The growing season in Ravenswood suffered a frost last night. The Bulldog’s resident weatherman, Tom Vollman, formerly of Kansas City weather fame, slipped through our clutches when we tried to pin him down about the temperatures that may have led to a frost during Friday morning’s rush hour. But there should be no doubt that early this morning the temperature and other characteristics of a frost were in the neighborhood.
We suffered a frost. (Vollman said if there was any doubt about the Friday event, there wouldn’t be about the Saturday frost. So I guess he has us there.)
According to the Weather Underground station located in Irving Park, about two miles west of most of the neighborhood, the temperature dipped below the freezing point for pure water, 32° F, at 2:08A this morning. Temperatures are rising back to 32° F this hour, 8:42A. Also, the wind was calm, with gusts below 10 miles per hour and the wind holding at no more than 6 mph, with many readings of 0 mph. That leaves us with 6:34 hours of below freezing temperatures.
The term freeze and frost, while similar, describe different weather conditions. With sustained low temperatures and light winds, this is most likely a frost. A freeze happens when the temperatures dip lower and the winds generally increase. A frost does not occur every year because sometimes a killing frost will happen first.
A Weather Underground station in Rogers Park, just north of the Rosehill Cemetery, also reports frost-like conditions, with calm winds and temperatures dropping below 32° F at 2:44A and remaining there till 8:33A this morning, a period of 5:49 hours.
The final local Weather Underground station in Uptown reports temperatures broke 32° F at 6:44A and remain at 31° F at the time of the most recent reading, 8:47A. That indicates a frost period of at least 2:03 hours. That station is located just east of Graceland Cemetery.
The current temperature at WGN TV studios is 32.9° F, indicating the frost has passed in that area.
Frost conditions are very localized. A frost happens when the winds are calm and the sky is clear. Both these conditions happened early this morning. In addition, the temperatures hold to just below freezing. Under these conditions colder air settles near the ground and warmer air rises.
The temperature may vary from ground-level freeze to a warmer temperature just two meters above the ground. A frost such as we had last night is likely to end the growing season of most small plants and containers. If you protected your plants last night they may continue to grow. Larger and hardier plants and plants that still have tree leaves protecting them may also have survived.
With the end of the frost, we enter a period of waiting for additional weather conditions: the killing freeze, a heavy frost and the first snowfall of the season. The first snowfall can take two forms, precipitation that does not cover the ground and ground cover.
National Weather Service, which maintains records of these events since 1885, says only two Novembers have had no snowfall, most recently in 2001, but also in 1999. Several have only reported trace snowfalls.
Here is what we wrote previously about this:
According to the Old Farmers Almanac, Chicago experiences its first frost, on average, October 26. The Illinois Climatologist says, on average, October 14.
The earliest killing frost in Chicago, according to the National Weather Service, happened on October 2, 1971. The earliest frost happened on September 22, 1974, while (sit down for this one) the earliest snowfall in the city happened on September 25, 1928, the earliest measurable snow: October 12, 1909.
According to the data from the Illinois State Weather Service, the latest frost at the Chicago Botanical Garden over the 20 year period from 1981-2000 happened on November 6.
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