In the Northeast

[doctorwayne]

I know the type of weathering to which you're referring, Reinhard. An easy way to achieve it, if you have an airbrush, is to apply strips of masking tape, horizontally around the silos. The tape should probably be about 1" wide: you don't have to cover each separate section, merely the area immediately above where each ring of discolouration starts. Mask-off the entire silo(s), then, starting at the bottommost ring, spray around the silo, aiming at the lower edge of the tape. Remove that tape, then move on to the next ring up, again spraying at the lower edge of the tape. The overspray will lightly weather the section below. Work your way up to the top in this manner - each spray will not only weather the section on which you're working, but the overspray will further darken the sections below, leaving the bottom one dirtiest, as it would be due to run-off. When you've finished, you can apply another light spray, working vertically, to soften the delineations between sections. I find that the paint mix should be quite thin, as with most weathering - between 70% and 90% thinner.
Here's a stairway enclosed in an unpainted steel - the rust from each overlapping sheet has bled down onto the sheet below, much like the weathering on the silos would:


Wayne