CoachA rain gauge is a container that collects the rain falling on its opening. The depth of water it collects is measured in millimetres: “5 millimetres of rain” means that the rain would have made a layer of water 5 millimetres deep on flat ground, if none of it had run off or soaked in.
Examples
Example 7.12 (A week of rain at school)
The class read the rain gauge every morning for a week and found: Monday 0, Tuesday 4, Wednesday 12, Thursday 7, Friday 0, Saturday 2, Sunday 5 millimetres. The wettest day was Wednesday. It did not rain on Monday nor on Friday. In the whole week, millimetres fell. Wednesday alone brought 12 of the 30 millimetres, that is of the week’s rain.