The clue for 15a appears to be grossly misleading. The answer has to be Kitakyushu ... but Kitakyushu did not exist in 1945 when the second bomb was dropped. It was formed in 1963 through the amalgamation of four other cities. Thus, it can hardly have been obscured by cloud ... can it?
Kokura is the name given in Wikipedia of the city obscured by cloud.I think the compiler has made a mistake here ..but..Kitakyushu is the only thing that fits.
From Wiki....
Kokura was the primary target of the nuclear weapon "Fat Man" on August 9, 1945, but since the city was obscured by clouds and Major Charles Sweeney of the Bockscar had orders to drop the bomb visually, they moved on to Nagasaki.