Is the weather system of World of Warcraft random?

No, I've been killing trumpets in Thorn Valley for a long time.

The general weather system is a three-hour cycle.

In other words, it must be sunny for 3 hours.

In Thorn Valley, it's raining, and in Neurisse, it's a sandstorm.

Stay long and you'll know.

But relatively speaking, the refresh time of some maps is 18 hours.

Such as barren land.