Sunlight 

"Sunlight is thought to bring on attacks, some patients link outbreaks to prolonged exposure to strong sunlight from late spring through to early autumn."
  Got a source? All I've heard and read indicates the opposite, that outbreaks occur most during the Winter months, and that "some patients" have found sunlight to the palms to be a good preventative. Unfortunately, I do not myself have a link ready.  It may be worth removing.