Boosters aren't ideal/shouldn't be used as a cure for poor signal ideally. Simply because a booster wont just amplify the signal if fed with a poor signal in the first place,it will also amplify any noise/interference that may be present.That old cliche rubbish in rubbish out applies here.
Booster's/amp's should be used (in an ideal world)as part of a distribution system,where you're splitting a signal to muliple tv's to account for any losses due to splitting the feed,connections and cabling.Or to account for any losses incurred due to long cable runs etc.
If you are going to use an amp due to poor signal,then a masthead amp will give better results as it amplifies at the source,i.e as the name suggest's on the aerial mast itself.
Rain should only cause problems if it's extreme,even then it shouldn't really be an issue if you live in a good signal area with a decent/properly set up aerial in good condition. It may well be that either your aerial or cabling is shot and letting in moisture when it rains and causing the break up.
My advice would be get an installer in and have it checked out,messing about with booster's may be just throwing good money away after bad.