| Section C | C index | 161-169 of 1157 terms |
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ceiling—1. After U.S. weather observing practice, the height ascribed to the lowest layer of clouds or obscuring phenomena when it is reported as broken, overcast, or obscuration and not classified “thin” or “partial.” The ceiling is termed unlimited when the foregoing conditions are not satisfied. Whenever the height of a cirriform cloud layer is unknown, a slant / is reported in lieu of a height value. At all other times, the ceiling is expressed in feet above the surface, which is a horizontal plane with an elevation above sea level equal to the airport elevation. At stations where this does not apply, “surface” refers to the ground elevation at the point of observation. For obscurations, the ceiling height represents vertical visibility into the obscuring phenomena rather than the height of the base, as in the case of clouds or obscuring phenomena aloft. In an aviation weather observation, the ceiling height is always preceded by a letter that designates the ceiling classification. 2. The greatest altitude to which an airborne object (aircraft, balloon, rocket, projectile, etc.) can rise, under a given set of conditions. The ceiling of an aircraft is that altitude where the stalling Mach number and the buffeting Mach number approach identical value.
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ceilometer—An automatic, active, remote-sensing instrument for detecting the presence of clouds overhead and measuring the height of their bases. For optically thin clouds, such as most cirrus, more than one layer may be detected, but when optically thick clouds, such as liquid water stratus, are present, the light beam is unlikely to penetrate much beyond the base of the lowest liquid layer. Laser ceilometers use intense pulses of light in a very narrowly collimated, vertically directed beam, and have collocated transmitter and receiver systems. The cloud base heights may be displayed in a variety of time-height section images or backscatter intensity profile plots. Some older ceilometers use separated transmitter and receiver units. The instruments are designed to work during the day or night.
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celestial sphere—The apparent sphere of infinite radius, having Earth as its center. It is upon the “inner surface” of this sphere that all heavenly bodies, the ecliptic, and the celestial equator appear. Disregarding the effects of topography and refraction near the horizon, for practical purposes half of this sphere may be considered visible from any point on the earth's surface at any time.
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cell—In radar usage, a local maximum in radar reflectivity that undergoes a life cycle of growth and decay. The rising portion of the reflectivity maximum is indicative of updraft, and the later descending portion is indicative of a precipitation downdraft. Cells in ordinary convective storms last from 20 to 30 min, but often form longer-lasting multicell convective storms. Cells in supercell storms are more steady and last considerably longer. See also thunderstorm cell.
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