Ceilometer

A Ceilometer is a ground-based active remote‑sensing instrument that measures the height of the cloud base (ceiling) and, in some configurations, vertical visibility. It operates by transmitting a pulsed laser or infrared beam vertically into the atmosphere and measuring the time‑of‑flight of the backscattered signal from cloud droplets or aerosol layers. The instrument calculates the cloud base height from the time delay between the transmitted pulse and the received return signal. Ceilometers are a critical part of aerodrome meteorological observation systems, providing data that determines approach minima, runway selection, and overall flight safety.

A ceilometer exists to answer a question a pilot on approach cannot answer for themselves until it may be too late: how low is the cloud base, and will the runway environment actually become visible in time to land safely. Because cloud ceiling directly determines whether a given approach category can be flown at all, an inaccurate or absent ceiling measurement risks either an unnecessary diversion or, worse, an approach continued below a safe decision height with the runway still obscured.

By measuring ceiling height continuously and automatically rather than relying on a human visual observation, the ceilometer removes both the delay and the subjectivity that come with judging cloud base by eye — giving air traffic control and pilots a consistent, defensible basis for approach minima and runway selection decisions, particularly in rapidly changing weather.

Installed at airfields as part of the meteorological observation system, typically near the runway touchdown zone or at a central location. The ceilometer provides continuous cloud base height measurements, which are used for weather reporting (METAR, TAF), ATC information, and pilot briefings. It is also used at heliports and military bases. The instrument is mounted on a stable base or mast, with the optical window oriented vertically to ensure a clear view of the sky, free from obstructions such as buildings or trees.

Sited on level, stable ground close to the runway touchdown zone, or in a central meteorological compound where a single instrument serves the airfield, on a concrete plinth or shelter roof that keeps the unit rigid and free of vibration. The optical window faces vertically upward with a clear, unobstructed cone of sky above it, clear of overhanging structures, aircraft movement areas, and anything that could cast a shadow or deposit exhaust residue on the lens, and the housing is orientated so the window can be reached safely for routine cleaning. Power and data cabling run underground in duct to the meteorological equipment shelter or AWOS cabinet, and mounting height keeps the optical window clear of standing water, snow accumulation, and vehicle strike zones while remaining accessible from a straightforward maintenance platform or ladder.

The ceilometer emits short pulses of near‑infrared (e.g., 905 nm) or visible laser light vertically upwards. As the pulse travels, it encounters particles (cloud droplets, aerosols, precipitation) that scatter a fraction of the light back toward the instrument. A sensitive receiver (a photodiode or avalanche photodiode) detects the backscattered signal. The time delay between the transmission and reception of the signal is measured, and since the speed of light is known, the range (height) to the scattering layer is calculated. The instrument records the backscatter profile over a range of heights (typically from 30 m to 7,500 m or more) and identifies the first significant increase in backscatter, which is reported as the cloud base height. Many modern ceilometers also calculate vertical visibility (up to 1,500 m) and can detect multiple cloud layers.

Like the anemometer, the ceilometer has no relationship to the airfield's constant current lighting circuit — it is a data source within the automated weather observation system, not a load on a lighting loop. The instrument's internal processing unit computes the backscatter profile and reported cloud base height itself, then passes that result over a serial (RS-232/RS-485) or Ethernet link to a data logger or data collection platform, which timestamps it and forwards it to the central AWOS/ASOS processor. There it is merged with other sensor inputs to generate the ceiling entries in METAR/TAF products, the controller's live weather display, and ATIS broadcasts. Because the ceilometer reports over an individually addressed data link rather than sharing a current loop with other equipment, a fault is reported against that specific instrument, with the built-in self-test continuously checking laser output power, receiver sensitivity, and window contamination (a dirty or iced optical window scatters the outgoing pulse and is flagged separately from a genuine low-cloud reading, since an uncorrected contamination fault can otherwise be mistaken for a very low ceiling). Where an airfield runs more than one ceilometer — for example alongside a transmissometer for full CAT II/III weather minima — each unit's reading is reported and validated independently before the AWOS/ASOS processor combines them into the published ceiling and visibility values.

A ceilometer is built as a self-contained, weatherproof optical unit housing the laser transmitter, receiver optics, and processing electronics within a single sealed enclosure, typically of powder-coated aluminium or stainless steel construction rated to IP66/IP67. The transmit and receive apertures are fitted with optical windows kept clear of condensation and contamination by internal heating elements and, in many designs, a window-blower or wiper mechanism, since any obscuration of the optics directly degrades the backscatter signal the instrument depends on. The laser source and detector are mounted on a rigid internal optical bench, isolated from vibration and thermal drift to keep the beam alignment stable over years of unattended operation. The unit is mounted on a fixed pedestal or plinth, precisely levelled so the beam is transmitted vertically, with the enclosure's internal electronics bay accessible via a sealed access panel for servicing without disturbing the optical alignment. Cable entries for power and data are sealed against moisture ingress, and the housing includes ventilation or climate control to keep internal components within their rated operating temperature range across extreme outdoor conditions.