The prepositions below and under can both mean lower than. But there are some differences.
Below is preferred when one thing is not directly under another.
- When the sun sets, it sinks below the horizon.
Below is used in measurements of temperature and height, and in other cases where we think of a vertical scale.
- The temperature is 10 degrees below zero.
- The Dead Sea is below sea level.
- She is below average in intelligence.
We prefer under when something is covered or hidden by what is over it.
- I think the cat is under the bed.
- The whole village was under water.
We usually use under, not below, to mean less than or younger than.
- There were under fifty people at the meeting.
- You cannot see this film if you are under 18.