It (i.e., the imagined smoke) was because, when the Quraish refused to obey the Prophet, he asked Allah to afflict them with years of famine similar to those of (Prophet) Joseph. So they were stricken with famine and fatigue, so much so that they ate even bones. A man would look towards the sky and imagine seeing something like smoke between him and the sky because of extreme fatigue. So Allah revealed:-- 'Then watch you for the Day that the sky will bring forth a kind of smoke plainly visible, covering the people; this is a painfull of torment.' (44.10-11) Then someone (Abu Sufyan) came to Allah's Messenger (ﷺ) and said, "O Allah's Messenger (ﷺ)! Invoke Allah to send rain for the tribes of Mudar for they are on the verge of destruction." On that the Prophet (ﷺ) said (astonishingly) "Shall I invoke Allah) for the tribes of Mudar? Verily, you are a brave man!" But the Prophet prayed for rain and it rained for them. Then the Verse was revealed. 'But truly you will return (to disbelief).' (44.15) (When the famine was over and) they restored prosperity and welfare, they reverted to their ways (of heathenism) whereupon Allah revealed: 'On the Day when We shall seize you with a Mighty Grasp. We will indeed (then) exact retribution.' (44.16) The narrator said, "That was the day of the Battle of Badr."