It's a mineral deposit of some sort, either from the water in the air or the whitening agent in the roof fabric (if you have an EPDM rubber roof). Have you tried the usual glass cleaners, e.g. an ammonia+water solution? Lime Away is another product that is often useful for removing common calcium-based mineral deposits. Mineral spirits works good if the deposit is oil or paint based.
And then there are always car polishes, or rubbing compound for extreme cases.