In the eleventh verse of Surat az-Zukhruf, rain is defined as water sent down in due measure. The verse is as follows:
"And He who sends down (from time to time) water from the sky in due measure, and We raise to life therewith a land that is dead. Even so will you be raised (from the dead) (Surat az -Zukhruf, 11)
This "measure" mentioned in the verse has to do with a couple of characteristics of rain.
Now the elusive search for the "meaning" of "measure" begins.