This is what the train window looked like through the polarising filter of my sun shades. It looks like the stress patterns produced by polarising filters. But I have remembered that these are normally produced by putting the plastic between two polarising filters. Could it be that the polarised sky acted asns the first filter? I didn't think of that at the time so I didn't check the direction. The rainbow colours are also interesting. Why do they repeat? They look lke the thin-film interference patterns. I didn't think to rotate the filter to see what affect the polarisation direction would have.
