What is a Shelf Cloud and How Does It Form?
So you are outside just before a storm rolls through and you see something odd- something that you have never seen before. An ominous, straight cloud sits draped across the sky. But what is it? How did it form? And should I be worried?
Well most likely you probably saw something like this:
What you are seeing is called a Shelf Cloud. And to answer one question; no, this cloud will not hurt you even though it can look very ominous. It is a cloud formed due to the scientific principle that warm air rises, and cool air sinks. As shown in the diagram below, cool air is pushed down underneath a storm and begins to move outward (away) from the storm. As this happens, warm air rises up over this cooler air and starts to condense, thus forming a cloud. This is the same principle that causes storms along a cold front, but on a much, much smaller scale. The result from this process is a long, skinny cloud that resembles a flat shelf (hence the name).
Now you might be sitting here and wondering how often this occurs. It may surprise you that this happens almost every day all over the US during the Spring, Summer, and Fall months when storms occur. It does not appear as defined in some storms as in others, so the Shelf Cloud might not be visible unless you are actively looking for it. One thing is for sure though, if you do happen to see one make sure to take a picture because they can be quite beautiful. Just make sure you aren't outside too long because there is probably a storm with lots of lightning close by!