John Gill Commentary 1 Kings 18:44

"And it came to pass at the seventh time, that he said, Behold, there ariseth a cloud out of the sea, as small as a man`s hand. And he said, Go up, say unto Ahab, Make ready [thy chariot], and get thee down, that the rain stop thee not." — 1 Kings 18:44 (ASV)

And it came to pass at the seventh time that he said, behold
there arises a little cloud out of the sea, like a man's hand
Either about the size or in the form of it; rain water comes out of the sea, and, being strained through the clouds and air, becomes fresh:

and he said, go up :
the meaning seems to be, that he should first go down from the mount, and then go up to that part of it where Ahab was:

say unto Ahab, prepare your chariot ;
bind or fasten the horses to it, as the phrase seems to signify:

and get you down ;
from the mountain where he was, to go to Jezreel, which lay low in a valley:

that the rain stop you not ;
on the road, that might be made impassable by it, signifying that such abundance should fall as would make it so.