Weather.

[Xirean]

You might just be getting unlucky when you load into an area. Weather changes rather frequently, but if you aren't in the zone very long you probably won't see it. I've had fog in a beast tribe zone every time I've been there for the past 2 weeks. My friend went there 5 minutes after I left and it was bright and sunny.

[Shriv]

The weather changes every 8 hours Eorzia time, midnight, 8am and 4 pm. (8hours ET = 23mins 20sec IRL time). There are certain activities in game that requires rain (Sightseeing Log, Fishing Log/quests, etc), so it is expected to see it steadily showing up.

You are probably just having a streak of bad timing to kind of always find yourself having to resort using an umbrella..