How dangerous are guerrilla rainstorms and heavy rain? Disaster risks based on precipitation
In recent years, we have had more opportunities to hear terms like "guerrilla rainstorms" and "linear precipitation zones."
There are many cases where sudden, intense rain causes roads to flood or landslides to occur.
So, what exactly do terms often heard in weather forecasts, such as "50mm of rain per hour" or "torrential rain exceeding 80mm," actually look like?
In this article, we summarize the guidelines for precipitation and disaster risks.
What is precipitation?
Precipitation represents the depth of water if rain that fell over a certain period were to accumulate on the ground as is.
The unit is expressed in "mm (millimeters)."
For example, 100mm of rain in one hour is equivalent to 100 liters of water falling per square meter.
Not only short-term heavy rain, but even relatively weak rain can lead to rising river levels or landslides if it continues for a long time[1].
Rain intensity and danger levels by hourly precipitation
Somewhat strong rain (10–20mm/hour)
This is rain that falls with a "whooshing" sound[2].
Your feet get wet from water splashing up from the ground
Puddles form all over the ground
It becomes difficult to hear conversations due to the sound of the rain
If it continues for a long time, it can cause road flooding.
Strong rain (20–30mm/hour)
This is the state of what is called a "downpour"[2].
You get wet even if you are using an umbrella
It is hard to see even with the wipers on high speed
Water levels in gutters and small rivers rise
Heavy rain (30–50 mm/hour)
Described as "rain like water being dumped from a bucket" [2].
Roads turn into rivers
Sewers and drainage facilities become overwhelmed
Landslides become more likely
During high-speed driving, a water film forms between the tires and the road, causing brakes to fail (hydroplaning phenomenon)
Very heavy rain (50–80 mm/hour)
Described as "rain falling like a waterfall" [2].
Umbrellas are almost useless
Driving a car becomes dangerous
Risk of flooding in underground facilities increases
Debris flows and flood damage are likely to occur
Visibility becomes poor as spray turns the surroundings white
This is a level of rain where you should avoid non-essential outings.
Violent rain (80 mm or more/hour)
The Japan Meteorological Agency describes it as "an oppressive feeling that makes it hard to breathe, and a sense of fear" [2].
River flooding
Widespread flooding
Sediment disasters
A life-threatening major disaster
is highly likely to occur.
Treat weather forecast terms as a 'sign to take action'
When terms like 'heavy rain,' 'very heavy rain,' or 'extremely heavy rain' are used in weather forecasts, it is a sign that the risk of a disaster occurring is increasing.
Furthermore, in situations where it has already been raining, landslides or rising river levels can occur even if the total precipitation is not particularly high.
Paying a little attention to the 'intensity of rain' reported in your regular weather forecast may be the first step toward protecting your own life and the lives of your family.