Clutter is a term used to describe any object that may generate unwanted radar returns that may interfere with normal radar operations. Clutter can be classified into two main categories: surface clutter and airborne or volume clutter. Surface clutter includes trees, vegetation, ground terrain, man-made structures, and sea surface. Volume clutter normally has a large extent and includes chaff, rain, birds, and insects. Surface clutter changes from one area to another, while volume clutter may be more predictable. Surface clutter includes both land and sea clutter, and is often called area clutter. Area clutter manifests itself in airborne radars in the look-down mode. Volume clutter has large extents and includes rain, chaff, birds, and insects. The volume clutter coefficient is normally expressed in square meters. Three factors affect the amount of clutter in the radar beam. They are the grazing angle, the surface roughness, and the radar wavelength.