In the Full mission builder in the object list, you have a tab called "local weather". You place it on map like any other object and then adjust the parameters. I like to use Cummulus and extend the range a lot. By default the clouds will be placed in a very small circle. To extend the area covered, you edit the first two numbers in the second tab. I usually add two zeros. the third number is altitude.
The third tab is all messed up but sometimes you an make some adjustments to cloud density, winds and few other things.
You can create multiple different layers and place them at different altitudes. It looks cool and the turbulence is crazy sometimes.
of course, that part of the game was declared "beta" from the start and it crashes some times.
Just play with it.