O’Hare International Airport and Chicago Executive Airport both recorded a low temperature above freezing overnight, but why is there frost on roofs and grass in Arlington Heights? Even standing water in Arlington Heights had a thin layer of ice on the surface.
Both airports are located a little closer to Lake Michigan, which tends to keep overnight lows a little warmer during cold months in Chicago. However, in warm months, the proximity to Lake Michigan might bring lower recorded overnight lows. That’s because the specific heat of water is higher compared to most other substances, such as soil and concrete. Specific heat is a scientific term describing the amount of heat required to raise the temperature of one gram of a substance one degree Celsius.
By the Lake during Spring? Chicago’s lakeshore is relatively warmer at night, but cooler during the day in Spring. Because of the specific heat of water, it takes longer for Lake Michigan to absorb heat (get warmer) compared to surrounding inland soil and air when considering daytime temperatures or Spring temperatures. It also takes lake water longer to cool below 32°F when inland temperatures are plummeting at night. That’s why it’s normally a little warmer near the lake at night in Spring. Inland temperatures can rise faster than lake shore temperatures on warm Spring days, so daytime temperatures near the lake are usually cooler than daytime inland air temperatures. If the wind is blowing from the east and carrying the cooled lake air further inland, then lake effect temperatures will stay cool further inland. The difference between Highland Park and Rockford can be as much as 10°F or more — with Rockford heating up faster during the day, while Lake Michigan keeps Highland Park cooler.
Here’s something a little frustrating for people who love warm temperatures in the Spring — the onshore breeze. When air on land heats up more than the air over the lake, the warmer air molecules inland rise because the air is less dense. The denser air over the lake then becomes more pressurized compared to the air over land, and the cooler lake air moves inland or creates an onshore breeze. Onshore breezes are common and more noticeable during warm May mornings in the Chicago area. May mornings often start out sunny and warm, but the onshore breeze that develops from the heated land air brings in cooler air and eventually the area clouds up and gets cooler by afternoon.
By the Lake during Fall? Because of the specific heat of water, it takes longer for Lake Michigan to lose heat (get colder) compared to surrounding inland soil and air when considering overnight temperatures or Fall temperatures. The Lake and lakeshore experiences relatively warmer temperatures at night and warmer air during the day if the atmosphere brings in cold air temperatures. If cold air invades the area, temperatures near the lake will be a little slower to drop.
In summary, whatever temperature direction the atmospheric air temperature is headed, the lake water and air temperature near the lake is going to be slower to respond. That means it tends to be a little warmer at night near Lake Michigan, and a little cooler near Lake Michigan during the day. On a grander scale, it tends to be a little warmer near Lake Michigan in the Fall and a little cooler near Lake Michigan in the Spring.
So what were last night’s lows that were recorded?
Chicago Executive Airport … 33.1°F
O’Hare International Airport … 34.0°F
Joliet … 28.4°F (South, but further inland)
Rockford … 28.9°F (further inland)
Waukegan … 32.0°F (lower than O’Hare because its further North, but warmer than Joliet, which is more inland)
And the latest day on record to freeze in Chicago?
The latest day on record to fall to 32°F was recorded on May 25, 1992.
The latest day on record to fall below 32°F was recorded on May 21, 2002 when the temperature fell to 31°F.
The latest day with an average low of 32°F at Chicago’s official weather station at O’Hare International Airport is March 29.
April 20 is the first day with an average high of 60°F. Chicago’s high temperature was only 43°F yesterday. A high of 49°F is forecast for today.CLEAR SKIES?  Weather Data for Thursday, April 21st, 2011
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