I've also been doing a lot of animating in Bryce 3D, and I think I may know what you're talking about. It sounds to me like it's probably not the clouds causing the problem in the example you mentioned, but more likely the material used on the city (I'm assuming the "twinkling" only happens on the city-object). I've found that, in the Materials Lab, setting the "Bump Height" value of the offending object's material to a very low value, or even to 0, will usually make a great improvement. It's also possible to make finer adjustments in the Deep Texture Editor with the individual component(s) driving the bump channel. Maybe you could try a test plop-render animation of the city with it's bump height set to 0 to see if that helps. Also, you should always use anti-aliasing when rendering animations, or things can get real messy (I'm assuming you are). -jakeh