Then suddenly I 'saw' that it were treetops. Was it also the form of the tree lines which were the wrong shape?
If I would have moved my head left and then right then I would have noticed that the black lines from the tree tops would have moved in relation to the cloud behind and would have been able to judge that those lines where nearer to me than the cloud, but I didn't think about that.
The next day I went to the same place and took a photo.
But is this seeing? In actual fact no. You can only think it as you see only shades of colour and because the patterns correspond with the pattern of a 'cloud with rain', you put the two together.
And in fact this is with many optical illusions. Mostly they are thinking mistakes or we are not aware of how our eyes work and in those instances we have to investigate (think and experiment) how the illusion comes about.
And anyway, how do we know it is an illusion? Once we understand it, it is not an illusion anymore!