Weatherz School: The seasons
How do you know when fall has begun? When the window A/C retires for the year, or when you switch your iced coffee order to hot? The technical answer is, fall begins on the autumnal equinox.
The seasons are defined by four points within a single trip around the sun; the vernal equinox, the summer solstice, the autumnal equinox, and the winter solstice.
The seasons as we know them are dependent on one major factor; the Earth’s tilt. The Earth’s axis is tilted at 23.5 degrees. Because of this, the North Pole never points directly at the sun.
The northernmost point that the center of the sun will ever be directly over is the Tropic of Cancer. This happens on the summer solstice, which is around June 21st. This is the longest day of the year in the Northern Hemisphere.
The southernmost point at which the sun can be directly overhead is the Tropic of Capricorn. This happens on the winter solstice, around December 21st. This is the shortest day in the Northern Hemisphere.
Then there’s the equinox. This is when the center of the sun passes directly over the equator. This happens two times of year; around March 21st and September 21st. On the equinox, the whole planet receives approximately 12 hours of daylight. The term equinox is Latin for “equal nights.”
None of these dates are fixed. This year, the sun will be directly over the equator at 8:03 pm on September 22nd to mark the beginning of autumn. But, if you ask a meteorologist, they’ll say autumn has already begun.
The changing dates of the astronomical seasons don’t work well for keeping track of seasonal averages. That’s why meteorologists break down the seasons into three month groups.
Winter is the three coldest months, December, January and February. Spring is March, April and May. Summer is June, July, and August. Meteorological fall began September 1st and takes us through November 30th.
Whichever convention you use to measure the seasons, fall is upon us.