零下

1. Origins, structure, and basic meaning

• Structure: 零下 (れいか) is a measurement noun indicating a temperature below zero.
• Basic usage: 零下まで下がる marks the below-zero range reached by falling temperature, while 零下五度 adds a number to form the complete value of five degrees below zero.

2. Common pitfalls and similar expressions

• Range label and concrete value
  • 零下 alone does not state the exact temperature. A specific value requires a following number, as in 零下五度, while まで emphasizes the range reached by the drop.