To take the next step with my poster, I added gradients. It did not take long though until I realized that I could not keep up my previous colour scheme. Just blue and white did not look good and also, it did not work with the white clouds. So I experimented with green, since I used that colour for the trees anyway and a lighter blue instead of white. After all, I was satisfied but not completely happy with the result.
Another thing I did was removing the lines in the third quarter and colouring the clouds depending on how close they are to raining from white to dark grey.