If temperatures overnight are already very close to the freezingpoint, then this drop of just a few degrees can take the temperature below freezing.
如果夜间的温度已经非常接近冰点，那么仅仅下降几度就可以使温度降到冰点以下。
2
The temperature remained below freezingpoint throughout the day.
全天气温一直在冰点以下。
《柯林斯英汉双解大词典》
3
Frozen type air dryer by lowering the temperature of compressed air to close to the freezingpoint of water, that extra water vapor condense into liquid water, thus and flow separation.