Clouds have to be one of my favourite things to look at. They are constantly changing, come in many forms and have an element of mystery to them. I find clouds to be one of the more challenging things to paint for a variety of reasons. I’ll try to explain some of these challenges and discuss observations I have made.
A little history: Until the nineteenth century, they were all just called “essences” as nobody thought to classify or name the different types. In 1802, a 30-year-old amateur meteorologist, Luke Howard, decided to categorize and name them. He presented his findings to the Askesian Society in London, his work was accepted and is what we still use today.
This post won’t get too far into the science that describes clouds, but I believe it is important to know about the basic cloud types in order to paint them well. The illustration below depicts the most common cloud types at their relative altitudes. (I have seen paintings that depict high altitude clouds placed too low in the scene. It’s all right if you want to do this for a certain effect, but if you are doing it accidentally, it can look really silly). I chose to paint this without colour to focus on the forms.
Usually, when I draw or paint I use a technique called the “Form Principle.” The Form Principle is a way of representing the way light and shadows fall on objects. This works well for painting solid objects as you think of the shape of the object you want to represent, then decide where the light source is. This principle will guide you as to where you put your light and shadows. Unfortunately, this technique has only limited success when painting clouds. The problem with this is you wind up with clouds that look solid – like lumps of plaster floating in the sky.
In the illustration above, I’m attempting to show how the properties of the material interact with light in different ways. For example, illustration three shows how the form principle still applies to a very shiny object. The centre light, highlights, and shadows are all in the same place as in the first illustration, only the highlights are much more defined and the reflections dominate the shadows.
Illustration four depicts a sphere that appears to be made of transparent glass. Again, the shadows and highlights are in the same places, only the highlight that appears on the top right of the sphere passes through to re-appear on at the lower right. Also, notice how the glass acts as a lens and brings a bright spot to the middle of the cast shadow. Another interesting note about representing glass is how the edges of the object appear to be a little darker and slightly more opaque than the centre of the object.
Illustration five depicts the same sphere as in illustration one, only under diffuse lighting conditions. Notice how the form shadow, cast shadow, and highlights are gone. The object is mainly shown in reflected light and only has a small occlusion shadow underneath. The object is rendered in such a way where areas closer to and facing the viewer are brighter while more distant areas and angled away from the viewer appear darker. This is known as ambient occlusion and is an important concept to use in drawing and painting. I often combine directional and ambient light in my work as it makes things look more three dimensional. Renaissance artists like Michelangelo used the ambient occlusion technique almost exclusively in their paintings to emphasize the forms of their figures.
For illustration six I have rendered this hypothetical sphere shaped cloud. When you observe clouds, the light not only hits the surface, it penetrates and scatters inside them; it is a phenomenon called sub-surface scattering. If you have ever put a flashlight behind your fingers and noticed the way the light makes the outer edges of your fingers glow you have experienced what subsurface scattering is. This is the key to making clouds look believable. Notice how the highlights are broad and wrap around towards the edges of the form. In less dense clouds, the light may pass through the form and spill into the core of the shadow area following the path of the light rays. The area near the terminator line is often the darkest part of the shadow.
Clouds as seen from a distance often appear to have sharp, defined edges, but like our glass sphere, the edges appear slightly darker. This phenomenon is subtle, especially when contrasted with cloudless areas in the sky, but observe how noticeable this effect is when a bright area appears behind it.
Believe it or not, clouds do have structure. I’ll update this post soon with some thoughts about the anatomy of clouds. Even at this point, though, you need to think about what kind of cloud it is an how its masses are formed. The density of water droplets and/or ice crystals inside them will change the way light interacts with them.