During a discussion at realclimate.org, I was prompted to investigate some of the ways of determining the radiation flux.
I came across an interesting NASA website, which shows an animation of the monthly radiation flux on earth from July 2006 to now. It also shows changes in cloud cover and snow together with the radiation flux. It's fascinating to look at the radiation changes through the seasons in the northern and southern hemisphere. And how some parts of earth have almost constant cloud cover, particularly over northern Africa. The changes in snow cover are interesting as well. The animation clearly shows the extensive snow over North America this winter (January 2010).
The changes are monitored by satellites: the Clouds and the Earth's Radiant Energy System (CERES) sensors on NASA’s Terra and Aqua satellites described here.
Ceres is the goddess of agriculture and motherly love and we may need to seek her help in coming years if we want to keep producing enough food to feed everyone.