After U. S. Weather observing practice, the ceiling classification applied to a ceiling height that is determined in any of the following ways: 1) by means of a convective-cloud height diagram or dewpoint formula; 2) from the known heights of unobscured portions of natural landmarks, or objects more than one and one-half nautical miles from any runway of the airport; 3) on the basis of observational experience, provided the sky is not obscured by surface-based hydrometeors or lithometeors, and other guides are lacking or considered unreliable; or 4) determined by ceilometer or ceiling light when the penetration of the light beam is in excess of normal for the particular height and type of layer, or when the elevation angle of the clinometer or ceilometer-detector scanner exceeds 84°. Such a ceiling is denoted E in aviation weather reports.