Changing of the Seasons: Astronomical vs. Meteorological
As I write this, we in the northern hemisphere are poised to welcome spring. It arrives on March 20, 2018 at 12:15 P.M. EDT. That’s when the sun crosses the equator on its journey to the Tropic of Cancer, which will usher in the summer season. The southern hemisphere experiences the same thing, but they’re welcoming autumn and saying goodbye to those lazy, hazy, crazy days of summer. For them, it’s the autumnal equinox. It happens at the same time worldwide, but the different time zones will dictate at what time spring or autumn arrives in any given locale.
The sun doesn’t actually cross the equator, of course. Because of the tilt of the earth on its axis (23.5 degrees for those of you keeping count) we actually cross under the sun, baring our equator to our benevolent star. For those of you who can stand at the equator, the sun will be directly overhead when spring arrives. We all enjoy about equal hours of daylight and darkness on the first day of spring.
Spring doesn’t always arrive on the same day every year. That’s because our orbit around the sun is elliptical, not circular, and because it takes 365.24 days for the earth to complete one orbit of the sun, requiring a day to be added every fourth year (Leap Year). So the arrival of spring varies by a day or two from year to year, fluctuating from March 19th to the 21st. That’s according to the astronomers.
Meteorologists See The Seasons Differently
Meteorological spring arrives about three weeks before astronomical spring, on March 1. Meteorological summer begins on June 1, meteorological autumn begins on September 1, and meteorological winter begins on December 1. Meteorologists and climatologists divide the year into four seasons, each of roughly three months duration, and more closely aligned to our civil calendar than are the astronomical seasons. In a non Leap Year, meteorological winter lasts 90 days while meteorological spring and summer last 92 days.
Weather professionals recognize winter to be the coldest time of the year and summer to be the hottest time of the year. Spring and autumn are considered transitional seasons. Meteorologists and climatologists find it much easier to compile monthly and seasonal weather statistics by dividing the season according to annual temperature cycles and the calendar instead of using the astronomical seasons.
The data compiled by weather scientists is an indispensible tool for many of the world’s industries. Largely dependent upon the weather are agricultural, transportation, tourism, and sports interests. Add to that: municipalities (from plowing roads to maintaining playgrounds), the military, construction, and commerce, and it’s a start.
Each earth year is comprised of two equinoxes (vernal and autumnal), discussed previously, and two solstices (summer and winter). Astronomical summer occurs when the sun “arrives” at the Tropic of Cancer, resulting in the longest day of the year (usually around June 21). Astronomical winter occurs when the sun “arrives” at the Tropic of Capricorn, resulting in the shortest day of the year (usually around December 21).
Most of the world goes by the astronomical seasons: vernal equinox (around March 21), summer solstice (around June 21) autumnal equinox (around September 21) and winter solstice (around December 21). But for meteorologists and climatologists, the seasons change on March 1, June 1, September 1 and Dec. 1.
National Oceanographic and Atmospheric Administration
National Centers for Environmental Information:
Meteorological Versus Astronomical Seasons