// Sensors
Weather instruments
The sensors behind the tower’s met display: anemometers next to the ILS glidepaths, the cloud base recorder, instrumented RVR, temperature and pressure.
What it is
- Anemometers measure wind; a cloud base recorder (ceilometer) measures cloud height with a laser; transmissometers or forward-scatter meters give instrumented RVR (IRVR) along the runway; there are temperature, humidity and pressure sensors too (CAP 746).
- Instrumented RVR measures how far along the runway a pilot can see. CAP 670 accuracy: ±10 m for 50–400 m, ±25 m for 400–800 m, ±10% above 800 m.
How controllers use it
- RVR is reported when visibility is below 1500 m, in UK steps of 25 m up to 400 m, 50 m up to 800 m and 100 m up to 1500 m.
- IRVR is reported as Touchdown / Mid-Point / Stop End and passed at each change of one step, e.g. “RVR runway 30 touchdown 550, mid-point 600, stop end 650.”
UK rules and practice
- Reporting rules: CAP 493 Section 3 Chapter 3.
At Prestwick
- The AIP places the anemometers next to both ILS glidepath aerials, and warns that low-level mechanical turbulence can make the reported wind lower than what pilots feel.
- Low-visibility procedures apply below 800 m visibility; below 600 m RVR only one movement is allowed on the manoeuvring area. The maximum reportable RVR for runway 30 is 1400 m.
- A cloud base recorder is mandatory at licensed aerodromes with precision-approach runways (CAP 746), which includes Prestwick’s ILS runways.
// Through the eras
| Era | Status | Notes |
|---|---|---|
| 1930s–40s | Typical UK practice | Hand-read instruments and observers from the met office are typical. |
| 1950s–60s | Typical UK practice | Cup anemometers with dial repeaters are typical. |
| 1980s–90s | Typical UK practice | Transmissometer RVR and ceilometers are typical at ILS airports. |
| Modern | At Prestwick (sourced) | Anemometers beside both glidepath aerials and RVR reporting (AIP); a cloud base recorder is mandatory (CAP 746). |