This can have an incredible impact on frequencies in both the VHF and UHF range. Weather fronts are usually the cause of this phenomenon and the radio waves are introduced between the inversion layers and the ground. The inversion layer is where cold and warm air layers meet. This forms a duct or tube in the atmosphere where radio signals enter and are bent. Signals then follow this duct and drop out at the other end.
Tropospheric effects can show up at any time, in any season.
However, late spring and early fall are the most favored periods. Signals from far outside normal communication range can suddenly appear causing interference with local repeaters. When this happens mobiles and hand helds will have problems trying to overcome these signals or their communication can be blocked altogether, depending on the strength of the interfering signals. It may linger for weeks, and cause periodic interruptions. In most radio applications there is usually a single transmitter or repeater used to give radio coverage in a given area.