for which? warband or native
weather affects only three things in the game so far, they are horse/mount, bow/crossbow(artillery) and visibility
so far the three weather patterns are
Rain-may decrease visibility, decreases mount speed slightly(unnoticeable), slightly decreases damage for bow and crossbows again not noticable
Fog-lowers players visibility however has no affect on ai's since their built with a gps or got heat sensors if you want to put it like that
Calm- has no affect on anything, greatest visibility