Speaker
Dr
Nonhlanhla Precious Maphaha
(National Metrology Insitute of South Africa)
Description
All calibrations of thermometers for meteorological or climatological applications are based on the International Temperature Scale of 1990, ITS-90. Based on the best science available in 1990, ITS-90 specifies procedures which enable cost-effective calibration of thermometers worldwide. In this paper we discuss the impact for meteorology of two recent developments: the forthcoming 2019 redefinition of the kelvin, and the emergence of techniques of primary thermometry that have revealed small errors in ITS-90.
The kelvin redefinition. Currently, the International System Units, the SI, defines the kelvin (and the degree Celsius) in terms of the temperature of the triple point of water, which is assigned the exact value of 273.16 K (0.01 °C). From 2019, the SI definitions of these units will change such that the kelvin (and the degree Celsius) will be defined in terms of the average amount of energy that the atoms and molecules of a substance possess at a given temperature. This will be achieved by specifying an exact value of the Boltzmann constant, kB, in units of joules per kelvin. Thus after 2019, measurements of temperature will become fundamentally measurements of the energy of molecular motion. However, because thermometers will continue to be calibrated according to the procedures specified in ITS-90, this will have no immediate effect on the practice of meteorology.
Errors in ITS-90. Since 1990, the primary thermometry technique known as acoustic thermometry has improved to an extraordinary extent. Acoustic thermometry measurements are now capable of detecting errors in the primary thermometry used to construct ITS-90, and hence in ITS-90 itself. Over the meteorological range these errors are small but they are present in every calibrated thermometer on Earth. The errors vary approximately linearly between ≈ +0.005 K at ~ +50 °C, and ≈ -0.003 K at ~-30 °C. Errors of this magnitude are unlikely to concern meteorologists, but if there is in future a shift away from ITS-90 to a new International Temperature Scale, then the concomitant shifts in practical temperature calibrations may be just detectable by homogenisation algorithms used in climate studies.
Level for award<br> (Hons, MSc, <br> PhD, N/A)?
NO
Apply to be<br> considered for a student <br> award (Yes / No)?
NO
Primary author
Dr
Nonhlanhla Precious Maphaha
(National Metrology Insitute of South Africa)
