Both Jane's WWII Fighters and Parsoft/Activision's Fighter Squadron:Screamin' Demons Over Europe had volumetric clouds. That is to say nice puffy 3D clouds that you could fly through and hide in. WWIIF's clouds weren't too friendly on framerates. FS:SDOE were good, but you have to remember that both were NOT massively multiplayer.
The main obsticle to overcome is having the clouds appear in the same place on everybodies system, assuming that they are somewhat dynamic and will move. Not much different than other strat elements but just adds more cpu cycles to the mix.
I am really looking forward to clouds in a online WWII flightsim, something long overdue. Not only will they provide a tactical element but they will also help convey a sense of space for us to fly in.
Wouldn't it be cool if the clouds were part of a simple weather model in which they would form, drift, and dissipate. Perhaps a cloud cover would form over an enemy field, something a reconnaissance mission would have to discover.
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Wicked
The Pale Horses
487th Lil Bastards