The autumnal equinox arrives tonight, September 22, at 8:05 p.m., marking the official start of astronomical fall in the Northern Hemisphere. At that precise moment, the sun will sit directly above Earth's equator, creating roughly equal day and night across most regions of the world. The word equinox comes from Latin, meaning 'equal night.' In the Southern Hemisphere, the same event marks the start of spring.
Astronomers note that summer ends September 22, making tonight the start of astronomical autumn. However, meteorologists and climatologists consider fall to have begun September 1, grouping the year into three-month seasons based on temperature cycles, with June, July, and August being the hottest months. The start of fall does not necessarily mean an immediate drop in temperature; the combination of El Niño and climate change makes warmer-than-normal temperatures likely this season.
Forecasters expect this El Niño to peak later this fall as one of the strongest on record. This summer already set global records, with August being the warmest month worldwide for combined land and ocean temperatures. Meanwhile, the contiguous United States has seen its warmest year-to-date on record, with 10 states - including Texas, California, and Arizona - experiencing their warmest start to a calendar year.
From now until the winter solstice, most U.S. states will lose 3 to 3.5 hours of daylight. Earth's 23.5-degree tilt means the Northern Hemisphere is gradually turning away from the sun, bringing longer and darker nights.