The Illinois State Water Survey maintains a network of 19 soil temperature sites across the state that measure temperatures at 4 and 8 inches. You can look at maps for 10 am, any hour of the day, high for the day, low for the day, under sod, and under bare soil. You can find all their data at this site: http://www.isws.illinois.edu/warm/soiltemp.asp
Here is the 4-inch soil temperature from yesterday. It’s always a day behind so that they can upload the data and do quality control checks. The data now arrive hourly. My mistake – they used to upload the data once a day and do QC but now it is more timely. As you would expect, soil temperatures change more slowly than the air temperatures.
And here is what it looked like two years ago after a record warm March. I chose April 2, 2012 for the same time of day and depth. As you can see, the soil temperatures were about 12 degrees warmer and USDA NASS reported that 5% of the corn crop had already been planted by that date.
Four inches is a reasonable depth for evaluating “soil weather” conditions for sprouting seedlings, but gives no information about geothermal/insulative effects that would be relevant for say, fish ponds and sunken aquaponics systems, sunken greenhouses, etc. Is there any place (esp. near Chicago) where temperatures at 2 to 3′ depth are monitored. I am guessing I could get close just by calculating the annual average at 8″ (or even 4″). Is there a way to simply download a full year’s worth of data (from St Charles in my case) in CSV or other spreadsheet-compatible form?