קל"ה - הלכות תפילה א:א
Dew and rain are made from one's Torah study. That is, when one learns Torah at night rain is made, and through one's learning during the day dew is made. Therefore, during the winter [when the nights are very long] when one mostly learns during the night time, we say [during Shemona Esrei] He who causes the rain to fall and during the summer time we say He who causes the dew to fall.
However, because of study that isn't befitting and doesn't rise up to the Shechinah, one causes the rains to stop, Heaven forbid.