Sub zero weather

Comment: the heat is on to find new ways to measure temperature

Equipment manufacturers are yet to come up with a practical alternative to mercury thermometers that are due to be phased out by 2020.

Weather affects almost every aspect of our daily lives. Extreme weather can have major economic impacts, as well as causing personal tragedies. In Britain alone, economists estimate that monitoring and forecasting weather accurately contributes £3bn a year to the economy. Over decades, subtle changes in patterns of weather form the basis of climate change, disrupting societies and economies.

Understanding the science of weather and climate, and harnessing this knowledge to provide accurate forecasts in time to allow a response, has become a major industry. Some of the world’s largest supercomputers are devoted to calculating the equations on which complex mathematical models of the weather are based.

Feeding these models requires continuous observation of parameters from barometric pressure to temperature, solar radiation, wind speed, direction and many more. In developed countries, and increasingly in developing nations, automatic weather stations feed a stream of data back to meteorological agencies or private firms.

One of the most important measurements is temperature, not only of the air itself but also of surfaces at ground level, in the soil and the surface and deeper layers of the ocean. For nearly 200 years, meteorological observers have refined temperature measurement to ensure that the body of data is consistent over time and across locations. Confidence that the long time series of temperature measurements are internally consistent makes it possible to identify trends that indicate changes in climate.

Alongside the sophisticated automatic weather stations, there is an army of individuals who collect measurements to fill in the gaps in the major networks, either as volunteers or as a small part of their ordinary job. Merchant ships take regular measurements of significant parameters over the course of their voyages. In the past, this was often the only record available for large parts of the Earth’s surface, but even today they provide ‘ground truth’ to calibrate remote sensors on Earth-observing satellites.

Most weather stations now rely on electronic sensors. Carefully constructed, calibrated and designed to operate reliably under sometimes adverse conditions, these can record temperature accurately to within a tenth of a degree. However, for many human observers, the humble mercury-in-glass thermometer remains the instrument of choice. This has been refined over the 500 years since it was invented and, when read carefully, can measure air temperature to within 0.2°C.

The mercury thermometer has other benefits besides accuracy. It is relatively cheap, fully waterproof and works reliably from -35°C, up to more than 60°C. It requires no batteries and can be read, with the aid of a torch, at night as well as in daylight. It has a suitable response speed to temperature changes. This simple device has supported meteorologists well for centuries, but that is about to change.

While the amount of mercury in the average thermometer is very small and sealed in glass, mercury is a serious hazard to human health. Hundreds of people died in the Minimata Bay area of Japan as a result of long-term exposure to mercury waste via contaminated fish. The enduring legacy of Minimata has been an international treaty to bring about the end of mercury use in industry and technology by 2020, so meteorologists must find an alternative.

The difficulty is that electronic designers have so far failed to provide a truly effective solution. Electronic temperature-measuring instruments can be found on the market, priced from several tens of pounds through to more than £1,000, but very few combine the desired accuracy with the practical features and functionality needed. A clear display, long battery life, environmental robustness, a daily min/max capability and reasonable cost are called for, in a compact package. These requirements taken together are challenging, but should be within the capability of any competent designer.

If such an instrument was to be brought to market, meteorologists around the world would breathe a sigh of relief. The gauntlet is thrown down and, hopefully, the electronic engineering and design community will rise to the challenge. 

Neil Mander is an IET member and QA Lab manager at the Met Office

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