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Determination of the Thermal Offset of the Eppley Precision Spectral PyranometerEppley's precision spectral pyranometer (PSP) is used in networks around the world to measure downwelling diffuse and global solar irradiance at the surface of the Earth. In recent years several studies have shown significant discrepancy between irradiances measured by pyranometers and those computed by atmospheric radiative transfer models. Pyranometer measurements have been questioned because observed diffuse irradiances sometimes are below theoretical minimum values for a pure molecular atmosphere, and at night the instruments often produce nonzero signals ranging between + 5 and - 10 W/sq m. We install thermistor sondes in the body of a PSP as well as on its inner dome to monitor the temperature gradients within the instrument, and we operate a pyrgeometer (PIR) instrument side by side with the PSP. We derive a relationship between the PSP output and thermal radiative exchange by the dome and the detector and a relationship between the PSP output and the PIR thermopile output (net-IR). We determine the true PSP offset by quickly capping the instrument at set time intervals. For a ventilated and shaded PSP, the thermal offset can reach - 15 W/sq m under clear skies, whereas it remains close to zero for low overcast clouds. We estimate the PSP thermal offset by two methods: (1) using the PSP temperatures and (2) using the PIR net-IR signal. The offset computed from the PSP temperatures yields a reliable estimate of the true offset (+/- 1 W/sq m). The offset computed from net-IR is consistent with the true offset at night and under overcast skies but predicts only part of the true range under clear skies.
Document ID
Document Type
Reprint (Version printed in journal)
Haeffelin, Martial (Virginia Polytechnic Inst. and State Univ. Blacksburg, VA United States)
Kato, Seiji (Hampton Univ. VA United States)
Smith, Amie M. (Virginia Polytechnic Inst. and State Univ. Blacksburg, VA United States)
Rutledge, C. Ken (Analytical Sciences Corp. United States)
Charlock, Thomas P. (NASA Langley Research Center Hampton, VA United States)
Mahan, J. Robert (Virginia Polytechnic Inst. and State Univ. Blacksburg, VA United States)
Date Acquired
August 20, 2013
Publication Date
February 1, 2001
Publication Information
Publication: Applied Optics
Volume: 40
Issue: 4
ISSN: 0003-6935
Subject Category
Solar Physics
Funding Number(s)
Distribution Limits