Closeness to the Shechinah at Night

קל"ה - הלכות תפילה א:ב

The Torah that one learns at night has a greater chance of rising to the Shechina, both from man's side and from the Shechinah's side.  This is because at night a person is better able to nullify and attach himself to Hashem than during the day.  Night is a time when the world is much more still and the concerns of this physical world are not being pursued.  Also, the Shechinah comes close to the person who is learning Torah at night and listens to his voice as it is written, "Arise, cry out in the night at the beginning of the watches, pour your heart out like water before Hashem's face." (Eichah 2:19)  Specifically Hashem's face, because the Shechinah is so close, and [the person learning Torah] he is literally in front of Hashem's face.  At night the Shechinah dwells in the lower worlds, and the essence of her comfort is through being close to one who is learning Torah.

What Creates Rain and Dew?

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.