Clear Sky Calculator 2.0.5

The Clear Sky Calculator serves as a tool to assess the necessity of recalibrating radiation sensors. Its precision is enhanced when utilized near solar noon during the summer months. The calculator estimates the incident solar radiation — whether total global shortwave radiation measured by pyranometers or global photosynthetic photon flux density captured by quantum sensors — on a horizontal surface at any time and location worldwide. The underlying equations are derived from the clear sky solar radiation model employed in calculating net radiation within the ASCE Standardized Reference Evapotranspiration Equation (http://www.kimberly.uidaho.edu/water/asceewri/index.html).

The only required inputs are site elevation, latitude, longitude, reference longitude, air temperature, and relative humidity measurements or estimates. These data points are generally readily accessible, making the Clear Sky Calculator a practical reference for estimating sensor accuracy and identifying calibration needs.

When applied around solar noon on multiple clear, unpolluted days during spring and summer, the calculator demonstrates an estimated accuracy of approximately ± 4 % across diverse climates and global locations. For instance, in Logan, Utah, on April 21, 2012, modeled incoming shortwave radiation (SWi) from the calculator closely aligned with actual measurements obtained from a heated and ventilated Kipp & Zonen CM21 pyranometer. The ratio of measured SWi to modeled SWi ranged from 1.00 to 1.05 between 9 AM and 6 PM (solar zenith angles less than 65°), with an average ratio of 1.02 ± 0.01 during a window spanning two hours before to two hours after solar noon (see http://www.apogeeinstruments.com/using-the-clear-sky-calculator/).

A comprehensive review of the accuracy of the Clear Sky Calculator is available at http://clearskycalculator.com/model_accuracy.htm, where the importance of precise input data is discussed in detail.

It is recommended that users leverage the Clear Sky Calculator to monitor the performance of pyranometers and quantum sensors regularly. Sensors exhibiting persistent discrepancies exceeding a few percent relative to the calculator should be evaluated for recalibration. Apogee encourages customers to contact us if significant deviations are observed, to ensure measurement integrity.