Follows is the official NWS recommendation on how to site an air temp gauge. They say the gauge should be freely exposed to air, sunshine and correctly shielded. I interpret that to mean the sensor should not be put under a tree, porch, shady side of house, etc. The manufacturers of such systems should remember to include NWS 10-1302 and either an effective temp shield or else instructions on how to build a Stevenson screen. Separate sensors are a good thing but they need to be correctly setup.
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NATIONAL WEATHER SERVICE INSTRUCTION 10-1302
NOVEMBER 14, 2014
Operations and Services
Surface Observing Program (Land), NDSPD 10-13
REQUIREMENTS AND STANDARDS FOR NWS CLIMATE OBSERVATIONS
........4 Air temperature. Air temperature is temperature of the free air conditions surrounding the
station at a height between 4 and 6 feet above ground level. The air should be freely exposed to
sunshine and wind and not close to or shielded by trees, buildings, or other obstructions.
4.1 Air Temperature Measurement Performance. The following temperature instruments are
used in the NWS cooperative observing program:
a. maximum / Minimum Temperature System (i.e., MMTS)
b. liquid-in-glass maximum and minimum thermometers
Temperature instruments will be shielded from the following elements:
a. precipitation
b. direct and reflected sunshine
c. direct and reflected thermal energy (i.e., Infrared)
All thermometers should be shielded with a thermoscreen or radiation shelter just large enough
to protect against the elements stated, and slotted sufficiently to allow air to advect naturally into
and out of the thermoscreen during calm air conditions. Powered aspirators are not required for
these instruments.
All temperature measuring instruments should be issued with a certificate confirming
compliance with the appropriate performance specification and accuracy; or be issued with a
calibration certificate which gives the corrections that are applied to meet the required accuracy.
This initial testing should be performed by an accredited calibration laboratory or a national
testing institution.......
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