How can I remove hard water stains on my windows?

My windows have what I think are hard water stains along the bottom edge (exterior side only). The fogginess cannot be removed by a simple washing and squeegee.

I suppose something acidic might remove them if they are indeed hard water stains. But I don’t want to use anything that might etch the windows or damage the paint on the walls. So, how can I get clean and clear windows again?