It’s a common elemental theme we see across stories, fantasies, etc. It seems like it is an element that should be part of that pantheon but maybe there is a scientific explanation of why it isn’t? Or is it simply, they did not have enough understanding of it at the time those 4 were suggested.
In the desert episode Aange did water bend a cloud into a bottle or something, right? I always thought that was water bending. There’s also this mist that they make in this White Lady episode, I thought that was also Katara?