The author refers to an intangible object and compares it to another from the word. The birth and evolution of the writing marks a dialectic between the images and the alphabetic writing from which, the images and the text take their own positions diverging or approaching to a greater or lesser extent.
This is correct because by using this type of description, the author not only brings a visual image to mind, but also what the clouds feel like or behave like which is a more detailed description.
