A cloud question

Flukey

I've often seen a phenomenom where a moving section of cloud seems to appear once it reaches one point and disappear having moved further on. A bit like a large band of clouds, where at any point in the middle you can see the solid clouds clearly moving forward, but the whole area of cloud never seems to move as such. If you watch the back edge you can see cloud continually forming as the clouds move forward, leaving a back edge always in the same place and the front edge always having its cloud disappear at that point, leaving a front edge always in the same place. There is more or less clear sky in front and behind this block. However there is a constant movement of cloud through this section of the sky, a bit like below:

.;~****************~;.
.;~****************~;.
.;~****************~;.

So you could watch a piece of cloud form at the back, becoming very solid looking, move through and then disappear at the front. What causes this? Is it a block of air with a different temperature to the air around it or something like that?

Snowbie

It moves into dry descending air.