is based on the natural rotation of Earth around the sun. On this calendar, we define the seasons with the two solstices and two equinoxes, which are determined by Earth’s tilt and the sun’s alignment over the equator.
Equinoxes occur when the sun passes directly above the equator, while the solstices are the time when the sun reaches its maximum or minimum declination, marked by the longest and shortest days. In the Northern Hemisphere we have the summer and winter solstice, occurring on or around June 21st and December 22nd respectively. The spring equinox and the autumnal equinox occur on March 21st and September 22nd respectively. In the Southern Hemisphere, the seasons are reversed but begin on the same dates.
Since it takes Earth 365.24 days to travel around the sun, an extra day is needed every fourth year. This can cause some variation in the exact date of the solstices and equinoxes. The elliptical shape of Earth’s orbit causes the length of the astronomical seasons to vary between 89 and 93 days, according to NOAA’s National Centers for Environmental Information. With all the possible variations in season start dates and season length it made it difficult for scientists to consistently compare climatological statistics from year to year. To compare data, you need to have an independent variable to correctly evaluate the changes in temperature over time. Thus, the meteorological seasons were born.
The Meteorological Seasons
Meteorologists and climatologists break the seasons down into groups of three months based on annual temperature cycle. In general, we tend to think of winter as the coldest time of the year, summer as the hottest time of the year, with spring and fall as the transitional seasons in between. The meteorological seasons in the Northern Hemisphere are broken down as follows:
Spring – March, April, and May.
Summer – June, July, and August.
Fall – September, October, and November.
Winter – December, January, and February.
Years of meteorological observation and forecasting led to the creation of these seasons. These are more closely tied to our monthly civil calendars than the astronomical seasons are. Meteorological seasons are also more consistent in length, ranging from 90 days for winter in a non-leap year to 92 days for spring and summer. With less variation in season start date and length, it becomes much easier to calculate seasonal statistics from the monthly statistics that are gathered. Both monthly and seasonal statistics are extremely useful for agriculture, commerce, and a variety of other purposes.
To sum it all up, the astronomical seasons are based on the position of Earth in relation to the sun, where meteorological seasons are based on the annual temperature cycles. So, if you want to celebrate the kickoff of meteorological fall today, get yourself a pumpkin spice latte – just make it iced!