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 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.