Below Zero
Sub Zero
noun, の adjective
Zero below is below zero. Sub zero.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
零下十度だろうが二十度だろうがに関係なく、洗い物はここでします。
It doesn’t matter if it’s ten or twenty below zero, we do our wash here.