When he made a decree for the rain. God "made a decree for the rain" when be placed the fall of rain under fixed and unalterable laws. In some countries rainy seasons begin almost regularly on a fixed day in the calendar, while for several months in the year it is almost certain that rain will not fall.
Even where there is no such exact regularity as this, the rainfall has its laws, since there are maxima and minima which are never exceeded. And a way for the lightning of the thunder. God gave laws to the electric current, and prescribed the "way" that it should take in its passage from heaven to earth, or from cloud to cloud, or from earth to heaven.
Everything was ruled beforehand by Infinite Wisdom.