Stupid noobie type question about ?clouds?

Put up a Wizards tower for the second time, but this time it starts at a ground level at about 100 meters, so now the top is at about 270 Linden meters. When I stand at the top two levels (I think 268 and 278 meters), there is a lighting effect like a security sweep in RL bonded warehouses. Couldn't find a source from the neighbors and finally guessed it is the effect of the clouds passing through. Can anyone confirm this or give a better theory? even accept facts, although they aren't as much fun...

You can choose not to see "Classic Clouds" by ticking a box in the sky settings.

I don't like to see them, myself, although for aviation it's fun to fly into and out of clouds.

Yes, it's an effect of the clouds, and I agree that it does look like a light sweep with a sharp edge.

It's a really good idea not to build anything habitable between 150 and 200 Meters altitude. The "Classic Clouds" fill that layer of the sky with huge, randomly moving particles. While you can turn off rendering them on your client, everyone else still sees them. Bring in the cloud layer inside a building is rather like being in a hotel room after someone lets off a smoke grenade. You can't get rid of the clouds or exclude them from an area. Being in the cloud layer can cause issues with camera control, trouble clicking on things, and can cause lag.