FM broadcast band signals are just one of the many radio services that can be affected by this summer-month ‘Ducting’ phenomenon. The lowest region of the earth’s atmosphere is called the Troposphere. This is the region where most weather occurs, and where VHF & UHF communications normally travel and die out, shortly beyond the horizon.
However, usually in September and October, (I’ve noticed it as early as mid-July) a condition develops, allowing radio signals to be heard hundreds of miles further than intended. Gulf coast states see it often, Atlantic seaboard, Great Lakes, and Mississippi valley areas see it occasionally. Although these ducts ordinarily form over water, they can form over land, as well, creating a great deal of listener confusion, and broadcaster dismay.
A weather anomaly called a Temperature Inversion is at the root of the problem. Normally air temperatures & pressures are highest at the Earths surface, and then decrease up and through the atmosphere. But, when a stable high pressure system of cooler air, becomes overrun by a warm air mass, a Tropospheric Duct is a result. Radio waves can become trapped below the warm air, traveling great distances through the cooler air, with little loss.
Other stations, from way out of the area, on or next to our frequency, that you could normally never hear, come banging in on top of us, with the intensity of a local station.
The degree and duration of the interference will vary. So, if you experience this symptom, and it is summer, think "Ducting".
- Eimite B. Wright
