TROPOSPHERIC BENDING

The lowest 13 to 20 km (8 to 12 mi) of the Earth’s atmosphere comprise the troposphere. This region has an effect on radio-wave propagation at certain frequencies. At wavelengths shorter than about 15 m (frequencies above 20 MHz), refraction and reflection can take place within and between air masses of different density. The air also produces some scattering of EM energy at wavelengths shorter than about 3 m (frequencies above 100 MHz). All these effects generally are known as tropospheric propagation, which can result in communication over distances of hundreds of kilometers.
A common type of tropospheric propagation takes place when radio waves are refracted in the lower atmosphere. This is most dramatic near weather fronts, where warm, relatively light air lies above cool, more dense air. The cooler air has a higher index of refraction than the warm air, causing EM fields to be bent downward at a considerable distance from the transmitter. This is tropospheric bending. It is often responsible for anomalies in reception of FM and TV broadcast signals.