A Weatherman is a member of the Weathermen Underground. (Inactive.) The Weathermen Underground emerged from the New Democratic Left and SNCC conference of 1968, comprising a small core group of activists (civil rights and peace activists, proponents of direct action and civil disobedience). The Weathermen Underground then banded into self-selected affinity groups---the core group much smaller than public image allowed---and organized direct actions. The group/movement took their name from the Dylan lyric, "you don't need a weatherman to know which way the wind blows," and claimed responsibility for a number of direct actions in protest of the Vietnam War.
Bernadine Horn is a high-profile activist and weatherman who was wanted by the FBI for over a decade. Bill Ayers is another. (After turning themselves in, in the 80's, both Ayers and Horn were exonerated.)