Why when we see the weather forecast, we always see that they measure rainfall in millimeters?

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

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

The construction of a rainfall measuring device is quite simple. Use a round cylinder with a flat bottom. On the wall of the pillar marked in corresponding measurement units such as mm or cm. When it rains, leave the cylinder outside and catch rain. So we have a rain gauge.

If the rainfall of 120mm is measured, it means that if the soil does not soak, after the rain it will be 120mm.

After the rain, the amount of rainfall that rises is what the amount of rain is measured. For example, 25mm or 110mm, etc.

One point to note is that the size of the cylinder does not matter. This is because the volume of a cylinder is calculated by its height multiplied by its bottom area. And the rain rate is calculated by the drop of rain per unit area. To sum up, the area of ​​the measuring device is no longer important.

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

Of course, if you choose a cylinder with a small mouth area, the measurement will be difficult and inaccurate. If the rainwater collector is used to enlarge the shape of a funnel, the water level in the measuring cylinder will increase rapidly and we must divide by the ratio of the funnel mouth.

So after the results, how do we know whether the rainfall is large or small. Suppose we measure a rainfall of 200mm, which means that the water will flood the field 20cm. If the yard has an area of ​​100m ² , the rainwater volume will be 100m ² times 0.2m and equal to 20m3.

Rainfall measuring device with funnel.

Recycle plastic bottles for measuring rainfall.

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