You should only lose the signal when a heavy rain or thunder storm rolls through your area. Rain drops are the same size as the 10GHz radio wave used to downlink the signal to your dish. The radio signal bounces off of the rain drops. Small rain showers should not effect the signal to the point of losing it.
If you have signal loss when it's not raining then you have a equipment setup problem. Thats a different can of worms.