Your car’s temperature gauge may read 38°C when the weather forecast suggests it’s a cool 26°C outside. This seems to suggest that your car is somehow malfunctioning, but it’s actually just measuring something different from the air temperature we all know and love.
The Short Answer
A car’s thermometer reads the temperature of the area immediately surrounding the sensor. In most cars this sensor is located low down near the front of the car, often in a recess near the lower front grille or beneath the bumper. It is only a few inches above the ground, and here the surface temperature of the dark tarmac can reach 10-20°C above the air temperature only a few inches above it on a very warm day.
Why Car Sensors Run Hot
Dark tarmac can reach 10 to 20°C above the temperature of the air a few inches above it on very sunny days. This sensor is mounted in a location that can have a lot of residual heat from the engine and from the exhaust system. In stop-start traffic, or crawling through town, this can even rise as the sensor is not subjected to any airflow and the sensor reading can even rise whilst stationary.
How Official Temperature Is Actually Measured
The air temperature that is reported by the Met Office is measured in a Stevenson screen – a white louvred box which contains a platinum resistance thermometer. This is placed in a shaded area of a station 1.25m above short grass. There are no buildings, trees or surfaces of concrete, brick or other materials which can radiate heat near to the thermometer. Air is able to pass through the louvres of the screen to be read by the thermometer. Full details of how temperatures are measured and reported by the Met Office can be found on their website.
Why A Garden Thermometer In Sunlight Misleads You
A thermometer on a fence post or patio can read a totally different temperature from the surrounding air. Not only will a thermometer on a fence or patio in the sun be higher than the Stevenson screen temperature, but it can also vary significantly from one minute to the next. Typically, a thermometer on a fence post or on a patio in the sun can read up to 10°C above the true temperature for the surrounding air on a clear day. This is because the thermometer and its casing is being heated by the sun and the temperature will drop sharply within minutes if the thermometer is moved to a shaded area.
Your Car vs. a Weather App (including a thermometer in your garden)
First, your phone app will be showing you a forecast or the current temperature for the nearest official Met Office weather station. That station could be miles away from you, at a different height and in a completely different location – perhaps by the coast or in open countryside. There are also large differences between urban areas and rural locations and even between a cold spot in a town and a nearby hot spot. The reading your app is showing is for somewhere completely different to where you are, even if it’s only down the road.
Why Your Car Will Never Set A Record
The car temperature reading can never form part of a UK temperature record. The official UK temperature records are produced from a network of over 1,200 Met Office stations across the country. Each station has to meet a set of stringent criteria regarding the height, exposure and surrounding environment of the thermometer used to measure temperature at that site. As mentioned above, the temperature reading from a car’s thermometer is measured in a very different environment. That’s why, although it may seem very high, it can never form part of a national temperature record. General guidance on dealing with a car fault reading that seems to be inaccurate is available on the Citizens Advice website.
Getting A More Trustworthy Outdoor Reading
Measure the temperature in your garden for accurate outdoor temperature reading. The thermometer should be placed to avoid direct sunlight and not be placed on surfaces that store heat such as paving, concrete or render. The best position is at head height on open ground such as grass. It will take a few minutes for the thermometer to reach the temperature of its new surroundings so wait a few minutes and then take a reading and compare with the temperature for the nearest Met Office weather station. General advice on how car sensors measure temperature as well as problems they can have and some general motoring advice can be found on the Citizens Advice website and other motoring websites.
How The Readings Compare
Met Office station: a shaded, ventilated screen at 1.25 metres above grass, housing the platinum resistance thermometer that produces the official air temperature.
Car dashboard: a sensor located near the road surface and/or engine bay, measuring high temperatures due to sunlight, and/or heat generated by traffic, in order to cool the car engine.
Garden thermometer in sunlight: This measures the temperature of the sunlight heated surface of the thermometer itself and NOT the air temperature!
Your phone app is showing a forecast (which can be wrong) or the current temperature for the nearest official weather station (which could be several miles away). Even if it is close to you, the station could be at a different height, or in a different sort of location (e.g. by the coast as opposed to in the middle of a town).
Because a car thermometer reading is never a record breaking temperature reading. Readings from your car’s thermometer are not to be taken seriously and will never be used to record a temperature for weather records. The thermometer in your car is located in a wrong place to record a temperature for weather records and is used by the car’s computer for other reasons.
