Cloud of the Day – Nimbostratus
I mentioned nimbostratus in the Cloud of the Day post on altostratus, but I think it deserves a post of its own. It is a stratiform cloud that typically forms on a warm front. As advancing warm air rises over the retreating colder, denser air, we see the classical progression of cloud types. As the warm front approaches, we see the highest clouds first. Wisps of cirrus, maybe a layer of cirrostratus, complete with a halo, and cirrocumulus. Then, the closer the warm front gets, the lower the clouds we see. The middle etage clouds, altocumulus, and altostratus, perhaps even altocumulus castellanus, are next. Finally come the stratocumulus, stratus and the various forms of cumulus.
It is the stratus clouds that are of interest in the case of nimbostratus, because nimbostratus is so thick that it stretches from near the ground up into the middle etage. It is stratus and altostratus combined into one thick layer. It is so thick that its bottom is very dark, even black. It is so laden with water that precipitation is inevitable. The “nimbus” in the name has many meanings, but in the case of the cloud, it indicates that it’s a rain cloud.
Nimbostratus is the cloud that gives sustained rain over a wide area. It is the bringer of those dark, gloomy, wet days. Often there are fractus clouds scudding about underneath it. It’s the kind of weather that’s good for the lawn, and good for watching from inside, warm and dry.