Raise a pumpkin spice latte to fall’s return — the autumnal equinox takes place at 8:03 p.m. tonight.
That’s when the sun shines equally on both the Northern and Southern hemispheres — making day and night nearly equal — before crossing the equator to head south. We will lose a little bit more sunlight each day until the winter solstice on Dec. 21, the shortest day and longest night of the year. The same phenomenon will take place again next year to mark the spring, or vernal, equinox on March 20, 2023.
Meteorologists, however, said goodbye to summer on Sept. 1, since they prefer to track weather in four, three-month increments instead of astronomical seasons, which are defined by equinoxes and solstices.
It’s the perfect time to observe Chicagohenge.
The rising and setting sun lines up perfectly with the city’s east-west street grid, creating spectacular photo opportunities as the sun is framed within Chicago’s skyline.
The earliest first freeze on record occurred Sept. 22, 1995, at O’Hare International Airport, the city’s official recording site. The latest was Nov. 24, 1931.
In 2021, the first freeze didn’t arrive until Nov. 2.
“The first freeze, which is when the temperature drops at or below 32 degrees, typically occurs between Oct. 11 and Oct. 12 across the Chicago suburbs and Oct. 21 to Oct. 30 in the city and along the lakeshore,” Brett Borchardt, meteorologist for the National Weather Service’s Chicago office, told the Tribune.
Chicago experiences higher temperatures longer than outlying suburbs due to the heat-island effect. Its location next to Lake Michigan’s warm waters explains why the city and nearby suburbs freeze later in the year than their farther-out counterparts.
Frost can develop on clear nights when the air temperature is in the mid-30s, but can be scattered. That’s why, WGN-TV chief meteorologist Tom Skilling says, the weather service “does not keep statistics regarding frost but instead uses the season’s first temperature of 32 (degrees) or lower to define the end of the growing season.”
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“The first trace or more of snow typically occurs on Oct. 30, but has occurred as early as Sept. 25 (1942) and as late as Dec. 5 (1999),” Borchardt said. “The first measurable snow (0.1 inch or more) typically occurs around Nov. 17.”
Last year, Chicago received its first measurable snowfall of the season on Dec. 28 — the latest date of a first snowfall in Chicago history going back to 1885.
La Niña is sticking around. The climate pattern that has a cooling effect on ocean surface temperatures is expected to remain in effect through the end of the year, according to the World Meteorological Organization. This is the third consecutive year — or a rare “triple dip” — of the weather phenomenon.
“It’s unusual for a La Niña to hang on through summer, and even more unusual to go into a third consecutive La Niña — which we would if it hung on into fall,” said Trent Ford, Illinois state climatologist.
Locally, the National Weather Service’s Climate Prediction Center is forecasting temperatures leaning above normal and “equal chances” of above or below precipitation from October through December.