We moved into a new house last year and had new service installed, we also noticed every time three drops of rain fell the signal would go out. I checked the signal meter and it was not that strong so we did call Dish to come back out and correct, the original installer left fasteners loose and did not have it aligned to get the strongest signal, they re-pointed it and tightened everything down and it has been fine since.
I knew enough to know that it should not lose signals as much as it had because we had Dish at our last house with a lot less problems, plus my experience with setting up the portable dish while camping I soon learned the lower the signal strength the quicker it would go out if the weather was bad.
Since you just had it installed call them to send someone back out and get it corrected. If they cannot get a clear line of sight from your property you should be able to be released from the contract as well.