A thermal fade is a phenomenon of wireless signal degradation (fading

Fading

In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...

) caused by temperature and relative humidity factors. As the prevailing environmental conditions change, for example, from hot to cool, humid to arid or day to night, the electromagnetic waves refract

Refraction

Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

 differently thus altering the power distribution and possibly the direction of the signal. This is the same process that causes visible light to "bend" on hot days over a road or other surfaces.

This is a serious problem for AM

Amplitude modulation

Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...

 broadcasting but less so with FM

Frequency modulation

In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...

broadcasting. Thermal fading can occur in almost any frequency band.