Tropospheric ducting allows an FM radio signal to travel hundreds of miles farther than the usual 70 to 100 miles. Tropospheric ducting occurs most often during the summer months and sometimes in the presence of large storm systems. This phenomenon can last for a few minutes or for several hours. Back in 1993 I was able to receive, by chance, a Florida radio station in Western PA. It came in loud and clear for over three hours and then it was gone in minutes. I've picked up police radio transmissions from Southern California. The Ohio Dept. of Transportation has had interference on it's radio frequencies from Gulfport, Mississippi and Ocean Springs, Maryland due to "tropo ducting."
Normally, radio waves in the VHF, UHF, and microwave frequencies
travel by line of sight with about a 15% extension past the horizon due to refraction. Certain weather conditions increase the refraction and can eventually produce ducting. When a warm moist air mass gets trapped between cool dry air above and below it a tunnel or duct is formed. This is more pronounced when the temperature rises with an increase in altitude, which is the opposite of normal atmospheric conditions, and if the humidity drops at the same time the temperature is increasing. The radio signal gets trapped inside the warm air mass and can get refracted or bounced up and down at a very shallow angle against the cool dry air above and below it. The signal can travel for hundreds of miles with very little loss in strength. Large slow moving high pressure systems allow for some of the best conditions of "tropo ducting."
The most common but not the only places this occurs is in the Southeastern US, the lower Midwest and one of the best places is the Eastern Pacific Ocean. Many contacts have been made by ham radio operators from 144 MHz up to 5.6 GHz over a distance of 2500 miles, from California to Hawaii. The record is held by Gordon West, WB6NOA, an amateur radio operator from Southern California.
Tropospheric ducting occurs within the troposphere, the lowest layer of the atmosphere, usually a thousand feet or less above the surface of the earth.
"Propagation in the Troposphere."The ARRL Handbook for Radio Amateurs.Newington CT: ARRL, 2001
Russell J. Anderson."What is: Tropospheric Ducting?"(http://www.dot.state.oh.us/dist4/Radio/tropo.htm)
Gordon West.Technician No-Code Plus Lincolnwood, IL: Master Publishing,Inc. 1997