After Effects qustions, solids?

[Espnetboy3]

I will do my best to explain my situation. I have a rendered terrain and cam movement about 3 seconds long. I used a mask in AE to do a sky replacement. I created storm clouds in PS and put this into the AE comp. I have 2 clouds one in front of the other. I wish to have lighning flash in between the clouds(SO it apears to be in the storm cloud). What is the best way to accomplish this? I just added the lightning effect to the back cloud so there would be one storm cloud in front. Do i need to use a solid or matte. I really dont know about using solids to create stuff. I also want the lightning to enter at 20 frames and leave at 28 frames and because its effecting my actual cloud layer i cant just stop the effect. Any help would be great thanks