I have read that some missions you can only do during the rainy season, and also some that you can only do during the dry season. Can anyone elaborate on this?

because some enemies are only around during the rains and some are around during the dry consider it like "duck season" or "rabbit season"

as it says,

some places [the wild yoy can say] like Giza plai, forests, snow mountain, and tchida plain, there whether changes from time to time.

its mstly random, exiting the whole area to a new area can change the condition of the whether, sometimes you need to give some time then return for the whether to change.

then there is a nother thing: if you were in a rainy place, sometimes if you enter to a new area at that place the rain stops....but that doesnt mean its dry time, there well be cloud but no rain, the WHOLE whether changes only when you exit the Big place then coming back,

also in the westerland, sand storm can be present at times, and at times not, this can be changed only exiting from the area then entering again, might not work in the 1st time, but eventually will work.

also note: IN RAIN TIME MONSTERS BECOME STRONGER, AND NEW MONSTERS APPEARS, ESPECIALLY ELEMENTS MONSTERS [please refer to the topic in here on how to defeat them at 1st time encounter].

thats all.

Cool Thanks