How do you measure the amount of rain that has fallen?

How do you measure the amount of rain that falls in a given area? And with what instrument are measurements made? Here are some notions of rainfall.

The measurement of the amount of rain that falls on the ground in a given area is of great importance. The data can in fact be used to evaluate the extent of a storm, to estimate what the flood of a river will be, to know the extent of the recharge of the aquifers or to evaluate if an agricultural field has received enough rainfall in a given period. On a large scale it allows us to evaluate, year by year, periods of drought or rains that are more abundant than average. And to establish the average rainfall in a given place.

The question, however, is: how is the amount of rain falling on the ground measured ? If we spontaneously use a meter to measure a length, just as we use a scale expressed in kilograms to measure a weight, the measurement of rain involves some more problems.

In Italy the measurement of rainfall takes place using millimeters (mm) and allows us to express the so-called “rainfall height”. Basically the amount of rain is imagined as a height. The reason is simple: starting from a data expressed in millimeters, it is easy to trace the volume of water that has fallen on a certain surface. This type of measurement is also used abroad. In Anglo-Saxon countries using the thumb ( inch ).

The pluviometric height is expressed in millimeters

One millimeter of accumulation is equal in quantity to a liter fallen on an area of ​​one square meter. For example, if we say that the amount of rain that has fallen in a certain place is 20 mm, we are saying that 20 liters of water have fallen in every square meter of surface.

The millimeter (mm) of precipitation is therefore equivalent to the unit of measurement l / m2, that is, liter per square meter. In Spain, for example, instead of millimeters we prefer to use this type of unit, exactly equivalent.

To have a measure of this type, it would be necessary to prepare containers with a surface of 1 square meter and then measure, at each precipitation, how many liters ended up inside.

Rain gauges allow you to measure the amount of rain

However, it would be a somewhat inconvenient system. The rain gauges however, the tools used to collect and measure the amount of rainfall, have always much smaller mouths of 1 square meter. The standard ones used by the regional hydrographic services have a fixed diameter of 36 cm. They are instruments calibrated in such a way as to give us already the value in millimeters of the rain collected.

The denser the mesh of rain gauges located in the area, the more accurate the information on the amount of rainfall will be.

Measurement of rainfall in millimeters makes it easy to calculate the volumes of rain that have fallen over large areas. The volumes will be expressed in cubic meters, and will give us valuable information on the amount of rain that has potentially recharged the aquifers, for example, or on the extent of a flood wave that could endanger a town. These data are used for emergency management and for territorial planning in the agricultural field, as well as for the regulation of the level of artificial reservoirs, just to give some examples.

Finally, the pure curiosity remains to know how much rain has spilled a certain rainy episode on the ground. There are hundreds of amateur rainfall weather stations scattered throughout Italy and thousands of them all over the world, thanks to which we can know how much it has rained, and with what intensity, in a given time interval.

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