The summer warmth is upon us and the numbers game fills our weather charts. But how are the actual values recorded and why doesn't your car temp count. Welcome to a Stevenson Screen
Sorry, but that doesn't count.
There have been a few days recently, when you're out in the sunshine and it feels scorching, yet the minute you are in the shade, it does feel quite cool. Top temperatures on the weather map might show just 18C but in the bright sun it really does feel hot.
Often people say “oh, it was 30C on my car thermometer today” or "28C in my (south facing) garden".
This can’t be taken as a true day temperature, as there are strict conditions for recording air temperature. A conservatory in a house will feel hot after been heated all day by the sun, whereas a north facing shaded kitchen will feel cooler, although the actual air temperature outside is the same for all.
Cars are essentially metal boxes, with stagnant air inside, lots of windows letting in the sunshine and so the still air held within can just heat up and up. As a result, they show an inflated value on your dashboard display.
The highest ever UK temp was on 10 August 2003, with Brogdale, near Faversham (Kent) reporting 38.5 °C (101.3F). This value actually took over a month to arrive at the Met Office with Gravesend in Kent reporting 38.1C (100.6F) officially that day. Some official reporting stations collate their data less frequently. The garden thermometer doesn’t count as it needs to be recorded in a Stevenson screen to make the conditions aross the UK the same. We used to have one in our Primary School playgroup, for a long time I thought it was a weird beehive.
The slats on the white box, allow air to circulate freely. This doesn’t happen in your car. The thermometer is sheltered from direct sunlight (and any rain etc). The screen is also set at a specific height off the ground 1.2m up, so as not to be influence by heat radiating up from the surface. If you think how hot sand can get in the sunshine, there is not point sitting a thermometer on that and then comparing elsewhere. It is also protected from the wind, although the air does move. The door of a Stevenson screen faces north, so that when the observer opens it up to read the temperatures (if this is not done remotely) then bright sunlight does not fall onto the thermoeters and give a false reading. Lots to think about.
If you have got temperatures, pressure, rainfall readings that you want to share, come onto the Netweather forum. In Community, Weather Discussion and Chat, there is the daily general weather chat, you can post your records in there. If you have a reading from your dashboard stick with #cartemp
