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The Quest for Accurate Air Temperature (Part 2)

In the conclusion to last week’s blog, Mark Blonquist, chief scientist at Apogee Instruments and air temperature measurement expert, explains the complexities of some proposed solutions to the problems that challenge accurate air temperature measurement.

A aspirated raditation shield by Apogee Instruments

An aspirated radiation shield manufactured by Apogee Instruments in Logan, Utah. Multiple models of passive and active shields are available from several manufacturers.

Solution:  Passive Radiation Shield

In addition to an accurate sensor, accurate air temperature measurement requires proper shielding and ventilation of the sensor.  Passive shields do not require power, making them simple and low-cost, but they warm above air temperature in low wind or high solar radiation. Warming is increased when there is snow on the ground due to increased solar radiation load from higher albedo and increased reflected solar radiation. Errors as high as 10 degrees C have been reported in passive shields over snow (Genthon et al., 2011; Huwald et al., 2009).  The figure below shows the differences in error for the two conditions.

Shortwave Radiation > 50 W m-2 diagram

Corrections for Passive Shields

Equations to correct air temperature measurements in passive shields have been proposed, but often require measurement of wind speed and solar radiation, and are applicable to a specific shield design.  Corrections that don’t require additional meteorological measurements have also been proposed, such as air temperature adjustment based on the difference between air temperature and interior plate temperature differences. Others have suggested modifying traditional multi-plate passive shields to include a small fan that can be operated under specific conditions, but using natural aspiration when wind speeds are above an established threshold.

Solution: Active Shields

Warming of air temperature sensors above actual air temperature is minimized with active shields, which are more accurate than passive shields under conditions of high solar radiation load or low wind, but power is required for the fan. The power requirement for active shields ranges from one to six watts (80-500 mA). For solar-powered weather stations, this can be a major fraction of power usage for the entire station and has typically required a large solar panel and large battery. Power requirement and cost are disadvantages of active shields (Table 3), and they have led to the use of less accurate passive shields on many solar-powered stations.

Also, the fan motor can heat air as it passes by. Active shields should be constructed to avoid recirculation of heated air back into the shield.  There is no reference standard for the elimination of radiation-induced temperature increase of a sensor for air temperature measurement, but well-designed active shields minimize this effect.

Advantages and disadvantages of passive and active radiation shield diagram

Table 3: Advantages and disadvantages of passive (naturally-aspirated) and active (fan-aspirated) radiation shields.

There is no reference standard for the elimination of radiation-induced temperature increase of a sensor for air temperature measurement, but well-designed active shields minimize this effect. Radiation-induced temperature increase was analyzed in long-term experiments over snow and grass surfaces by comparing temperature measurements from three models of active radiation shields (the same temperature sensor was used in all shields and were matched before deployment). Continuous measurements for one year indicated that mean differences among shield models were less than 0.1 C over grass and less than 0.3 C over snow. Differences increased with increasing solar radiation, particularly during winter months when there was snow (high reflectivity) on the ground.

Air Temperature: a Complex Measurement

The properties of materials and nearly all biological, chemical, and physical processes are temperature dependent. As a result, air temperature is perhaps the most widely measured environmental variable. Accurate air temperature measurement is essential for weather monitoring and climate research worldwide. The road to accuracy is complex, however, and will continue to be challenging given the trade-off between accuracy and power consumption with passive and active shields.

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