Clouds come in layers!
While writing a story for Smithsonian in 1997 I interviewed a tower hand who recalled climbing into a grim, gray old overcast to do some broadcast-tower maintenance work. But after a few minutes in the murk, he emerged to see a brilliant blue sky above. He stopped to stare at the cloudscape, mist still clinging to his boots. No other people were in view. It was a world made new.
Interesting visual effects emerge when a high cloud casts a shadow on a low cloud. From the ground, sometimes these shadows appear as fuzzy silhouettes. The one on the right looks like a stylized thunderbird.
The following illustrates how altostratus can be extremely thin and well defined, like a sheet of tinted glass. I took this over Wisconsin from an airliner at 32,000 feet. It was overshadowing stratocumulus undulatus below.
In the next photo, it's a case of undershadowing: a low cloud (a jet contrail in this case) throwing its profile on a higher one at sunset. I took this in Minnesota last November.
Note the rough similarity to the infamous California contrail. It's coincidental!