Source: https://www.bo-sai.co.jp/typoon.htm


In Japan, we commonly use millimeters per hour (mm/h) to measure rainfall and wind speed in weather forecasts.

Rainfall

Mm/h expresses the amount of rainfall in millimeters per hour. However, sometimes, it’s difficult to know whether 35 mm (= 3.5 cm = 1.36 inches) of rain in one hour is too much or too little, even if they say there will be 35 mm/h, for example. Therefore, the following is a rough estimate of rainfall.

~ 15 mm: Slightly heavy rain (you can hear rainfall)
~ 20 mm: Heavy rain (puddles form and it’s hard to hear what you’re saying because of how loud the rainfall is)
~ 30 mm: Heavier rain (pouring so much that you’ll get wet even if you have an umbrella)
~ 50 mm: Very heavy rain (like an overturned bucket)

In Japan, the Japan Meteorological Agency issues warnings for the following

heavy rain advisory:
  -  Three-hour rainfall is 40 mm or more, one-hour rainfall exceeds 20 mm, and total rainfall is expected to reach 50 mm; or
  -  When 24-hour rainfall is expected to reach 70 mm or more in flat areas and 100 mm or more in mountainous areas.

Heavy Rain Warning
  -  Rainfall of 40 mm or more per hour is forecasted, or
  -  Total rainfall is expected to reach 100 mm or more, or
  -  Rainfall of 70 mm or more is forecasted in 3 hours, or
  -  Rainfall is expected to reach 150 mm or more in 24 hours in flat areas and 200 mm or more in 24 hours in mountainous areas.

Flood Alert
  -  When one-hour rainfall is forecast to reach 30 mm or more, or
  -  Rainfall in three hours is expected to reach 50 mm or more, or
  -  Rainfall in 24 hours is expected to reach 100 mm or more.

Flood Warning
  -  When rainfall of 40 mm or more is forecasted in one hour and total rainfall is expected to reach 100 mm or more, or
  -  Rainfall of 70 mm or more is expected in three hours, or
  -  When rainfall is expected to be 150㎜ or more in 24 hours in flat areas and 200㎜ or more in 24 hours in mountainous areas.

Now, when you look at Japanese weather forecasts or typhoon information, it will be easier to understand the information about rainfall. I think there are many Japanese people who don’t know this information.

Source: https://www.bo-sai.co.jp/typoon.htm 




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