When discussing the changing seasons, people often refer to “fall” as the period marked by the autumnal equinox. However, this is not the only way to define the season. Meteorologists and climatologists employ a distinct method for categorizing the seasons, including fall, that differs from the commonly understood astronomical definitions. This alternative approach provides a consistent framework for analyzing weather patterns and climate data.
Defining Meteorological Fall
Meteorological fall is a fixed period consisting of three full months. In the Northern Hemisphere, meteorological fall spans from September 1st through November 30th. This period groups together months that typically experience similar temperature trends as the Northern Hemisphere transitions from summer warmth to winter cold.
For the Southern Hemisphere, the equivalent period for meteorological fall occurs from March 1st to May 31st. This definition aligns with the annual temperature cycle, where these months represent the autumn season in the Southern Hemisphere.
Meteorological vs. Astronomical Fall
The distinction between meteorological and astronomical fall lies in their fundamental basis. Astronomical fall is determined by the Earth’s orbit around the sun and its axial tilt. It officially begins with the autumnal equinox, which occurs around September 22nd in the Northern Hemisphere and March 20th in the Southern Hemisphere. The exact date of the equinox can vary slightly each year due to the Earth’s elliptical orbit and leap years.
Meteorological fall, conversely, uses fixed dates that do not change annually. While astronomical fall is tied to celestial events like the autumnal equinox, meteorological fall is based on the annual temperature cycle and the Gregorian calendar. Both definitions are valid, but they serve different analytical purposes.
Why Meteorologists Use Fixed Dates
Meteorologists and climatologists favor fixed dates for seasons to ensure consistency in data analysis and forecasting. By using three-month blocks (e.g., September, October, November for fall in the Northern Hemisphere), it becomes simpler to compare seasonal weather patterns year-over-year. This standardization streamlines the aggregation of monthly statistics into seasonal averages, which is valuable for climate analysis.
The fixed periods also align more closely with the natural temperature progression experienced on Earth, especially in temperate regions. This makes it easier to track and study climate trends, as the start and end dates of the seasons remain constant. This approach provides a practical framework for meteorological observation and long-term climate studies.