Based heavily off of a photograph I shot out of my window about 3 days ago of the clouds during sunrise. The two largest clouds were trace-overs of the clouds in my photograph initially, but I hid the photograph halfway through the drawing to force myself to make all of the in-fill.

The sky gradient is roughly based off of the real colour of the sky at the time, but I had to darken it to give the clouds a bit more contrast. Really, only the main shape of the nearly-central cloud was directly based off of the photograph, as were many of the initial colours in the image.