We always have issues with ours when it rains and so do our in-laws who live about an hour away but use the same receiver.
The signal is of borderline acceptable strength. At the frequencies used for UHF TV, the damp atmosphere can result in a small but significant amount of attenuation to the signal when it rains. If the strength is only
just good enough during dry weather, that's why you'll get the poor reception when it's wet. With analog TV, a slight reduction in signal strength just gives rise to a slightly less perfect picture, with more noise. With digital, there's a "cliff edge" effect, where it's perfect down to a certain point, then parts of the digital signal start getting lost and it breaks up.
Alternatively, you might even have water getting into the connections at the antenna when it rains causing attenuation. Either way, improving your antenna system should provide a cure.