4. Thermometer Screens

Abstract

In meteorological observations the temperature of the air is of the first importance, and it is a subject to which a great deal of attention is given, thousands of temperature observations being made and recorded every day. It is therefore desirable that these observations shall be as correct as possible. At first sight nothing seems more simple than to take the temperature of the air. All that appears necessary is to hang up a correct thermometer of any construction, anywhere out of the sun, and the thermometer will then indicate the temperature of the air where the instrument is placed. We shall presently see that this is very far from being the case; and not only so, but we shall find that it is difficult to get two thermometers which will give the same readings, when hung near each other in the open air, even though they agree perfectly with each other when placed in water.

Ventilation effects on humidity measurements in thermometer screens

“…Ventilation affects the lag times of screen thermometers in terms of their ability to measure air temperature (Aitken, 1921;Harrison, 2010), but it also strongly affects the psychrometer coefficient needed in the calculation of RH. The ventilation of a psychrometer can be very poor, since the air speed measured within a screen is several times smaller than the wind speed conventionally observed outside it (Keil, 1996).…”

LiG Metrology, Correlated Error, and the Integrity of the Global Surface Air-Temperature Record

“…All the examined calibration-experiment error sets revealed a warm bias and a no normal distribution. Additional calibration experiments, not discussed here, have yield similar air-temperature-measurement uncertainties that invariably arise with the use naturally ventilated shields [34,35,44,45,68]. A high-quality Siemens thermistor housed a naturally ventilated Stevenson screen produced a non-normally distributed measu ment error (N = 144) (Figure S9 in the Supplementary Materials) [35].…”

“…Field calibrations of land-surface air-temperature sensors deployed in the United States Historical Climatology Network (USHCN) have invariably revealed systematic temperature measurement errors deriving from uncontrolled environmental variables [8,[31][32][33][34][35][36][37][38][39][40]. These environmental variables include direct solar irradiance or surface albedo reflectance, which cause heating within naturally ventilated louvered thermometer screens, or low wind speed (<5 m/s) that is insufficient to ventilate the sensor with external ambient air [40][41][42][43][44][45]. Systematic errors under field conditions vary unknowably across sign and magnitude, and have a negative impact on measurement accuracy [30,[46][47][48].…”

Lag‐time effects on a naturally ventilated large thermometer screen

“…Naturally ventilated thermometer screens are widely used for air temperature measurement (HMSO, 1981;Strangeways, 2003), but their deficiencies under poor ventilation conditions have long been recognised (Aitken, 1884a(Aitken, ,b, 1886(Aitken, , 1887(Aitken, , 1921Middleton, 1966). In general, naturally ventilated thermometer screens have difficulties at low winds speeds due to increased lag times; these can be up to 30 min for a Stevenson screen under calm conditions (Bryant, 1986).…”