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Why a window thermometer looks inaccurate

Updated: October 2026

Your window thermometer looks inaccurate at noon: the dial says something like 40 °C, and the forecast for your town says the high teens. Before you bin it, look at where it is stuck. A plastic case on glass in the sun is warming itself. The Products Compared editorial team set this against the Met Office method, Royal Meteorological Society guidance, and the UK reviews in the six amazon.co.uk listings compared on 15 October 2026. There was no laboratory test. If you buy through our Amazon links, we may earn a commission at no extra cost to you. The listings are on the [window thermometer comparison](/hub/climate-measuring/window-thermometer). This page is what the number means, and where to move the gauge.

Your window thermometer looks inaccurate at noon: the dial says something like 40 °C, and the forecast for your town says the high teens. Before you bin it, look at where it is stuck.

Our top picks

Faithfull Faithfull stick-on window thermometer

Best overall

Faithfull Faithfull stick-on window thermometer
7.2/10about £4.31
View offer
Brannan Brannan outside window thermometer

Rank 2

Brannan Brannan outside window thermometer
7.6/10about £5.95
View offer
Youshiko Youshiko YC779 digital window thermometer

Alternative

Youshiko Youshiko YC779 digital window thermometer
5.7/10about £18.99
View offer
On this page
  1. 1.What the forecast is measuring
  2. 2.What sun does to a dial on glass
  3. 3.Room heat through the glazing
  4. 4.Sun error or a dead unit
  5. 5.Move it before you bin it

What the forecast is measuring

The forecast is not reading your pane. The Met Office measures air temperature in a white louvred Stevenson screen at 1.25 m above the ground, shaded and ventilated, not glued to a window. Its schools guidance says the screen faces north so the sun does not heat the sensor. Stop treating a south-facing lunchtime dial as if it were that screen. The Royal Meteorological Society's MetLink notes make the same split. Keep a thermometer out of direct sun. A north-facing wall is a last resort, and the instrument should stand off the wall rather than sit on it. A display that shows 0.1 °C is precision on the screen, not accuracy of a tenth of a degree. If the sun can heat the instrument, you are no longer measuring the air.

What sun does to a dial on glass

UK reviews of the Brannan 14/419/3 report about 40 °C or about 50 °C in direct sun when the air was not that hot. They say the dial stays usable on a shaded window. The maker sheet still states ±2.0 °C at ambient, on a range of −20 °C to +50 °C. Ambient is the condition. Noon sun on the case is outside it. Faithfull reviews should not be averaged into one bias. Some say 5–10 degrees high. Others write wilder misses, including 32 shown as 10. Thermometer World notes that hot sun makes this class of stick-on dial inaccurate. Take the sun off the glass before you decide the coil is scrap.
Faithfull Faithfull stick-on window thermometer

Testsieger

Faithfull Faithfull stick-on window thermometer
7.2/10about £4.31
View offer

Room heat through the glazing

Eva Solo's sheet says a thermometer on the pane is affected by heat from the room, and that how much depends on the glazing. Double glazing reduces that leak compared with a single pane. It does not turn the outside of the glass into a Stevenson screen. None of the six listings states a correction, so there is no number here to subtract. A north-facing window in shade will usually sit closer to the air temperature than a south-facing pane at noon. Closer is the right word. Cleaning and new pads, if you do move it, are on the how to fit page.

Sun error or a dead unit

A Brannan that shows about 40 °C only while the sun is on it, and a plausible figure once the pane is in shade, is siting. Move it. A reading that is high all day on a shaded pane can still be a poor individual dial. Compare it for a day in the shade before you call the forecast wrong. One Youshiko review stayed at 50 °C and was called faulty. If the digits never change from a cool morning to the afternoon, you are looking at a unit that is not measuring. The cell and the contrast are on the digital or analogue page.

Move it before you bin it

If the number soars only while the sun is on the case, move the gauge to a shaded pane you can still read from indoors. Shade stops you measuring a warmed case. It does not turn the glass into a Met Office screen at 1.25 m. If the display never changes from morning to afternoon, treat that as a fault, not as sunshine.

Common questions

Does a north-facing window fix it?
It helps, and it does not finish the job. A north-facing pane avoids the midday sun that produces those 40 °C reviews. MetLink still treats a north-facing wall as a last resort. You get closer to air temperature. You do not get a Met Office reading.
Is 2 °C of disagreement normal?
On the Brannan sheet, ±2.0 °C is the stated tolerance at ambient for model 14/419/3, not a test of the other five listings. In the shade, a disagreement of about that size is inside that line. In the sun, reviewers are nowhere near it.
What if it shows 50 °C and never changes?
If it never moves, treat it as a fault rather than as sunshine. A Youshiko review registered 50 °C and was called faulty. A Brannan that hits about 50 °C only while the sun is on the case, and falls once the pane is shaded, is siting. No change at all is a dead unit.
Is a digital less affected by sun than a dial?
Not because it is digital. The Youshiko and the dials sit on the same glass, and the sun heats the case either way. An LCD can be harder to read in daylight, which is a contrast problem, not immunity. A unit stuck at one number is a fault. A number that soars only at noon is siting, on both kinds.
Can you adjust the pointer?
One Faithfull review mentions a small adjustment screw, and says the packaging does not. The Brannan sheet does not invite you to bend the pointer to match the forecast. If you do find a screw, set it in the shade, or you will correct a sun error into the dial. Most of these gauges have nothing to turn.

Related pages

Window thermometer buying guide

Window thermometer

Best window thermometer UK, one listing for each job

Digital vs analogue window thermometer: which to buy

How to stick a thermometer on outside window glass

Sources

Public UK sources used for this comparison. Listing prices, stars, and review counts are the amazon.co.uk dump dated 15 October 2026.

  1. Met Office — how we measure temperature (retrieved 15 October 2026)
  2. Met Office — make your own thermometer box (retrieved 15 October 2026)
  3. GOV.UK — controls on mercury-added products (retrieved 15 October 2026)
  4. Consumer Rights Act 2015, section 22 (retrieved 15 October 2026)
  5. Brannan data sheet 14/419/3 (retrieved 15 October 2026)
  6. Brannan — dial window thermometer (retrieved 15 October 2026)
  7. RHS — heating greenhouses efficiently (retrieved 15 October 2026)
  8. Thermometer World — dial window thermometer (retrieved 15 October 2026)