Jamieson, Fausset and Brown on Nahum 1:3

3. slow to anger, and great in power--that is, but great in power, so as to be able in a moment, if He pleases, to destroy the wicked. His long-suffering is not from want of power to punish (Ex 34:6, 7).

not at all acquit--literally, "will not acquitting acquit," or treat as innocent.

Lord hath his way in the whirlwind--From this to Na 1:5, inclusive, is a description of His power exhibited in the phenomena of nature, especially when He is wroth. His vengeance shall sweep away the Assyrian foe like a whirlwind (Pr 10:25).

clouds are the dust of his feet--Large as they are, He treads on them, as a man would on the small dust; He is Lord of the clouds, and uses them as He pleases.