Hi Luke...!
I generated a thread dealing with setting up the Weather some time ago...
...but as always....I can never find it... :'(...!
Anyway, try this sequence...>>>
Settings:
Weather:
Clear Skies: Start with clear skies...
User Defined Weather:
Customise Weather: Set the conditions to suit...
Advanced Weather: Set the conditions to suit...
Ok
OK
OK
Set up each of those options to suit yourself...
Sorted...!
...lots of dense 3D clouds will trash your frame-rates.
Remember, most flying days in GA aircraft, the weather is fine, a few whispy clouds, slightly misty, and visible distance distance set at around 10 miles...
Lock your frame rate to your average frame rate, (Min-Max)...!
If you frame rate averages between 25 and 40 FPS, lock it at 30 FPS for smooth running...

...!
Cheers Luke...!
...don't go overboard with impressive weather if you want decent frame rates and smooth flying...
...unless you really want to impress you mates with fantastic screen shots...

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...

...!
Remember. If it rains, your aircraft will get wet...!
Paul....

...!
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.