How do you measure the percentage of possible sunshine in Chicago?

by: WGN-TV Weather Team

Job :


Dear Tom,
You’re still referring to Chicago’s possible sunshine percentage. How is this measured?
Tim S. Schaumburg

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

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