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.
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 not random.
time lapse => smoke, sinks, sources, etc. fluids