Radiation screens and their impact on air temperature measurement: a review

Outdoor air temperature measurements are essential to be applied to agriculture, meteorology, hydrology, and urban planning. Precise measurements are particularly needed in the lower troposphere where the existence of large temperature gradients is prevalent. Urban temperature observation has been an area of research concern in recent years, particularly to study 1the urban heat island effect, local overheat, and heat reduction measures. Temperature readings most often employ sensors housed in protective shields. Low thermal inertia and fastresponding thermocouples and platinum probes, though less accurate, are two sensor types that are common. Shielding is used to minimize error by protecting sensors from precipitation and direct sunlight and allowing ventilation. Poor shielding can generate large temperature differences, over +4°C. Different shielding designs, such as Stevenson screens, Gill shields, and aspirated shelters, are investigated. Shielding efficiency depends upon their optical property, ventilation, and design. Correction methods like empirical corrections, numerical models including CFD, and neural network are also being considered to maximize data accuracy over urban areas. This paper offers an overview of studies that investigate radiation shields used for the measurement of external air temperatures and compare their performance in terms of various attributes. The methodology for selecting studies is presented first, encompassing field measurements, laboratory experiments, and numerical models. An inter-comparison of literature is then conducted, distinguishing commercially manufactured and home-made shields, ventilated and non-ventilated designs, and other significant features such as shape, size, optical properties, and materials. Errors in measurements, their reasons, and the procedures for correcting them are treated with special care.