Have to ask other than raising the humidity how can a lot of rain affect the hatch .? don't put a lot of time in monitoring heat and humidity. throw in the eggs turn it on and see how it comes out . If lower heat is one of the causes of insufficient fluid loss during incubation then I'm about to have a bunch of sticky chicks. Have to say it is nice when you do one thing and things get better . but of course the next set it all goes south again .
Rain can affect it, because it affects the natural surroundings around the bator, so that humidity from outside comes inside and that's where the bator is..