<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>Example sources:</p>
<p>Tage Andersson and Ingemar Mattisson, 1992. A field test of thermometer screens. SMHI RMK No62. <a href="http://www.diva-portal.org/smash/get/diva2:947985/FULLTEXT01.pdf" rel="noreferrer">http://www.diva-portal.org/smash/get/diva2:947985/FULLTEXT01.pdf</a></p>
<p>Apogee instruments. Comparison of Three Fan-Aspirated Solar Radiation Shields. <a href="https://www.apogeeinstruments.com/comparison-of-three-fan-aspirated-solar-radiation-shields/" rel="noreferrer">https://www.apogeeinstruments.com/comparison-of-three-fan-aspirated-solar-radiation-shields/</a></p>
<p>Comparison studies of helical vs. multi-plate radiation shields. <a href="https://allmeteo.com/radiation-shield-comparisons" rel="noreferrer">https://allmeteo.com/radiation-shield-comparisons</a></p>

CC BY-SA 4.0
2022-05-05T16:17:38.403Z
2024-05-15T23:54:34.097Z
2024-05-15T23:54:34.097Z
Peter Jansson
<p>I'd like to know how the state-of-the-art meteorological direct, localized measurements of the temperature of the air is measured.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>So I'd like to ask:</p>
<p><strong>Question:</strong> How do meteorologists accurately measure outdoor air temperature?</p>
<p>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.</p>

CC BY-SA 4.0
2022-04-29T03:50:28.973Z
https://en.wikipedia.org/wiki/Stevenson_screen
trond hansen
CC BY-SA 4.0
2022-04-29T04:08:58.077Z
@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](https://library.wmo.int/doc_num.php?explnum_id=4939) 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.
uhoh
CC BY-SA 4.0
2022-04-29T07:54:19.117Z
@trondhansen the Stevenson screen is rather outdated. I think it is more common to use smaller devices like the Vaisala DTR protection line for temperature measurements. See here: https://www.vaisala.com/sites/default/files/documents/DTR13-DTR15-Datasheet-B210335EN-B.pdf . No expert however... So maybe someone who actually does measurements can elaborate.
Joscha Fregin
CC BY-SA 4.0
2022-04-29T08:07:21.157Z
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.
gerrit
CC BY-SA 4.0
2022-04-29T08:55:36.550Z
@gerrit then in principle at least said book chapter is likely to already exist somewhere. Hopefully it can be found :-)
uhoh
CC BY-SA 4.0
2022-04-29T13:41:02.137Z
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.
Poutnik
CC BY-SA 4.0
2022-04-29T18:45:51.867Z
@Poutnik "It is not so much important how it is done..." I am not sure that's true. If visible sunlight and/or infrared radiative heating/cooling has some impact on the attempted air temperature measurement, then changes in cloud cover over decades could induce a small shift in absolute temperature.
uhoh
CC BY-SA 4.0
2022-04-29T18:57:12.567Z
What I have said stays. Sunlight and IR radiation have in the first place impact on surface and near immediately the air temperature as well. Note that I have studied meteorology for 4 semesters in the military department at university as then mandatory preparation for being later  enlisted meteorologist on the military airfield.
Poutnik
CC BY-SA 4.0
2022-04-29T19:01:39.753Z
@Poutnik that's excellent! But we should always review and question what implicit assumptions might be found behind what we are taught. When "X doesn't matter" becomes doctrine without quantitative proof, there is a possibility that it should be reexamined quantitatively when X becomes important in a new and different context. Science is constantly reevaluating, updating, revising and correcting itself and occasionally turning itself on its head. We must question everything all the time.
uhoh
CC BY-SA 4.0
2022-05-03T07:07:44.157Z
@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.
Poutnik
CC BY-SA 4.0
2022-05-09T23:39:59.237Z
Consistency of method used can be more important than absolute precision - for many uses it isn't the exact temperature that matters most, it is the measuring of change between days, seasons, years, decades. Like Poutnik says doing it the same way allows the values to be compared. In a complex landscape there won't be any one absolute temperature.
Ken Fabian
CC BY-SA 4.0
2022-05-09T23:44:54.877Z
@KenFabian but if the method confounds one thing with another - direct sunlight or radiation with air temperature and the confounding influences change, then just doing it wrong *consistently* looks a lot less optimal.
uhoh
CC BY-SA 4.0
2022-05-11T07:33:32.297Z
@uhoh - the methods in regular use do make efforts to avoid factors like direct sunlight, proximity to the ground, buildings, lack of breeze and other local factors and appear to do so reasonably well but if you are eg looking for evidence of change in average temperatures over time (a significant use) it is more important to have consistent measurement methods than an absolute and precise temperature. There is too much potential variability in absolute temps - a shift of breeze carrying hotter air from sunlit concrete or cooler from nearby vegetation will change temps from moment to moment.
Ken Fabian
CC BY-SA 4.0
2022-05-11T08:08:56.520Z
@KenFabian nothing I wrote says otherwise. I'm just saying that just because A has been felt to be "more important" than B in the past doesn't mean it will be true now and in the future.
uhoh
CC BY-SA 4.0
2022-05-15T01:15:41.377Z
@uhoh -  I suspect the way to get better temp records is shifting from daily max/min to continuous temperature recording and include wind speed and direction, humidity and insolation measurements. With close to real time transmission/reporting.
Ken Fabian
CC BY-SA 4.0
2022-05-15T01:42:09.793Z
@KenFabian The question's title is "How do meteorologists accurately measure outdoor air temperature? There's sunlight, local trapping of air, radiant heat, precipitation" Certainly having additional parameters would be helpful to correct errors in air temperature measurement due to correlations *if those instrumental correlations are quantified.* Have there been efforts to do so? If so, that would be the basis of an excellent answer post to this question as-written.
uhoh
CC BY-SA 4.0
2024-07-11T19:02:02.283Z
The ultimate solution to radiative error is my patented Radiation Compensated Thermometer/bolometer. It tracks the sun's transit using a larger white shield on top and a smaller white shield on the bottom. Inside the shields are painted black which acts as a heat sink/adsorption isotherm. The RCT tracks daytime sunlight at various angles much like a telescope. When compared to a
Gill plate shield it revealed a 1 degree error at solar noon and up to a 5 degree error before and after. Ventilated shield data does not compensate for
the humidity in the air sample creating friction as per ASHRAE st
david Bergstein
<meteorology><atmosphere><instrumentation><air>
How do meteorologists accurately measure outdoor air temperature? There's sunlight, local trapping of air, radiant heat, precipitation
uhoh
# How do meteorologists accurately measure outdoor air temperature? There's sunlight, local trapping of air, radiant heat, precipitation

