Abstract
Abstract. Climate science depends upon accurate measurements of air temperature and
humidity, the majority of which are still derived from sensors exposed
within passively ventilated louvred Stevenson-type thermometer screens. It
is well-documented that, under certain circumstances, air temperatures
measured within such screens can differ significantly from “true” air
temperatures measured by other methods, such as aspirated sensors.
Passively ventilated screens depend upon wind motion to provide ventilation
within the screen and thus airflow over the sensors contained therein.
Consequently, instances of anomalous temperatures occur most often during
light winds when airflow through the screen is weakest, particularly when in
combination with strong or low-angle incident solar radiation. Adequate
ventilation is essential for reliable and consistent measurements of both
air temperature and humidity, yet very few systematic comparisons to
quantify relationships between external wind speed and airflow within a
thermometer screen have been made. This paper addresses that gap by
summarizing the results of a 3-month field experiment in which airflow
within a UK-standard Stevenson screen was measured using a sensitive sonic
anemometer and comparisons made with simultaneous wind speed and direction
records from the same site. The mean in-screen ventilation rate was found to
be 0.2 m s−1 (median 0.18 m s−1), well below the 1 m s−1
minimum assumed in meteorological and design standard references, and only
about 7 % of the scalar mean wind speed at 10 m. The implications of low
in-screen ventilation on the uncertainty of air temperature and humidity
measurements from Stevenson-type thermometer screens are discussed,
particularly those due to the differing response times of dry- and wet-bulb
temperature sensors and ambiguity in the value of the psychrometric
coefficient.
Subject
Atmospheric Science,Geology,Oceanography
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