Why is rain measured in millimeters? Manual of the World explains

The intensity of a rain, whether in reports after the storm or in the weather forecast announcing precipitation, is usually classified by a measurement in “millimeters of rain”.

Because of the strangeness that this measure can cause, Manual do Mundo, the largest Science and Technology channel in Latin America, made a very educational video to show how rain is measured by the column of water it generates, and not by its volume. .

To better understand, first it is necessary to explain what millimeters of rain are. This measurement is actually done using a device called a rain gauge, but understanding what it does is very simple.

Imagine that rainwater falls into a plastic box in the backyard of a house, which is empty. As the rain falls, this box fills with water. At the end of the rain, a person can take a ruler and measure the height of this accumulated water. That's basically what a rain gauge does.

This is possible because the height of this water column does not change, regardless of the size of this box. If the box is small, it will capture fewer water droplets. If it is larger, it will accumulate more drops.

In the end, the liquid accumulated in both boxes will be the same height. This is why measuring in millimeters makes more sense, because it is a standardized way of measuring how much rain has fallen in that region.

Rain intensity

The problem with measuring in millimeters is that it is not very logical to give the size of the rain. So there are two ways to better understand what this number means.

If the information is that a 100 millimeter rain occurred, you need to imagine that in your entire neighborhood, if the water had nowhere to run off, a gigantic puddle of water 100 millimeters high would have formed.

It is also possible to convert this measurement to liters. The rule is: every 1 millimeter of rain is one liter raining on an area of ​​1 square meter.

It is important to say that measuring rain intensity has to be considered in relation to time. Rain of 100 millimeters in one hour means it was very intense rain. However, if it rains 100 millimeters over the course of an entire day, this can be considered moderate rain.

According to the National Institute of Meteorology, Inmet, precipitation intensity is classified as follows:

• Light rain: less than 2.5 mm/h (millimeters per hour)
• Moderate rain: between 2.5 mm/h and 10 mm/h
• Heavy rain: between 10 mm/h and 50 mm/h
• Violent rain: above 50 mm/h