Why were the enemy planes not flying in Hemingway's "Old Man at the Bridge"?

The narrator reveals at the end of the story that the enemy Fascist planes were not flying because it was an overcast day with a low cloud ceiling. It strikes the narrator, a soldier fighting against the Fascists, that the bad weather along with the fact that cats could take care of themselves (and therefore didn't need the old man's help) was "all the good luck the old man would ever have."
The Fascist troops are on the offensive, and the man, unable to move any further from exhaustion, will be caught up in their advance. If planes had been able to take to the air, they probably would have bombed the town of San Carlos where the old man had lived. They may have even strafed the road as he tried to escape, shepherding the animals he could take with him. The irony is that he is worried about the deaths of the animals, when he himself will very likely be killed. Indeed, they are probably more likely to survive than he is.