DAVID ZALAZNIK/JOURNAL STAR 
Ed Swigris, 75, turns his boat from the small dock the Illinois Valley Oarsmen maintain to head into the fog shrouding the Illinois River on Sept. 18. Ed Shimon, meteorologist at the National Weather Service in Lincoln, said the annual fall return of cool nights, warm days and light winds contribute to cold air, being heavier, settling into valleys laden with moisture.