Rain without any working of the elements (sine operatione principiorum naturae) would be a miracle of the first rank, which we do not venture to pray for when we pray for rain. No doubt, St Thomas means 'without any working of the elemental powers sufficient of itself to produce the effect.' There seems to be a certain law of parsimony about miracles, God using natural causes so far as they will go, and eking them out by divine or angelic power, when of themselves they would go no further to His purpose. In my Oxford and Cambridge Conferences, First Series, 1897-1899, there is a discussion on the compatibility of miracle with the invariability of the laws of nature.
Of God and His Creatures: 3.101