Sounds like you would need a video switcher. The way the weather works is that the person is shot in front of a greenscreen and the green is removed either using the switcher itself or another production time keying technology like ultimatte. This becomes your first video feed. The graphics of the weather or whatever are created separately and shot on a second camera or feed in through a computer system. The Switcher can them take both streams and superimpose them to create the image you see on TV. Usually the person bign shot is looking at a video feed so that can know where to point.
