In Weather > Advanced Weather > select various cloud types and set different levels for each type by using the + and - icons and drag the boxes to size using the mouse to increase coverage.
If you drag a box nearer to ground level the clouds will be lower.
Note that setting "Deviation" towards zero results in more, (denser), clouds.
Its fun to experiment with the "Advanced Weather" settings in FS 2002...

....!
Cirrus = High level clouds.
Cumulus = Medium level clouds.
Stratus = Low level clouds.
Thunderstorm = Forget flying....LOL...!
Note that under average flying conditions , visibility will be between 20 - 30 miles, generally less, rarely more!
Cheers...
Paul.
(England).
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.