What is a ceilometer?

It measures the height of the base of the clouds above the aerodrome. Most modern units report up to three cloud layers with their amount, and report vertical visibility when fog, heavy rain or snow hides the sky. The height of the lowest layer covering more than half the sky is the ceiling, which is one of the values used to decide whether approach minima are met.

It sends very short pulses of infrared laser light straight up. Cloud droplets scatter a little of that light back to a receiver beside the transmitter. The instrument times the echo, and the height is the speed of light multiplied by the round-trip time, divided by two. Software then processes many pulses to pick out the cloud layers and ignore rain, haze and noise.

When fog, heavy precipitation or blowing snow hides the sky, there is no clear cloud base to measure. The ceilometer then reports the vertical visibility: how far up an observer could see into the obscuring layer. It is reported as VV in a METAR, and it counts as the ceiling in that situation.

No. A ceilometer looks straight up and reports cloud height. A visibility sensor looks sideways through the air and reports the meteorological optical range, which is the input to RVR. Fog can make horizontal visibility very poor while the ceilometer reports a very low vertical visibility, so the two instruments are used together in low-visibility operations. See entry 11 for the visibility sensor; the RVR sensor is entry 13.

For approach and landing, ICAO Annex 3 guidance places the cloud-height sensor at the ILS middle-marker site (roughly 900 to 1,200 m before the threshold) or an equivalent position on the approach path, where the cloud that matters to an arriving aircraft is. The meteorological authority decides the number and positions. The mounting must stay clear of the runway protected areas or be frangible, and the vertical beam needs a clear path.

It is a separate meteorological system and not part of the series circuit, but it shares the airfield. It needs power, earthing and surge protection, and its cable needs recording and protection like any other airfield cable. Its output goes to the AWOS, the ATC display and the low-visibility procedures; at some aerodromes ceiling and visibility together trigger those procedures and the related lighting settings, but practice varies, so follow the aerodrome's procedures.

Typical faults are a dirty, wet or frosted window, bird droppings, insects or debris on the optics, a blocked blower or failed heater, laser ageing and a unit that is no longer level. A contaminated window can report false low cloud or miss real cloud. Ceilometer lasers are normally eye-safe, but do not look into the beam through binoculars or other optics, and follow the manufacturer's laser-safety instructions. Work near the approach path needs the permit-to-work and aircraft-movement controls, and because the ceiling affects approach minima, tell ATC and the meteorological authority before taking a sensor out of service.