AERMOD Tech Guide
8.2 Estimation of Solar Insolation for Small Solar Elevation Angles
If the sun is above the horizon but the elevation is less than about 2┬░ ,eq.(5) estimates the clear sky insolation, Ro , to be negative. To avoid this situation, Ro is approximated for sola elevations less than 10 ┬░ by linearly interpolating between 0.0 W m-2 (=0┬░ ) and 141.91 W m- -2 2 (=10 ┬░ ).