What is a temperature sensor?
The shielded probe that measures air temperature and, with a humidity sensor, dew point — and why the radiation shield and the siting decide whether the reading can be trusted.
A temperature sensor measures the air temperature at the aerodrome and, with a humidity sensor, the dew point. The usual probe is a platinum resistance thermometer (Pt100): its resistance rises with temperature in a known way, and the electronics convert it to degrees Celsius. Dew point is the temperature to which the air must cool, at constant pressure and moisture content, to become saturated.
Both values go into the aerodrome weather report in whole degrees Celsius. The gap between them matters: when air temperature and dew point are close (as a rule of thumb, within about 2 to 3 °C), fog, low cloud or frost become possible, which is early warning for low-visibility operations and ties the sensor to the visibility sensor and RVR chain.
The probe sits in a radiation shield with the sensing element about 1.25 to 2 m above natural ground. A probe in direct sun, or next to a wall, apron or engine exhaust, reads too warm, so shield and siting decide accuracy as much as the probe does. A weather station (entry 16) usually groups it with the wind, pressure and visibility sensors.
Frequently Asked Questions
Q1What does an airfield temperature sensor measure?
It measures the air temperature at the aerodrome. Together with a humidity sensor it gives the dew point, which is the temperature to which the air must cool to become saturated. Both values are part of the aerodrome weather report and are normally reported in whole degrees Celsius.
Q2How does a Pt100 temperature probe work?
A Pt100 is a small platinum resistance thermometer. Its electrical resistance rises with temperature in a known, repeatable way, about 100 ohms at 0 degrees Celsius. The electronics measure that resistance and convert it to temperature. Connecting it with three or four wires lets the instrument cancel the resistance of the cable, which matters on long airfield runs.
Q3Why is dew point reported as well?
The gap between air temperature and dew point shows how close the air is to saturation. When the two are close, fog, low cloud or frost become possible, particularly on clear, calm nights. That gives early warning of low-visibility conditions, which is also why the sensor belongs in the same weather chain as the visibility sensor and the RVR system.
Q4Why does the sensor sit in a radiation shield?
Sunshine and heat radiated from the ground would warm a bare probe, so it would read higher than the air. A radiation shield, made of stacked louvred plates or a ventilated tube, blocks the sun while letting air flow through. A fan-aspirated shield improves this in calm, sunny conditions. Without a shield the reading can be wrong by several degrees.
Q5Where is it placed?
Where the air is representative of the aerodrome, over natural ground and clear of buildings, hard surfaces, apron and engine exhaust, at a height of about 1.25 to 2 m above ground as recommended by WMO. The meteorological authority decides the position, which is often near the runway or in a weather-station enclosure. Where a mast has to stand in the runway strip or another area where aircraft could run off, Annex 14 asks for frangible, low-mounted equipment, so confirm against the aerodrome safeguarding rules.
Q6How does it connect to the AGL system?
It is a separate meteorological system and not part of the series circuit, but it shares the airfield. The sensor needs power, earthing and surge protection, and its cable needs recording and protection like any other airfield cable. Its readings go to the AWOS and ATC displays. Maintenance teams can also use cold-weather readings to plan for frost, snow and ice on fixtures and signs, but practice varies by aerodrome.
Q7What goes wrong, and is it safe to work on?
Typical faults are a heated or contaminated shield, a blocked or failed fan, water in the probe or connector, cable damage, wrong siting, calibration drift and an unprotected mast hit by surge. The result is a reading that is too warm, too cold or stuck. Work near the runway needs the permit-to-work and aircraft-movement controls, and because the readings go into the weather report, tell ATC and the meteorological authority before taking the sensor out of service.
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