# Why when looking at the weather forecast, we always see they measure the amount of rain in mm?

In recent months, storms have brought rain to our country, flooding the whole country. Whenever we forecast the weather, we often hear about a few hundred or tens of millimeters of rain. So what do those numbers mean and how is precipitation measured?

If 200mm of rain is measured, it means that if the soil does not absorb water, after the rain it will be flooded by 200mm, or 20cm. In fact, the soil will absorb water and run off into rivers and streams. That’s why we rarely feel the rainfall directly.

The fabrication of a precipitation measuring device is quite simple. Use a round cylinder with a flat bottom. On the wall of the cylinder mark in the corresponding unit of measure such as mm or cm. When it rains, leave this cylinder outside and catch the rain. Thus we have a device to measure precipitation.

If 120mm of rain is measured, it means that if the soil does not absorb water, after the rain it will be flooded by 120mm.

After the rain, the amount of rainwater that rises to which line is the measured amount of rain. For example, 25mm or 110mm, etc.

One point to note is that the size of the cylinder is not important. That’s because the volume of the cylinder is calculated as the height times the area of the base. And the rate of rain is calculated in raindrops per unit area. In sum, the area of the measuring device is no longer important.

If you choose a cylinder with a small mouth area, the measurement will be very difficult and inaccurate.

Of course, if you choose a cylinder with a small mouth area, the measurement will be very difficult and less accurate. If the rainwater collection mouth is enlarged like a funnel, the water level in the measuring cylinder will increase rapidly and we have to divide it by the ratio of the funnel mouth.

So after having the results, how do we know if the amount of rain is big or small? Let’s say we measure a rainfall of 200mm, which means that the water will cover the field by 20cm. If the yard has an area of 100m 2 , the volume of rain water will be 100m 2 times 0.2m and equal to 20m 3 .

Precipitation device with funnel mouth.

Recycle plastic bottles as rain gauges.

With a rainfall of 200mm and a yard of 100m 2 , you have collected 20m 3 of water. Imagine if it rained on a large area over many provinces and cities, the amount of water flowing to the rivers and streams would be terrible.