Air temperature sounds like the easiest measurement in meteorology and it is the one most often taken wrongly. The reading you want is the temperature of the air, and a sensor in direct sunlight gives you the temperature of the sensor instead — which on a still, bright day can be five to ten degrees higher. Getting it right needs three things: shade, free airflow, and a sensible height above the ground.
This section covers every instrument here that reads temperature, from a £15 indoor station and a window thermometer to professional aspirated shields, logging instruments and specialist gauges for ponds, animal housing and vehicle transport. Below you will find how the measurement is properly taken and what accuracy to expect.
How air temperature is properly measured
The reference standard is a sensor in a ventilated white enclosure, 1.25 to 2 metres above short grass, well away from buildings, paving and anything that stores or radiates heat. The traditional enclosure is the Stevenson screen — the familiar louvred white box — and the modern equivalent is a stack of white plastic plates called a radiation shield. Both do the same two jobs: keep direct and reflected sunlight off the sensor, and let air move freely past it.
Passive shields work well when there is a breeze and less well on still, sunny days, when they can still read a degree or so high. A fan-aspirated shield draws air past the sensor continuously and removes that error almost entirely, which is why it is the choice wherever the record has to be defensible.
Two more siting rules save most of the common mistakes. Never mount the sensor on a south-facing wall, which acts as a heat store all afternoon. And keep it well clear of tarmac, gravel and decking, which re-radiate heat upwards long after sunset.
How the sensors work
Almost all consumer instruments use a thermistor: a semiconductor whose electrical resistance changes sharply and predictably with temperature. They are cheap, small and fast, and calibrated against a lookup curve in firmware.
Professional and industrial instruments generally use a platinum resistance thermometer, most commonly the PT100, in which the resistance of a fine platinum element rises very nearly linearly with temperature. It is more stable over years and is what you want when a calibration certificate is involved. Thermocouples, which generate a small voltage at the junction of two dissimilar metals, cover much wider ranges but with lower resolution, so they belong in ovens, kilns and flues rather than in weather work.
Accuracy, resolution and response time
These are three different things. Resolution is the smallest change the display shows, usually 0.1°C. Accuracy is how close the reading is to the truth, typically plus or minus 0.5°C on a decent consumer instrument and plus or minus 0.1 to 0.3°C on a professional one. A display showing 0.1°C steps does not mean it is accurate to 0.1°C. Response time matters too: a heavily shielded sensor lags real air temperature by minutes, which is deliberate and correct, because you want air temperature rather than every passing thermal.
Derived figures: wind chill, heat index and min/max
Most stations report more than the raw number. Minimum and maximum since reset are the traditional daily record. Wind chill combines temperature and wind speed to express how fast exposed skin loses heat, and only means anything below about 10°C. Heat index, sometimes called apparent temperature, combines temperature and humidity to express how hard it is to cool by sweating, and only means anything above about 25°C. Both are calculated, not measured, so a station can only report them if it also reads wind or humidity.
What we recommend
For indoor and outdoor together with no fuss, the Technoline WS9450 Temperature & Humidity Station and TFA Ratio Wireless Indoor & Outdoor Min/Max Thermometer are the two we sell most of. If you want two outdoor points on one display, the TFA DOUBLE-CHECK Wireless Thermometer with Two Sensors does it. The Bresser Window Thermometer Translucidus WT is the simplest possible answer and needs no power at all. For water, the TFA PALMA Wireless Pool/Pond Thermometer reads a submerged sensor remotely. Where you need a recorded history, the TFA KlimaLogg Pro with Data Logger or the Fourtec LITE5032L USB Temperature Logger both download to a PC. For a properly shielded outdoor record, the TechnoLine WS 2816 Complete Weather Station covers domestic use and the Davis Wireless Vantage Pro2 with 24-Hr Fan Aspirated shield is the instrument to specify when the figures have to stand up.
Specialist temperature monitoring
Not every temperature question is a weather question. We hold dedicated ranges for Fish Pond Temperature Gauges, Pet and Animal Welfare monitoring and Monitoring Pets During Car Transportation, all of which use the same sensing technology applied to a specific duty of care.
Related measurements
Dedicated instruments are grouped in Thermometers, combined units in Temperature & Humidity and Rain & Temperature, and recording instruments in Temperature Loggers and Data Loggers. Because comfort depends on moisture as well as heat, see Humidity and Hygrometers. Radiation shields and replacement sensors are in Sensors, Spares & Accessories, complete systems in Home Weather Stations and Weather Stations, and the other elements under Elements.