空

1. Origins, structure, and basic meaning

• Structure: 空 is read as そら and is a noun for the sky above the ground where clouds, rainbows, and birds are seen.
• Basic usage: A location of appearance takes に in 空に虹が現れる, while a traversed route takes を in 空を横切る.

2. Common pitfalls and similar expressions

• に and を
  • に treats the sky as the location where something appears; を treats it as the space through which something moves.
• Reading caution
  • The kanji has other readings in words about emptiness or space, but this standalone sky sense is read そら.