The following graph shows the diurnal (night and day) cycle of solar radiation from August 19th to August 21st (the day of the eclipse). The light orange clear sky solar radiation is the theoretical radiation on a day with no clouds. The dark orange displays the radiation observed by the pyranometer at our Silver Lake station.
Satellite images
Noon, Aug 19
Noon, Aug 20
Noon, Aug 21
The images above show the cloud cover at noon (CDT) for each day. Note how the progression from clear sky to clouds over the three days corresponds with the progression from smooth to jagged solar radiation curves. Images are sourced from NOAA.
Why are the curves offset?
Notice that the observed radiation increases earlier in the morning than the clear sky radiation. This probably indicates that the pyranometer isn't exactly level. In this case, it is probably tilted slightly to the east, since it catches the sun earlier. Tilt can result from birds landing on the platform, among other reasons. We check the level at least twice per year during routine maintenance.
What is Solar Radiation?
According the The American Meteorological Society, solar radiation is defined as the total electromagnetic radiation emitted by the sun. About one-half of the total energy in the solar beam is contained within the visible spectrum from 0.4 to 0.7 μm, and most of the other half lies in the near-infrared, a small additional portion lying in the ultraviolet.
The pyranometer used in the mesonet measures global solar radiation. That is the total solar radiation, direct and diffuse, measured on a horizontal surface.
Clear Sky vs. Observed
Clear sky solar radiation is the theoretical incoming radiation without any cloud interference or reflection. The model used on this page comes from the Department of Energy.
Observed radiation, on the other hand, shows what our pyranometer actually measured. Since Northeastern Kansas had partly to mostly cloudy skies, the observed radiation fluctuates wildly. Radiation is diminished by clouds blocking the sun and returns to normal when the clouds move away.
In some cases, the incoming radiation it can actually exceed clear sky levels when adjacent clouds reflect additional radiation toward the pyranometer. See the Resources section for further information on comparing clear sky models to observed radiation.