Weather, n. The climate of an hour. A permanent topic of conversations among persons whom it does not interest, but who have inherited the tendency to chatter about it from naked arboreal ancestors whom it keenly concerned. The setting up of official weather bureaus and their maintenance in mendacity prove that even governments are accessible to suasion by the rude forefathers of the jungle.
The rest of Bierce's entry (a poem) is hilarious and can be found on Manjusha's site.
2005 Update: The variable on which a farmer focuses attention and activity.