The highest frequency that can be received via sporadic-E depends on how intensely ionized the cloud is. Next affected are TV channels 3 and 4, followed by TV channels 5 and 6, and then the FM broadcast band. Only in very rare cases will sporadic-E propagation reach TV channel 7 or higher. This means, for example, that you could be seeing distant TV stations on channels 2 and 3 yet conditions might be normal on other channels and the FM band. During sporadic-E propagation, signals can abruptly appear or disappear.
Signals are usually very strong during sporadic-E .
Tropo propagation is caused by temperature inversions in the troposphere, the region of the atmosphere closest to Earth where all weather takes place. A temperature inversion is a point where air temperature actually increases with elevation. These often form along stationary or occluded weather fronts. The effects of tropo propagation increase with frequency. Tropo is far more common on the FM band and TV channels 7 and higher than it is on channels 6 and lower.