What does Psalm 18:9 mean?

A poetic description of God's powerful response (Psalm 18:7–8) to David's pleas continues. David portrays the Lord as descending from heaven like a black cloud. The imagery suggests the sky was dropping down to the earth, engulfing it in an ominous darkness. Earlier passages compared God's wrath to an earthquake, and possibly to a volcanic eruption. This verse might follow that pattern, depicting the dust, smoke, and ash of such an event. An alternative image might be that of a massive tornado.

When God judges the wicked in the tribulation, the seven-year period that follows His removal of Christians from the earth (1 Thessalonians 4:13–17), thick darkness will fall upon the earth. Joel 2:2 describes this period as "a day of darkness and gloom, a day of clouds and thick darkness." Revelation 16:10 prophesies that during the fifth bowl judgment the kingdom of the beast will be plunged into darkness and people will gnaw their tongues in anguish. Whereas the events of Revelation 16:10 are at least partially literal, David's description of God's descending like a black cloud is figurative. Nevertheless, both descriptions provide a terrifying account of God's furious judgment.