Tropospheric Bending:

The lowest 13 to 20 km (i.e., 8 to 12 mi) of the Earth's environment include the troposphere. This area has an effect on radio-wave propagation at certain frequencies. At wavelengths shorter than around 15 m (frequencies above 20 MHz), reflection and refraction can occur in and among air masses of various density. The air also generates some dispersion of EM energy at wavelengths shorter than around 3 m (i.e., frequencies above 100 MHz). All these effects commonly are termed as tropospheric propagation, that can outcome in communication over distances of hundreds of kilometers.

A general type of tropospheric propagation occurs whenever radio waves are refracted in the lower environment. This is most dramatic close to weather fronts, where warm, associatively light air lies above cool, denser air. The cooler air has a high index of refraction than the hot air, causing EM fields to be bending downward at a significant distance from the transmitter. This is tropospheric bending. It is frequently responsible for anomalies in the reception of FM & TV broadcast signals.