7 But this man rightly understood what mercy he should pray for from God. “Your Mercy, O Lord, is in the Heavens; and Your Truth reaches even to the clouds.” That is, the Mercy which You give to Your Saints, is Heavenly, not earthly; is Eternal, not temporal. And how couldest Thou declare it unto men? Because “Your Truth reaches even unto the clouds.” For who could know the Heavenly Mercy of God, unless God should declare it unto men? How did He declare it? By sending His truth even unto the clouds. What are the clouds? The Preachers of the Word of God....Truth reached even to the clouds: therefore unto us could be declared the Mercy of God, which is in Heaven and not in earth. And truly, Brethren, the clouds are the Preachers of the Word of Truth. When God threatens through His Preachers, He thunders through the clouds. When God works miracles through His Preachers, He lightens through the clouds, He terrifies through the clouds, and waters by the rain. Those Preachers, then, by whom is preached the Gospel of God, are the clouds of God. Let us then hope for Mercy, but for that which is in the Heavens.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)