Record-high solar radiation in October

[November 13, 2010]  CHAMPAIGN -- Did it seem to you that the weather in October was quite a bit sunnier than in Octobers of the past? Do you remember last October as being rather cloudy?

If these are your impressions, you are correct, according to Bob Scott, director of the Water and Atmospheric Resources Monitoring program at the Illinois State Water Survey, a division of the Institute of Natural Resource Sustainability. Scott operates an array of weather sites across the state called the Illinois Climate Network, and one of the sensors on the stations measures solar radiation.

Solar radiation totals for October 2010 were higher than in any past October since sites were installed in 1989, while the solar radiation totals for 2009 were the lowest.

"Our sensors measure what we call direct solar radiation," Scott said. "This includes pure sunshine and reflected sunshine from clouds and blue sky."

October typically is drier than the spring and summer months that precede it, and sunshine in October is in a seasonal decline, with fewer daylight hours and the sun getting progressively lower in the sky.

"But this October was much drier than normal," Scott said. "Consequently, there were many more days with bright sunshine and fewer days with clouds and rain, as opposed to last October, which had many rain events."

During the 20-year history of the weather station in Champaign prior to last year, sunshine in October has averaged about 379 megajoules per square meter.

If you ignore the units, Scott said, the range of solar radiation values in October in Champaign through 2008 varied from a low of 328 in 2004 to a high of 414 in 1992. The very wet October of 2009 had a total of 264 units of solar radiation, while the very dry October of 2010 reported 449.

"Both prior maximum and minimum values were slammed in the past two years," Scott reported.

The current dry conditions have now continued into November, with strong indications that it too will see above-normal solar radiation.

As far as a cause for these events, "there is none," said Scott. "Our data by themselves are far too short of a record to suggest a cause, and they are on opposite sides of the scale from each other. Without more information, these data must be categorized right now as simple natural variability."

[Text from file received from the Illinois State Water Survey]