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I'd like to know how the state-of-the-art meteorological direct, localized measurements of the temperature of the air is measured.

If you put a thermometer in sunlight it may read high. If you put it in a box with some air vents it may also read high during the day and exhibit a time lag due to local trapping of air when there's no wind and due to radiation exchange with the thermal cavity in which it now sits.

If there's suspended droplets of water in the air it can condense or simply collect on surfaces and later evaporate which can act as a source or sink of heat with the thermometer.

The rate of heat exchange between the air and the thermometer is pretty low due to the relatively low density of air, so these other effects can significantly compete.

So I'd like to ask:

Question: How do meteorologists accurately measure outdoor air temperature?

Of course there are other ways to determine the average temperature of a geographically large amount of air of some significant thickness in height, but I'm asking about localized measurements at say a meteorological data collection station at an airport or other fixed site, collecting data for archival purposes or weather predictions, or even air temperature measurements from a system affixed to a weather balloon or aircraft used to collect data.

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    $\begingroup$ en.wikipedia.org/wiki/Stevenson_screen $\endgroup$
    – trond hansen
    Commented Apr 29, 2022 at 3:50
  • $\begingroup$ @trondhansen I wonder how they confirm that this actually works to provide accurate air temperature beyond simply "Hmm... makes sense, it looks like it should work."? I see that on after page 47 in Wikipedia's linked Commission for Instruments and Methods of Observation, Fifteenth session, Helsinki, 2–8 September 2010 they do mention some screens, scenarios and potential magnitude of temperature errors associated with them. I'll bet there is solid research behind this, findable somewhere. $\endgroup$
    – uhoh
    Commented Apr 29, 2022 at 4:08
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    $\begingroup$ One could write an entire book chapter to answer this question, but check WMO guides — they provide for minimum standards for officially approved temperature measuremens. $\endgroup$
    – gerrit ♦
    Commented Apr 29, 2022 at 8:07
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    $\begingroup$ There is conventional way to do so, having unified white-painted box with lamel walls to allow air flow, 2 m height, above the cut grass surface. It is not so much important how it is done, it is important it is done the same way everywhere for values to be comparable. $\endgroup$
    – Poutnik
    Commented Apr 29, 2022 at 13:41
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    $\begingroup$ @ohoh Note that meteorology is an applied science, where there is many trade offs for scientific accuracy, statistical applicability, technical and financial availability. Measuring has to be consistent both spatially and temporally. Measuring temperatures more accurately is wrong, if it breaks both these continuities. $\endgroup$
    – Poutnik
    Commented May 3, 2022 at 7:07

1 Answer 1

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True air temperature may be an elusive quantity to measure but shielding thermometers and sensors from direct solar radiation has been the main approach for a long time. The best results are provided by so-called aspirated radiation covers where sensors sit inside a radiation cover which is augmented by a fan that allows fresh air to pass across the sensor and therefore allowing fresh air, not air warmed by the shield, to be the basis for the measurement.

There are lots of reports on comparisons between shields provided by scientists and makers of radiations shields (as simple search will give numerous sources). These indicate that deviations of up to 7 degrees C can occur if non-aspirated shields heat up during zero wind conditions. That aside the Swedish Meteorological and Hydrological Institute use the Vaisala shields, linked in a comment to the question, as a standard.

At the Tarfala Research Station we have compared the old wood Stevenson screens with Young plastic screens and aspirated sensors and the old Stevenson screen actually performs better that the smaller plastic screens. There may be several reasons for this such as larger thermal inertia in the wooden screens and also larger volume of air inside the screen and likely larger air flow through those screens than the smaller plastic screens. Nevertheless, the aspirated screens are superior because they more quickly reflect changes in air temperature.

One could always argue what is the true air temperature. This will depend on where measurements are made relative to ground and also wind conditions. So any temperature measurement will likely be associated with errors. At the same time one could ask oneself what accuracy is required and for what reason we measure. Changing measurement method may introduce offsets that are simply instrumental and not necessarily climatological so great care has to be taken when interpreting temperature records involving changes in recording environment.

Example sources:

Tage Andersson and Ingemar Mattisson, 1992. A field test of thermometer screens. SMHI RMK No62. http://www.diva-portal.org/smash/get/diva2:947985/FULLTEXT01.pdf

Apogee instruments. Comparison of Three Fan-Aspirated Solar Radiation Shields. https://www.apogeeinstruments.com/comparison-of-three-fan-aspirated-solar-radiation-shields/

Comparison studies of helical vs. multi-plate radiation shields. https://allmeteo.com/radiation-shield-comparisons

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