Jamieson, Fausset and Brown on 1 Kings 18:44

44. Behold, there ariseth a little cloud out of the sea, like a man's hand--The clearness of the sky renders the smallest speck distinctly visible; and this is in Palestine the uniform precursor of rain. It rises higher and higher, and becomes larger and larger with astonishing celerity, till the whole heaven is black, and the cloud bursts in a deluge of rain.

Prepare thy chariot, and get thee down, that the rain stop thee not--either by the river Kishon being suddenly so swollen as to be impassable, or from the deep layer of dust in the arid plain being turned into thick mud, so as to impede the wheels.