That’s what I would have told you (I am an engineer).
The most likely cause of the “interference” that we have had the past couple of mornings is more of an issue with temperature and inversion layers within the atmosphere and not impurities in the air. Days when we have a cold dry air mass the likelihood for ducting is significant when a warm air mass moves our way. If that warm layer is such that it gets sandwiched between two cold dry masses a duct is developed that can extend the range of a transmitter site by up to a hundred miles or more.
Today, I was able to identify the station that was interfering with WSMD 98.3; it was being extended from Clarksville, VA (south center of the state on the NC line). Matching that with weather data an inversion layer was apparent at the 15,000-foot level and extended to the 18,000-foot level with temperatures about 10 degrees higher then the layers immediately above and below it. This formed the duct that brought the interference up here that was simply more powerful then the local station's direct broadcast. As the day wears on and there are other variations with the temperature aloft the ducting will subside and normal reception will be once again available.
