ASK TOM: Measuring sunshine

Dear Tom,

I’ve read that July of 1916 was the sunniest month ever recorded in Chicago, logging 95 percent of possible sunshine. Did the sunshine recorders in use then follow use today’s standards?

–George Ballas, Morton Grove


Dear George,


Retired Chicago weather observer Paul Kubecka found that the instrument for measuring solar irradiance in 1916, the Maring-Marvin sunshine recorder (in use in Chicago until 1953) was less sensitive than today’s version. It required .37 langleys to initiate the recording of sunshine compared with to the current World Meteorological Organization standard of .17 langleys. It also had a slower response time, taking up to 10 minutes before beginning to log sunshine at sunrise and sunset and during passing cloudy intervals. If weather conditions were identical, today’s instrument would have recorded a higher sunshine percentage than the 1916 version.