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There are a couple of mechanisms that will affect VHF and UHF propagation. The first involves weather, specifically temperature. For several reasons (fronts, heating over a cold surface, etc.) a layer of air of one temperature will trap a layer of a different temperature. Since the density of air depends on temperature, at the boundary between the two layers will be a rapid change in density. TV and FM signals approaching this region will get bent back towards earth instead of continuing out to space. This is tropospheric scattering. Sometimes, signals can get trapped in this region until another abrupt change knocks them out. The signal has travelled a lot farther than normal, and this is called ducting. Both tropo scatter and ducting affect a wide range of frequencies at the same time.
The other mechanism involves the ionosphere. Normally, ion density is not sufficient to bend VHF/UHF signals back down to earth. Occasionally, extra clouds of ionization form at the bottom of what's called the E-layer that are sufficient to bend back these signals. This effect is called sporadic-E propagation, and sets up very long distance reception, as in getting Kansas stations here. Often, local stations will be wiped out by the distant one, as the distant one has suffered very little loss along the way. This tends to be a VHF only event, and cuts off very abruptly above a certain frequency.
Rain and snow will affect UHF propagation.
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