Temperature measurement tech still effective 160 years on

Victorian thermometer boxes can still deliver remarkably accurate temperature readings almost two centuries after they were first introduced, a new study has shown.

A research team from the University of Reading found that Stevenson screens, which are still used in weather stations across the world, are consistently producing accurate temperature readings for vital meteorological data collection. 

Misrepresented temperature recordings from Stevenson screens are rare, and the difference typically very small, according to the study published this month in Atmospheric Science Letters. The research team behind the study highlighted that these small errors only occur on calm, clear days.

Lead researcher Professor Giles Harrison said: "Improved methods of air temperature measurement are becoming available but need to be evaluated against the traditional methods to ensure consistency of the climate record. We compared a modern aspirated thermometer system with a traditional Stevenson screen, concentrating on daily maximum and minimum temperatures.

“We saw some differences during calm, clear nights in the minimum temperatures, and, surprisingly, on some winter days with low sun angles in the maximum temperatures. Daytime maximum temperatures in summer were less affected, when winds are typically stronger.

“Generally, the Stevenson screen has served us well and continues to. Significant discrepancies between its readings and the modern method were only occasional, and our findings in no way challenge the important understanding that global climate is warming overall."

How it works

Stevenson screens are a highly recognisable feature of weather stations across the UK. Thomas Stevenson, lighthouse engineer and father of the author Robert Louis Stevenson first described his “…box for holding thermometers” in the Journal of the Scottish Meteorological Society in 1864. Stevenson screens have louvered sides to permit the wind to ventilate the thermometers or other temperature sensors they enclose, whilst also offering protection from rain and direct sunshine.

The more modern approach, one increasingly preferred worldwide, is to ‘aspirate’ the temperature sensor – using such equipment, air is forced over the sensor day and night using a small fan. Aspiration avoids the stagnation of warm or cold air which can occasionally happen in Stevenson screens.

Advancing technology

The study analysed more than 1.5 million points of temperature readings, comparing a conventional Stevenson screen with a state-of-the-art aspirated thermometer. While differences were typically minimal, researchers found that in some rare weather conditions, traditional screens showed biases with a magnitude exceeding 1 °C in daily maximum and minimum temperatures.

The research estimates that across Europe, conditions potentially affecting measurements in traditional screens occur for about 12% of minimum temperatures and 4% of maximum temperatures. The researchers stress that these findings do not undermine the clear trend of global warming, and highlight the ongoing advances in temperature measurement technology.

Dr Stephen Burt, of the University of Reading and co-author of the study, said: “As meteorological techniques advance, we're able to capture temperature data with increasing precision and accuracy. This study highlights how the field is continuously improving, allowing for more consistent tracking of climate patterns and extreme weather events."

Read Professor Giles Harrison’s commentary on the new study on The Conversation: Victorian technology for measuring the weather is still remarkably accurate – new research

Harrison, R. G., & Burt, S. D. Accuracy of daily extreme air temperatures under natural variations in thermometer screen ventilation. Atmospheric Science Letters, e1256. https://doi.org/10.1002/asl.1256