I've selected this on some maps but what dose it do. It never seems to change throughout the time I'm fishing
The dynamic weather system is supposed to randomly change the weather while you're fishing. I'm not sure if it works or is bugged. But in truth it may be working. There are only three weather types in the game. If it keeps switching from sunny to cloudy you won't notice to much of a difference. That said, I've fished for hours and hours on dynamic and I've never once seen it randomly start raining.
True you wouldn't notice from clear to cloudy at all. Like you tho I've never seen it rain. Just wish they would do the same with the time of day. Would be ace to fish from the morning to the night