The best available figure I found is that Hiroshima’s average annual temperature in 1945 was about 14.3 C (57.7 F). For Japan as a whole, the available source I found does not provide a single nationwide 1945 average temperature, but it does show August 1945 temperatures in major cities were generally close to normal.

What I found

  • Hiroshima: average annual temperature about 14.3 C (57.7 F).
  • Tokyo in August 1945: 26.7 C versus 26.4 C in previous years.
  • Fukuoka in August 1945: 27.2 C versus 28.1 C in previous years.

Important note

A single “average temperature in Japan in 1945” is not clearly stated in the sources I found, because climate data is usually reported by city or station rather than one national annual average.

If you want, I can help narrow it to:

  • a specific city like Tokyo, Hiroshima, or Nagasaki,
  • a specific month in 1945,
  • or a national approximation using available station data.