2008 GLOS Mooring deployment
Shortwave radiation (don't be put off by the word "radiation") is simply energy recieved by the sun. Technically, this is all electromagnetic energy between about 0.3 and 3.0 um. This is the dominant source of heat energy to the lake and is largely responsible for the lake warming up in the summer (when the sun is high in the sky and we recieve a lot of SW radiation). In the winter, losses to other heat transfer mechanisms win out over the small amount of SW radiation recieved and the lake cools off.
We can make an estimate of "clear-sky" shortwave radiation for a location based on simple geometry; plotting the two together:
you can see that the geometric prediction (in green) does a pretty good job (this is how we predict sunrises and sunsets), but of course does not predict cloud cover, which accounts for the difference between the two curves. Cloud cover is a another major player in determining how much heat gets to the surface.
Here's what the sensor looks like: