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The thermometer doesn't lie - or does it?

Jen Bartram
Digital News Editor

Monday, July 14, 2014, 08:53 GMT -

The mercury is expected to soar across the UK later in the week, with temperatures in some areas likely to hit 30C or more – which would be the highest temperature this year so far.

That’s if you believe the meteorologists. If you rely on your car or garden thermometer, then you may have already observed temperatures above 30C this year.

So can you trust the weather forecast and, if so, why the discrepancy?

First of all, it’s important to know that the temperature given on a standard forecast, unless indicated otherwise, is the air temperature. Weather forecasters observe air temperature in a very precise and, crucially, uniform way, in order to be able to make accurate comparisons day after day, all around the world.

Air temperature measurements are taken using a Stevenson screen; a wooden or plastic box that protects the instruments inside from the elements. Painted white on the outside and black on the inside, it has a complex system of ventilation to allow free circulation of air and to reduce the amount of heat conducted from its outer walls.

Thermometers within Stevenson screens are placed at a height of 1.25m above ground and, within the ‘box’, they are sheltered from strong winds and rain. They are placed away from trees, fences, buildings, concrete and tarmac in order to ensure that air can flow freely. Even the door is north facing, so that direct sunlight will not adversely influence the temperatures when it is opened to take a reading.

If your garden thermometer, for example, is mounted on a south-facing wall, in bright sunshine, then it is extremely likely that your own meteorological observations will show a temperature much higher than that declared on your weather forecast.

A car thermometer is even more likely to give a temperature reading that doesn’t match with the one a weather forecaster will give you.

Cars are made of metal, which absorbs much more heat than the air around it, and warms up. If a car is outside on a sunny, warm day, heat will build up inside and the air will become stagnant. What’s more, the engine will give off heat as it is used. If you add heat from the road surfaces, then there are already a number of different factors, which can affect the reading given by your car’s thermometer.

So why have it at all? A car thermometer is a very useful tool during the winter. Often located behind the front bumper, it will give an indication to drivers as to when road surface temperatures are getting near to freezing – which could be a danger if roads become icy.

The highest official air temperature measured in the UK was taken on 10 August 2003, near Faversham in Kent, where the air was measured at 38.5C – but many people’s cars or gardens will have felt warmer than that in the direct sunshine. Although your thermometer doesn’t lie, make sure it’s measuring the temperature of the air, or, failing that, check out your local weather forecast here!

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