https://cgcookie.com/courses/painting-the-elements

This course will take you through the entire process of crafting the traditional elements of our world - air, earth, fire, and water. Each element is broken down into different sections, so you'll see how each one can appear in a variety of ways. By studying everything from lens flares to volcanoes and rainstorms, you'll come away with a better understanding of the world around you, and how to better recreate those effects with digital ink.