In a previous post, I talked about installing a new pyranometer to my weather station. Below is a graph of today’s solar radiation flux in W/m^2.
The graph clearly shows when the instrument was in shade (7:30 – 9:30 AM EST). Maximum value was after noon due to the fact that the longitude is about 3 deg west of the 75th meridian and because the equation of time for today is 3 minutes. The maximum solar elevation angle was about 28 deg. The optical depth of the atmosphere is significantly higher at such a low angle than if the sun was at zenith.
Stormstown Dispatch 