What conditions are needed to have the 'shafts of light' in the sky as seen in paintings?
Partly cloudy skies are needed, preferably with the puffy clouds called 'cumulus' clouds. Sunlight streams through the gaps, providing the 'shafts' or 'rays' of light. It's an effect called Crepuscular rays. The rays are really parallel...they only seem to spread out because of optical effects. If you stand at the end of a road, at the far end it looks like the sides of the road meet, but become increasingly separate or 'spread out' as they approach you. The same effect produces the impression of 'rays of light', together with the gaps in the cloudcover.