Clouds are forms but they are not solid forms (they have to look like you can fly through them.) This means that the planes suggesting the forms can not be flat one hue, one tonal value passages. There has to be a lot of variety that is clear enough to suggest that you can fly through the plane but close enough to still suggest one overall form. This is very hard to do if you try to do it all at once.
I think the best way to start with a cloud is with a geometric form and then modify it. The basic reason is that it allows for getting the perspective right, and that perspective can get complicated. Since clouds are overhead, we are often dealing with three-point perspective. That means that we are looking at the bottom of the cloud as well as the sides, that’s the so-called worm’s eye view.
What looks like the side of the cloud, if you think of the sky as a theatrical backdrop, wall or curtain, is really the bottom if you correctly see the sky as a ceiling.
This calls for three vanishing points: a left vanishing point (LVP) and right vanishing point (RVP) and a zenith. See diagram below, but afterward check out my trick for how to do this.
This looks complicated to do, but there’s a neat shortcut. Notice that the bottom is a diamond-shape. Do that first. Then the side lines going up converge, and the top slants. I find that, if I think of the diamond, I can get the rest as long as I remember all the lines slant (no line is parallel to the horizon or the edges of the paper.)
I should mention that the form should probably be more complicated than the one pictured above with several clouds or several parts of the same cloud having their own form. So a good cloud at this stage look like we’re looking up at several block-like forms.
pencil lines don't work (see below)
The next step is to make a note of the values which side is dark, etc. In all likelihood the bottom diamond-shape will be dark and one of the sides will be dark but probably not the same value.
The problem is that even faint pencil lines usually show in the relatively light sky and don’t work if they do so. Therefore you have to have a mental picture of the form. The important thing, as I’ve already said, is to get the squashed diamond-shaped bottom of the cloud which is usually darker than the rest of the cloud.
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