With all the rainfall figures coming in from the recent floods a theme was noticed – many of you measure rainfall in millilitres not millimetres.

We thought we’d go straight to the experts to get an explanation. Who better than former MetService Weather Ambassador Bob McDavitt.

Bob says rain is measured in a rain gauge its depth is measured in millimetres (mm).

“A mm of rain spread out over a square metre weighs around 1Kg and takes up 1 litre in volume. if we spread the same depth of rain over a hectare (100m x 100m) then it weighs a tonne / 1000kg”

So 1 mm of rain = 1L per square M.

“Millilitres of rain is probably what hydrologists and roof builders work with” says Bob.  “If you know the square metres of your roof (when reduced to a flat surface) then multiply that by the mm of rain to get the litres going into your rain storage tank, so if your place got 50mm of rain last night (two inches) and your (flattened) roof is 50 sq metres then that is 50 x 50 = 2,500 litres into the storage tank, or a quarter of a 10,000 litre tank”. 

As to how some people switched from mm to ml? Bob has his theory: “I think that’s happened because the slang term for millimetres of rain is mils of rain, and it is so easy to from mils to ml.”

The correct measurement for rainfall is in millimetres (mm).

– WeatherWatch.co.nz