Albert Barnes Commentary Psalms 36:5

"Thy lovingkindness, O Jehovah, is in the heavens; Thy faithfulness [reacheth] unto the skies." — Psalms 36:5 (ASV)

Your mercy, O Lord, is in the heavens — This commences the second part of the psalm: the description of the character of God in contrast with the character of the wicked man.

The meaning here is evidently that the mercy of God is very exalted; it reaches to the very heavens, as high as the highest object that one can conceive.

Thus, we speak of virtue as “exalted,” or virtue of the “highest kind.”

The idea is not that the mercy of God is “manifested” in heaven, for mercy, being favor shown to the guilty, has no occasion there. Nor is the idea that mercy, as shown to humankind, has its “origin” in heaven (which is indeed true in itself). Instead, as explained above, it is of the most exalted nature, as high as one can conceive.

And Your faithfulness — Your “truthfulness;” Your fidelity to Your promises and to Your friends.

Reaches “to the clouds.” The clouds are among the highest objects. They rise above the loftiest trees, ascend above the mountains, and seem to lie or roll along the sky. The idea here, therefore, as in the first part of the verse, is that it is elevated or exalted.