> Method of construction: (Measuring vessel)

… Bearing in mind the need to achieve the maximum definition of measurement, choose a cylindrical vessel of suitable dimensions (I used an empty Sainsbury's nail-polish remover that my wife had finished with).

Attach / glue a flat stand to this vessel (I used the discarded circular plastic from the small washing-up bowl/above), so that it will not overbalance easily.

Basic principle of rainfall measuring

… The basic principle involved when measuring & reporting rainfall is that you assess the depth of water that would be covering the ground after a period of rain, assuming none evaporated, ran-off or percolated away.

Of course you could just put a flat tray with raised edges (of a suitable height) down on the ground and get down on your hands & knees with a fine ruler and try and measure the rainfall as soon as it has fallen. In practice, this is not convenient, and it is better to
(i) contain the rainfall within a near-totally closed container (to reduce evaporation & eliminate run-off & "splash-in") and
(ii), maximise the ratio of the diameter of the collecting funnel (D) to the diameter of the measuring vessel (d), so that small amounts of rainfall will give large depths in the latter vessel.

The depth of rainfall in the measuring flask needs to be corrected for the ratio of the squares of the diameters (D^2/d^2), i.e. the ratio of the area of the funnel (collecting the rainfall) & the area of the measuring vessel.

Thus, for a 12cm (diameter) gauge, using a 3 cm (diameter) vessel, the ratio is (144:9) or 16.

So 1 mm of rainfall collected in the main gauge is represented by 16 mm in the measuring vessel; 2 mm by 32 mm and so on.

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