On Irradiance Measurement Errors at Sea Due to Tilt of Pyranometers

Abstract

Calculations are made of errors in the measurement of solar irradiance at the sea surface. which occur when pyranometers are tilled. Tilts due to buoy or ship motion caused by wave action or a preferential tilt due to drag are considered. Errors depend on sky condition and are maximum for clear skies. They depend on soar zenith angle and on the relative azimuth of sun and tilt directions, and vary, therefore, with latitude and season or time of day. Errors as large as ±10% to ±20% in the daily average (with sign dependent on azimuth of the tilt) could be encountered for clear skies poleward of 45°N or S in the winter half of the year for a 10° preferential tilt. Instantaneous errors due to wave action can be as large. Because of the difficulty of making corrections a posteriori, gimbal mounting of pyranometers is recommended.