Rain Intensity in mm per Hour

Rainfall intensity: classifying how hard it rains

Rainfall intensity tells you how fast water is falling at a given moment, measured in millimetres per hour. The WMO splits it into four bands: light < 2.5 mm/h, moderate 2.5–7.6 mm/h, heavy 7.6–50 mm/h and extreme > 50 mm/h. Inside a hurricane eyewall you can see more than 100 mm/h. Tubarão (SC) lived through something far worse in December 2008, when over 150 mm dropped in one hour and set off catastrophic flash floods. The same number also feeds the IDF curves (Intensity-Duration-Frequency) that engineers lean on when they design storm-water systems.

Applications

It shows up in urban drainage design (ABNT NBR 10844 for roof drainage, NBR 12266 for storm sewers), in sizing highways and culverts, and across hydrological modelling. Defesa Civil and CEMADEN use it to push real-time flash-flood and landslide alerts. You also find it behind reservoir spillway design, airport operations and aviation safety, and erosion-control engineering.

FAQ

How is intensity measured? Tipping-bucket pluviometers, disdrometers and weather radar do the job. They sample about once a minute and then report the equivalent hourly rate.

Why do extreme events use shorter durations? The peak intensity over a 5, 10 or 15 minute window can run several times higher than the hourly average. That brief spike is what actually floods cities.

What is an IDF curve? An Intensity-Duration-Frequency curve ties rainfall intensity to duration and return period. Engineers read it to size drainage works for a storm of whatever rarity they're planning around.