At that time. — i.e., when the lion and destroyer of Jeremiah 4:7 should begin his work of destruction.
A dry wind. — Literally, a clear wind, the simoom, the scorching blast from the desert, coming clear and without clouds. Other winds might be utilised for the threshing-floor, but this made all such work impossible, and was simply devastating, and was therefore a fit symbol of the terrible invader.