Dear Tom,
You are always referring to Chicago’s percent of possible sunshine. How is that measured?
Tim S. Schaumburg

Dear Tim,
Sunshine data is measured by an instrument called a sunshine recorder. It consists of two photoelectric cells, one exposed to sunlight and the other shaded. An electric signal is generated when the sun shines on the exposed cell, a signal that is not balanced by the shaded cell. This imbalance trips a relay and activates a recorder that tallies the daily minutes of sunlight. The daily percent of possible sunshine is the ratio of the actual minutes of recorded sun to the total possible sunshine minutes from sunrise to sunset. December has the least amount of possible sunshine with 17,130 minutes, while July has the greatest with 27,642 minutes. Over the course of a year, the city historically receives about 54 percent of its possible sun. Chicago climatologist Frank Wachowski operates the Midway Airport sunshine recorder, the only one in Chicago, and provides the information. The city’s sunshine records began in late 1893.