(WTNH) — Have you ever wondered what the differences are between meteorological & astronomical seasons?

In just a few weeks on December 21 the calendar tells us it’s the first day of winter, well that’s beginning of astronomical winter. However, you may have heard a Meteorologist use the term “meteorological winter”. Well you will today because in the meteorological world, winter starts today!

But why do meteorological and astronomical winter start on different days?

Astronomical seasons are based on the position of the Earth in relation to the sun, which is what most people grew up learning. In my world, meteorological seasons are based on the annual temperature cycle.

The natural rotation of the Earth around the sun, which started happening well before you or I were born, forms the basis for the astronomical calendar. We know them as seasons and they are defined by two solstices and two equinoxes. They are determined based on the Earth’s tilt and the sun’s alignment over the equator.

During the solstices the sun’s annual path is farthest, north or south, from the Earth’s equator. During either equinox, the sun passes directly above the equator. “In the Northern Hemisphere, the summer solstice falls on or around June 21, and the winter solstice on or around December 22, the vernal (spring) equinox on or around March 21, and the autumnal equinox on or around September 23.

These seasons are reversed but begin on the same dates in the Southern Hemisphere. Ever wonder why there’s a leap year? Because the Earth actually travels around the sun in 365.24 days, so an extra day is needed every fourth year!

Here’s where it gets interesting:

Meteorologists and climatologists break the seasons down into 3 month groupings which are based on the annual temperature cycle. Generally we think of winter as the coldest time of the year and summer obviously as the warmest. Spring and fall being the transition seasons, and that is what the meteorological seasons are based on.

“Meteorological spring includes March, April, and May; meteorological summer includes June, July, and August; meteorological fall includes September, October, and November; and meteorological winter includes December, January, and February.

According to NOAA, “The meteorological seasons were created to make observing and forecasting purposes easier. To make things easier, the length of the seasons is also more consistent for the meteorological seasons, ranging from 90 days for winter of a non-leap year to 92 days for spring and summer. It makes it 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.”

Thanks for reading!

-Meteorologist Kevin Arnone

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