Things to Know Before Disaster Prevention: Understanding Rainfall in "Millimeters"

When you hear "50 millimeters of rain per hour" in a weather forecast, I think few people can immediately visualize how intense that rain actually is.

To be honest, I was the same way before. I was like that until just a moment ago.

Minamioguni Town is a town that has experienced heavy rain disasters many times.

We are currently putting effort into training disaster prevention specialists, but I feel that the first step to protecting ourselves in an emergency is for everyone to understand what the rainfall numbers actually mean.

So today, I will try to explain rainfall "millimeters" by relating them to things as familiar as possible.

1 millimeter of rainfall is "one milk carton's worth"

To begin with, what is a millimeter of rainfall?

In short, it is the depth of water if you assume that the fallen rain neither flows away nor soaks into the ground, but accumulates right where it fell.

Since that is hard to grasp, let's think about it in terms of area.

If 1 millimeter of rain falls over 1 square meter—roughly the size of a slightly large doormat—the accumulated water is exactly 1 liter.

That is one milk carton's worth.

If it is 2 millimeters, it is one plastic bottle; if it is 10 millimeters, it is 10 milk cartons' worth.

Even just 1 millimeter is one milk carton.

When you think about it that way, you realize that rain is much heavier than you might imagine.

It is easier to grasp if you think about a parking space

Let's increase the scale a little.

A parking space for one car is about 2.5 meters by 5 meters, or roughly 12.5 square meters.

If 30 millimeters of rain per hour, which is considered "heavy rain," falls here, the water that falls is approximately 375 liters.

Since a household bathtub holds about 200 liters, that is nearly two full bathtubs.

Two bathtubs worth of water in one hour over the space of one car.

When you imagine that pouring down over the entire road and the entire town, I think it becomes clear why roads can turn into rivers in the blink of an eye.

In terms of 10-liter buckets, that is a calculation of 37 buckets falling onto a single car space in one hour.

The thing to look at is whether it is "per hour"

There is one important point here that is easy to overlook.

The "number of millimeters" in weather forecasts usually refers to the "amount that falls in one hour."

And the guidelines provided by the Japan Meteorological Agency are also based on the premise of what would happen if that intensity of rain continued for one hour.

In other words, even with the same "10 millimeters," the meaning changes completely depending on whether it falls all at once in one hour or slowly over the course of a day.

When looking at the numbers, it is important to always pair them with "over how much time."

How many millimeters is normal rain?

Before talking about heavy rain, let's align our sense of the rain we usually experience.

Rain of about 1 millimeter per hour is a light drizzle, where you might hesitate about whether to open an umbrella.

When it reaches around 5 millimeters per hour, it is common rain where you definitely need an umbrella.

This is usually when you feel "Oh, it's started raining" while shopping or picking someone up.

Once it exceeds 10 millimeters per hour from here, the atmosphere changes.

Your feet get wet with an umbrella, and you start wanting rain boots.

Beyond that is the classification by the Japan Meteorological Agency that I will introduce next.

Let's line up the Japan Meteorological Agency's classifications with our physical sensations

The Japan Meteorological Agency divides the intensity of rain into five levels based on hourly rainfall. When lined up with familiar sensations, it looks like this.

10–20 mm is "somewhat heavy rain." It pours down, and puddles begin to form on the ground.

Even indoors, it becomes difficult to hear conversations due to the sound of the rain.

20–30 mm is "heavy rain."

This is what we call a downpour, and you will get wet even with an umbrella.

It becomes hard to see even with the car wipers on high speed.

30–50 mm is "violent rain."
As the saying goes, it's like a bucket being overturned, and roads turn into rivers.

50–80 mm is "very violent rain."

It falls like a waterfall, and umbrellas are completely useless.

It is a calculation of about a quarter of a bathtub's worth of water falling in one hour over an area the size of a single doormat.

And 80 mm or more is "torrential rain."
There is a suffocating sense of pressure, and the risk of large-scale disasters increases all at once.

What is truly scary is "prolonged rain"

We have looked at the intensity per hour so far, but disasters do not only happen when heavy rain falls all at once.

Even with "somewhat heavy rain," if it continues for many hours, the ground loosens and river water levels gradually rise.

That is precisely why it is very important to look not only at the hourly rainfall but also at the total amount from the start of the rain, known as the total rainfall.

For example, when the total rainfall reaches 400 millimeters, that is 400 liters per square meter.

That is a calculation of two bathtubs' worth of water falling onto a single doormat.

If you think about a single rice field of about 1,000 square meters, that is 500 bathtubs' worth.

Even a 25-meter school swimming pool would be filled by the rain that fell on just a few rice fields.

When you think about it that way, perhaps you can imagine the terror of a massive amount of rain falling on the mountains and gathering in the valleys.

In terrain surrounded by mountains like Minamioguni, the fallen rain gathers in low-lying areas all at once.

Even if it looks like light rain where you are, it might be raining much more intensely in the mountains upstream.

If this much water gathers in the mountains and rivers, it would not be strange for landslides or river flooding to occur.

And what you want to remember is that advisories and warnings are issued for each municipality based on an index that takes into account the state of the soil and rivers, not just the rainfall itself.

Checking the warnings for the area where you live, along with the numbers, is the most reliable thing to do.

Make the numbers personal

Rainfall in millimeters is not just a number in a weather forecast.

When you replace it with milk cartons or bathtubs, it suddenly becomes personal, showing you how much water it will bring into your life.

And when the numbers become personal, your actions change.

"The forecast is 50 millimeters per hour today. That's waterfall-like rain, so I'll finish my errands early."

In that way, the numbers in the news and your own day naturally connect.

Disaster prevention sounds like special preparation, but I think the entrance to it is unexpectedly found in these small "re-interpretations."

Being able to correctly read rain numbers is, I believe, a small but certain step.

Try talking with your family or neighbors, saying, "I wonder how many bathtubs' worth of rain that was."

Even just doing that moves your preparation for an emergency a little bit forward.

Minamioguni is a town blessed with rich nature.
At the same time, it is also a town that has lived while facing that nature.

So that we can continue to live with peace of mind in this beautiful mountain village while continuing to prepare.

That was the kind of rainy night it was.