The Nagasaki bomb was dropped by radar due to cloud cover.
Check my follow-up post #54.
The sources I eventually tracked down appear to say that radar was
almost used...but a last-second break in the cloud allowed for a visual.
Of course, I may be misled by the sources...maybe the radar was used,
but they had a visual confirmation?
Having been stung by the apparent errant story of using the cathedral as
the aimpoint...I'm now gunshy on the whole topic.