Dear Tom,
How do meteorologists measure “possible sun” each day? What instruments do you use?
—Jim Daly, Mt. Prospect
Dear Jim,
Every day has a maximum amount of possible sun; the total minutes between sunrise and sunset. In Chicago, these values range from 549 minutes at the winter solstice to 913 on the summer solstice. Chicago’s sunshine is measured by a sunshine detector at Midway, operated and maintained by Frank Wachowski. It consists of two photoelectric sensors, one exposed to the direct rays of the sun and another shielded. When sun shines on the exposed cell, an electric signal is generated—a signal that is not balanced by the shaded cell. This electrical imbalance trips a relay, activating a recorder that tallies the minutes of sun. The actual minutes of sun compared to the maximum possible yields the percent of possible sun.