Weather
US Weather This Week: Fall Officially Begins Tuesday, Sept. 22
Chicago, IL – Fall is almost here. The autumnal equinox arrives Tuesday evening, officially ending astronomical summer and beginning fall across Illinois and the rest of the Northern Hemisphere.
By Evan Cross · 2026-09-22T20:21:43.880Z
Chicago, IL – Fall is almost here. The autumnal equinox arrives Tuesday evening, officially ending astronomical summer and beginning fall across Illinois and the rest of the Northern Hemisphere.
According to the National Weather Service, astronomical fall begins Tuesday, Sept. 22, at 7:05 p.m. CDT. The seasonal milestone occurs during the September equinox, when the sun crosses the celestial equator heading south.
Around the equinox, locations across the United States experience close to 12 hours of daylight and 12 hours of darkness. Day and night are not exactly equal everywhere because of factors including atmospheric refraction and how sunrise and sunset are measured.
After Tuesday, the amount of daylight will continue decreasing across the Northern Hemisphere. That trend continues until the winter solstice in December, which brings the shortest period of daylight and longest night of the year.
The equinox also highlights how Earth's seasons work in opposite directions across the globe. While the United States and other Northern Hemisphere locations welcome fall, astronomical spring begins in the Southern Hemisphere.
Weather forecasters use a different calendar for tracking seasons. Meteorological fall began Sept. 1 and continues through Nov. 30, providing consistent three month periods for comparing weather and climate statistics.
The astronomical calendar instead follows Earth's position relative to the sun.
For Americans ready for autumn, Tuesday evening is the official turning point. From there, daylight continues to fade as the Northern Hemisphere moves deeper into fall and toward winter.