What is the significance of the dust and earth that will descend from the sky?
Rashi and Targum Yonasan: The (strong) 1 wind that blows after the inadequate rainfall will blow the dust that has not been settled by the rain, on to the crops which are still wet from the rain, turning it into mud, which will ultimately dry and make them rot.
Rashbam: Instead of the wind blowing down rain from the sky, it will blow down on you dust and earth 2 from the mountain-tops, which are dry. And the dust and the earth will pierce your eyes and blind you, and will force you to flee before your enemies (as the following Pasuk 25 explains).
See Torah Temimah, note 21.
Rashbam: Because "Matar" is a general term, applicable to things that descend (rain down), and not exclusively to rain, as we find in Vayeira Bereishis, 19:24 and in Tehilim, 11:6.
Since when do dust and earth descend from the sky?
Ta'anis, 9b: This is not meant literally - only since the wind raises them into the air, it appears as if they come down from the sky.
