What’s the origin of the metaphor “cloud”?

Today we use the term “cloud” without thinking about it. We use it mainly in terms of storage online, far away and unreachable. We can’t plug in a USB memory in the cloud to download data. We plug the USB into our own computer, connect to the cloud and download, as if it was a local storage.

In the two great Wikipedia articles Cloud storage and File hosting service there are a lot of information on architecture, potential threats and costs, but nothing really on the origin of the word cloud.

Searching online you get a lot of answers of the meaning of cloud, some better than others. One of the more fun comes from Rebecca J. Rosen’s article Clouds: The Most Useful Metaphor of All Time?

. . . when engineers would map out all the various components of their networks, but then loosely sketch the unknown networks (like the Internet) theirs was hooked into. What does a rough blob of undefined nodes look like? A cloud. And, helpfully, clouds are something that take little skill to draw. It's a squiggly line formed into a rough ellipse. Over time, clouds were adopted as the stand-in image for the part of a computer or telephone network outside one's own.

However amusing, it still doesn’t answer the question – where did it come from? Who coined it? What’s the origin of the metaphor cloud?