Fall Starts Today, Scientists Say

Tuesday marks the start of fall for weather scientists. This is called meteorological fall. It follows the calendar, not the sun. Real fall, called astronomical fall, starts later this year around September 22.

Scientists split the year into four seasons of three months each. This makes it easier to track weather over time. Meteorological fall runs from September 1 to November 30. This time brings cooler days and busy weeks for storms. Astronomical fall depends on how Earth tilts toward the sun. Both systems help people understand changing seasons in different ways.

September 1 marks the beginning of what scientists call meteorological fall. This system tracks seasons by the calendar rather than the position of the sun. Meteorological fall runs from September 1 through November 30 every year. Astronomical fall, by contrast, does not begin until the September equinox, which falls around September 22 this year.

Meteorologists and climatologists created this calendar-based system decades ago. Their goal was to simplify weather record-keeping across the globe. Meteorological seasons always span the same three full months each year. That consistency lets scientists compare temperature and rainfall data from year to year. Astronomical seasons shift by a day or two annually because they depend on Earth's exact position relative to the sun.

The switch to meteorological fall often brings noticeable changes across the country. Average temperatures begin to drop in many regions during these months. This period also includes some of the busiest weeks of the Atlantic hurricane season. That season typically peaks in early to mid-September. Farmers, schools, and event planners often follow the meteorological calendar informally, even without using the scientific term.

Both calendars are considered scientifically valid, and neither one is more correct than the other. They simply serve different purposes for different audiences. Meteorological seasons help scientists track and compare climate data consistently. Astronomical seasons mark the precise moment Earth's tilt produces certain lengths of day and night. For most families, the shift to September simply means shorter evenings and cooler mornings are on the way.