Dark clouds are a threat of rain or storm, used frequently in setting the scene for movies and books.
Winnie the Pooh knows rain clouds are black.
I'd think that clouds, especially isolated ones would be white. I tried to get a picture of the phenomenon I am going to try to describe, but I was driving alone on the Interstate and by the time I could pull off, conditions had changed and the cloud situation had vanished. Here goes:
A very clear summer day a couple days ago. Completely clear where I was in west central Wisconsin, beautiful summer late morning. Severe Clear as they say. But about 35 to 40 miles north of me a solitary storm was swelling up, producing a beautiful cumulo-nimbus. The boiling cloud was visibly different every minute or so, even at that distance. It was completely white, as a good cloud should be, except on the easterly side where some shadows were starting to develop as it rose.
Just before it started to cap over some of the more rapidly rising portions, very dark stratus at several altitudes began to appear. The were between me and the cloud, so no shadows from the thunderhead itself, and again, there were no high altitude overcast or stratus that I could see. But the very dramatic black clouds which should have been in the sun were as distinct and dark as if you'd taken a soft drawing pencil and sketched them in.
I've seen these type of stratus develop quickly, and almost as quickly fade in a few minutes. I'm not sure why they develop when the nearby storm itself is clearly ascending rapidly, and why they are black, with no surrounding or higher level clouds to shade the sun and lose their white reflective basic cloud color.
Simple minds are curious to know. I wish I were an artist so I could try to sketch what I saw, and I'd loved to have had a time lapse video of the phenomenon. I watch a lot of late afternoon and evening thunderheads, especially with the marked couple of weeks of isolated storms and non-organized patterns develop. I will make every (safe) effort to photograph one of these if it pops up again, but would like to hear anyone's ideas on why this may have happened.
And of course the dark green of a violent hail filled storm that approaches is another topic another time.
Dale
