In Chapter 3 of A Separate Peace, why is Finny described as being like the weather?

In Chapter 3, Gene says, "Although he was rarely conscious of it, Phineas was always being watched, like the weather".

Finny has a way about him which attracts the attention of others.  His impulsive creativity draws others to him, and, like Gene, they find themselves following his lead despite themselves.  When Finny begins to hatch the idea of a new game called "blitzball" in his mind, the boys who are "up the field...at badminton (sense) a shift in the wind; their voices (carry) down to us, calling us...When we (don't) come, they (begin) gradually to come down to us". 

Although he exercises a tremendous influence over the other boys, Finny is the opposite of manipulative.  Like the weather, he just is; his life, "ruled by inspiration and anarchy", holds an undeniable attraction to the others.  When he begins one night "to talk abstractedly about...the Super Suicide Society of the Summer Session...the half-dozen friends, who (are) there in (the) room listening (begin) to bring up small questions on details without ever quite saying that they (have) never heard of such a club".  Consequently, they "(sign) up as 'trainees' on the spot", and faithfully attend the club's meetings every night.  As Gene observes in his case, even though he is "acting against every instinct of (his) nature, (he goes) without a thought of protest", night after night, captive to the inexplicable influence of Phineas (Chapter 3).