After 112 years of measuring daily sunlight in the Twin Cities, the National Weather Service is going out of that business Thursday, pulling the plug on the sunshine detector at the Chanhassen office.

Actually, the nearly 60-year-old Foster-Foskett sunshine switch conked out Sept. 17, illustrating part of the reason why it, and 32 others of its vintage in the Weather Service's central U.S. region, are becoming weather history.

"We just don't have the old parts to fix it with," said Michelle Margraf, meteorologist and something of a personal attendant for the sunshine switch at the Chanhassen Weather Service office. "Not a lot of people use the data, and we didn't want to spend money to refurbish it."

Maybe more important, the significance of sunshine itself has changed in recent decades. Daily minutes of sunlight have long interested farmers, chambers of commerce in the South and, of course, vampires. But today it's the energy in the sunlight that matters for solar power, water and air quality, forestry, crop research and other pursuits.

In effect, the sunshine detectors only measured whether the sun was on or off. Where buildings were in the way, the accounting was often tweaked by individual observers, resulting in what Tom Townsend, the Weather Service's regional observations program manager in Kansas City, called "fuzzy data."

Today there are thousands of far more sophisticated solar devices across the United States measuring sunlight's intensity, with all its minute variations. That might include sunlight reflected off clouds, one reason a day with clouds might actually bring more solar radiation than a day with a clear blue sky. The sunlight switch at the Weather Service might not detect that.

The U.S. Climate Reference Network, a sister agency of the Weather Service, has 114 stations measuring solar radiation and is one of several enterprises that have overtaken the job of tracking sunlight.

DNR may inherit instrument

The local Weather Service sunshine detector, which combines a sort of solar panel with wires and a time counter Margraf calls an "odometer," will probably be turned over to the Minnesota Department of Natural Resources' climatology office. That agency has been tracking solar radiation with its own device since 1962, and won't be restoring the sunlight switch.

Assistant state climatologist Pete Boulay noted that for years the sunlight switch, while not as precise as some scientists would like, was ironically valuable when the weather got gloomy. For example, it put a measurement on the Twin Cities' longest sunless stretch: 15 days, from Oct. 30 to Nov. 13, 1972. Technically, that meant more than two weeks passed during which the sun never shone more intensely than 120 watts per square meter, the sensor's threshold for "on."

"Basically, that record might not ever get broken," Boulay said. "Maybe that's a good thing."

Bill McAuliffe • 612-673-7646