`How every fool can play upon a word," remarks Lorenzo in The Merchant of Venice. . Let me illustrate the point by listing some of the countless weather metaphors used to convey ideas which, in themselves, may have no connection with the elements.
Many are related to the sea. Having one's "weather eye open", for example, has no connection with reading a certain column in The Irish Times; ; it is to observe the elements very closely, and by implication, to look out for squalls - or in everyday life, to have one's wits about one. To "get the weathergage" is also nautical in origin, and means to get the upper hand. In the days of sail, a fighting ship that had the weathergage of the enemy was on the windward side of the opposing vessel, and therefore had a tactical advantage. Walter Scott in his poem Rokeby has a suitor trying to best his rival:
Were the line of Rokeby once combined with mine, I gain the weather-gage of fate. To "keep the weather of" means something similar - to get around, successfully persuade, or overcome. Shakespeare in Troilus and Cressida has Cassandra try to dissuade the ill-fated Hector from going into battle - but to no avail: Mine honour keeps the weather of my fate;
Life every man holds dear; but the dear man
Holds honour far more precious dear than life.
A person who is fickle is often figuratively described, for obvious reasons, as a "weathercock" - as for example in the case of Geoffrey Chaucer's donna mobile:
There is no faith that may your heart embrace: But, as a weathercock that turns his face with every wind, you change.
"Fair weather friends", in this sense, are weathercocks. They are those who stick close by when times are good, but are nowhere to be seen when fortunes change, and one is left to "weather the storm" all on one's own. Their absence, of course, may be due to the fact that we are making "heavy weather" of a situation, as when we seem to find a simple task unnecessarily hard. The reason for this, in turn, may be that we are "under the weather" - unwell or out of sorts, often because we have indulged too much the night before.
A fair-weather friend, on the other hand, may sometimes "make fair weather", which is to flatter or conciliate - as Shakespeare's Duke of York proposed to do: "But I must make fair weather yet awhile," he says, "till Henry be more weak, and I more strong."