Sunday, Sept. 22, 2013, marked the autumnal equinox and the first day of fall in the Northern Hemisphere. The way fall is defined is based on two methods: the meteorological and astronomical methods.
The meteorological method simply defines the season of fall as the months of September, October and November, according to AccuWeather Senior Meteorologist Dave Samuel.
The meteorological method is based on looking at the normal high temperatures and low temperatures of the different months. Based on those normal temperatures, months are grouped into seasons.
"You are dividing up the seasons by looking at the core three months in time that relate to those normal temperatures," AccuWeather Expert Senior Meteorologist Dan Kottlowski said.
Following that method, summer is composed of the three warmest months, winter is composed of the three coldest months and fall and spring are somewhere in between.
The astronomical definition of fall is based on the position of the sun with respect to the earth.
"When the direct rays of the sun pass over the equator, you will switch from summer to fall," Kottlowski said.
The autumnal equinox, the day the direct rays from the sun pass over the equator, is usually either Sept. 21 or 22. This is the event that occurred Sunday that started the astronomical fall, and causes the day and night to be equal in length.
There are two different ways to define fall because there is a delay in the cooling temperatures of the land and the ocean.
This Accuweather image shows fall patterns of temperature and storm activity
"The water contains so much energy and warmth that it delays the cool down; land masses will be cooling off quicker, and water just takes so much longer to release the heat," Samuel said.
This delay causes people on land to experience changes in normal temperatures sooner, since the land heats up and cools down much quicker than the water.
By Accuweather.com Staff Writer Rachelle Gaynor
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