秋晴れ

1. Origins, structure, and basic meaning

• Structure: 秋晴れ (あきばれ) is a compound noun for clear, pleasant weather characteristic of autumn.
• Basic usage: 秋晴れの下で開く modifies the clear-sky setting of a sports day, while 秋晴れになる describes the weather changing into a bright autumn condition from morning.

2. Common pitfalls and similar expressions

• Weather setting and change of state
  • 秋晴れの下 uses established weather as a setting, while in 秋晴れになる, に marks the resulting clear condition. It does not mean the season itself changes into autumn.