When is Fall 2017?
Fall 2017 starts on Friday, September 22, 2017
What is Fall?
Fall, usually called autumn outside of North America, is one of the four seasons that make up the year. It is the intervening period between the warmest time of the year, summer, and the coldest time of the year, winter.
Astronomical fall vs. meteorological fall
There are various ways to define the start and end dates of fall. The two most commonly used ones are based on the astronomical calendar and the meteorological calendar. Astronomical and meteorological fall start and end on different dates.
Astronomical fall
In the astronomical calendar the start and end dates of fall are based on the changing position of Earth in relation to the sun and the resulting solar events of equinoxes and solstices. In the Northern Hemisphere summer ends and fall starts at the moment of the September equinox, which occurs every year on September 22 or 23. Fall ends and winter starts at the moment of the December solstice, which occurs every year between December 20 and December 23. In the Southern Hemisphere fall lasts from the March equinox (March 19 to 21) to the June solstice (June 20 to 22).
The start and end dates for fall given on this page are the dates of astronomical fall in the Northern Hemisphere and are based on Coordinated Universal Time (UTC), which for practical purposes is equivalent to Greenwich Mean Time (GMT). While fall starts and ends at the same moment in time all over the Northern Hemisphere, the date and local time differ from place to place depending on the year and a location's time zone. For locations that are ahead of UTC (further east) it may fall on the day after, and for locations that are behind UTC (further west) it may fall on the day before. To find out the exact date and time of fall 2017 in your area use this seasons calculator.
Meteorological fall
Meteorologists, on the other hand, define seasons based on climatic conditions and the annual temperature cycle. It is important for them to be able to compare the same period of time in different years. The length of the astronomical seasons varies between 89 and 93 days, while the length of the meteorological seasons is less variable and is fixed at 90 days for winter in a non-leap year (91 days in a leap year), 92 days for spring and summer, and 91 days for fall. While the exact definition of a season's timing and length can differ in different areas based on local conditions, in most of the Northern Hemisphere meteorological fall is generally defined as the three months of September, October and November, with the season starting on September 1 and ending on November 30.
There is no "official rule" which definition of fall to use, and different countries adhere to different conventions. Most of North America and Europe use astronomical fall, while Australia and New Zealand use meteorological fall (however, as these countries are in the Southern Hemisphere, the seasons are opposed and fall there lasts from March to May). In other cultures, eg. in the traditional Chinese calendar and in Celtic traditions, the September equinox is considered to be roughly the middle of fall.