Weathervanes are used for showing the direction of the wind. Atop turrets in Capitol Hill a “true” reading would be hard to get. In order to work properly a weathervane most be located away from other buildings, trees and other objects that could interfere with the wind. If used properly a Weathervane could help a you make simple short range weather predictions. It’s safe to say the ones we saw were put up for decoration only.
Not that we can blame the homeowners – there are tons of styles to choose from - in ever conceivable price range. Now that we know they are out there we see them all the time. It’s hard to believe we never noticed them before.