After U. S. Weather observing practice, the ceiling classification that is applied when the ceiling value has been determined by means of 1) a ceiling light or ceilometer, provided that penetration of the beam is not in excess of that normally experienced for the height and type of layer and that the elevation angle indicated by the clinometer or ceilometer detector does not exceed 84°; 2) the timed disappearance of a radiosonde balloon with its height computed; 3) the known heights of unobscured portions of objects, other than natural landmarks, within 1½ nautical miles of any runway of the airport. A measured ceiling pertains only to clouds or to obscuring phenomena aloft. It is designated M in aviation weather observations.