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."
Sahih al-Bukhari Book 65, Hadith 343
← Previous Hadith Five things have passed, i.e. the smoke, the defeat of the Romans, the splitting of the moon, Al-Batsha (the defeat of the infidels in the battle of Badr) and Al-Lizam (the punishment)'.

Sahih al-Bukhari Book 65, Hadith 342 Next Hadith → It is a sign of having knowledge that, when you do not know something, you say: 'Allah knows better.' Allah said to his Prophet: 'Say: No wage do I ask of you for this (Qur'an), nor am I one of the pretenders (a person who pretends things which do not exist)' (38.86) When the Quraish troubled and stood against the Prophet (ﷺ) he said, "O Allah! Help me against them by afflicting them with seven years of famine like the seven years of Joseph." So they were stricken with a year of famine during which they ate bones and dead animals because of too much suffering, and one of them would see something like smoke between him and the sky because of hunger. Then they said: Our Lord! Remove the torment from us, really we are believers. (44.12) And then it was said to the Prophet (by Allah), "If we remove it from them. they will revert to their ways (of heathenism)." So the Prophet (ﷺ) invoked his Lord, who removed the punishment from them, but later they reverted (to heathenism), whereupon Allah punished them on the day of the Battle of Badr, and that is what Allah's Statement indicates: 'Then watch for the day that the sky will bring forth a kind of smoke plainly visible...we will indeed (then) exact retribution.' (44.10).

Sahih al-Bukhari Book 65, Hadith 344