Not completely technically literate, but i have had it explaine to me in the past. Something about water being kind of like an insulator. The more there is, the more it blocks. Fog will make it fuzzy, heavy rain will cut the signal. As vegitation between the source and reciever dries out, the signal will return. At least thats my uneducated understanding.
Because DVB reception in Cambridge(shire) is, generally, shite. You could try getting a bigger aerial, pointing your aerial more directly at the transmitter (Sandy Heath, probably - there's a site out there that will tell you what bearing to be at; let me know if you want me to dig it out), or a masthead amp.