(Asked by: username_0 on 2022-04-29)

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.

**Tags:** meteorology, atmosphere, instrumentation, air

> **Comments:**
> **username_1**: https://en.wikipedia.org/wiki/Stevenson_screen  
> **username_0**: @username_1 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](https://library.wmo.int/doc_num.php?explnum_id=4939) 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.  
> **username_2**: @username_1 the Stevenson screen is rather outdated. I think it is more common to use smaller devices like the Vaisala DTR protection line for temperature measurements. See here: https://www.vaisala.com/sites/default/files/documents/DTR13-DTR15-Datasheet-B210335EN-B.pdf . No expert however... So maybe someone who actually does measurements can elaborate.  
> **username_3**: 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.  
> **username_0**: @username_3 then in principle at least said book chapter is likely to already exist somewhere. Hopefully it can be found :-)  
> **username_4**: 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.  
> **username_0**: @username_4 "It is not so much important how it is done..." I am not sure that's true. If visible sunlight and/or infrared radiative heating/cooling has some impact on the attempted air temperature measurement, then changes in cloud cover over decades could induce a small shift in absolute temperature.  
> **username_4**: What I have said stays. Sunlight and IR radiation have in the first place impact on surface and near immediately the air temperature as well. Note that I have studied meteorology for 4 semesters in the military department at university as then mandatory preparation for being later  enlisted meteorologist on the military airfield.  
> **username_0**: @username_4 that's excellent! But we should always review and question what implicit assumptions might be found behind what we are taught. When "X doesn't matter" becomes doctrine without quantitative proof, there is a possibility that it should be reexamined quantitatively when X becomes important in a new and different context. Science is constantly reevaluating, updating, revising and correcting itself and occasionally turning itself on its head. We must question everything all the time.  
> **username_4**: @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.  
> **username_5**: Consistency of method used can be more important than absolute precision - for many uses it isn't the exact temperature that matters most, it is the measuring of change between days, seasons, years, decades. Like username_4 says doing it the same way allows the values to be compared. In a complex landscape there won't be any one absolute temperature.  
> **username_0**: @username_5 but if the method confounds one thing with another - direct sunlight or radiation with air temperature and the confounding influences change, then just doing it wrong *consistently* looks a lot less optimal.  
> **username_5**: @username_0 - the methods in regular use do make efforts to avoid factors like direct sunlight, proximity to the ground, buildings, lack of breeze and other local factors and appear to do so reasonably well but if you are eg looking for evidence of change in average temperatures over time (a significant use) it is more important to have consistent measurement methods than an absolute and precise temperature. There is too much potential variability in absolute temps - a shift of breeze carrying hotter air from sunlit concrete or cooler from nearby vegetation will change temps from moment to moment.  
> **username_0**: @username_5 nothing I wrote says otherwise. I'm just saying that just because A has been felt to be "more important" than B in the past doesn't mean it will be true now and in the future.  
> **username_5**: @username_0 -  I suspect the way to get better temp records is shifting from daily max/min to continuous temperature recording and include wind speed and direction, humidity and insolation measurements. With close to real time transmission/reporting.  
> **username_0**: @username_5 The question's title is "How do meteorologists accurately measure outdoor air temperature? There's sunlight, local trapping of air, radiant heat, precipitation" Certainly having additional parameters would be helpful to correct errors in air temperature measurement due to correlations *if those instrumental correlations are quantified.* Have there been efforts to do so? If so, that would be the basis of an excellent answer post to this question as-written.  
> **username_6**: The ultimate solution to radiative error is my patented Radiation Compensated Thermometer/bolometer. It tracks the sun's transit using a larger white shield on top and a smaller white shield on the bottom. Inside the shields are painted black which acts as a heat sink/adsorption isotherm. The RCT tracks daytime sunlight at various angles much like a telescope. When compared to a Gill plate shield it revealed a 1 degree error at solar noon and up to a 5 degree error before and after. Ventilated shield data does not compensate for the humidity in the air sample creating friction as per ASHRAE st  

---

## 1 answers

### Answer by username_7 on 2022-05-05. Score: 9 | Accepted answer

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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