I have loved and studied clouds for a pretty long time now. I was taking pictures of sunsets from my bedroom's window when I was 15, I'm now 35 and nothing has changed : I'm still amazed by the variety and colors and shapes that cloud bring to an otherwise "sad" blue sky.
My favorites are storm clouds : the humongous cumulo-nimbus.
When it comes to rendering, you have 2 difficulties to overcome : lighting and shape. Both are difficult and very important.
Contents
Cloud Types
You may underestimate the importance of the shape of a cloud, especially because they look random so you might think a random shape is okay. But then you are plain wrong, because clouds are all but random.
Shape
If you watch a time-lapse video of a cloud, you can see how clouds move, how they are created and destroyed. One main observation to make is that accelerated clouds motion make them look like regular smoke. This is an important fractal pattern : clouds are like smoke, but bigger. And as they are orders of magnitude larger, the time scale is orders of magnitude smaller. It will take several minutes for the huge mass of a cloud to accomplish the same motion as smoke. A bit like the time for humans seems like slow motion as compared to insects.
You can see in the example video above how the cloud is "pulsed" from hot air pockets below. You can also see how there are "sources" and "sinks" that make the cloud appear and disappear.
All these obey the complex laws of thermodynamics : hot humid air rises up and evaporates while cold air falls down and liquefies, all these occurring at precise temperature and pressure, depending on the altitude.
Clouds are composed mainly of water in all its states : vapor, liquid (i.e. small droplets) and solid (i.e. snow and ice crystals). Clouds also need "nuclei" particles to form, like dust or aerosol molecules : despite large humidity quantities in the air, if there is no nuclei (i.e. the air is too clean), there won't be any cloud.
Every Altitude its Cloud
Most of the clouds are very compact and fit in a "tiny" altitude range, although the storm cumulo-nimbi clouds span the entire range of altitudes, some of them going up to 15 kilometers (!!).
The scales in these clouds vary dramatically. Thin clouds are easily traversed by light while thick clouds are mostly reflective. Light scattering in clouds is really important because of the scales browsed by clouds : a photon doesn't go in a straight line for a long time when pacing through the many hundreds of meters inside a cloud.
Several ways for modeling clouds
Alto stratus
These are thin vaporous high-altitude clouds. They are the easiest and most commonly simulated clouds as they are very thin and can be easily mapped to a plane.