Rainfall intensity is classified according to the rate of precipitation, which is given as the amount of rainfall per unit of time. According to MANOBS (Manual of Surface Weather Observations), the following general categories are used to classify rainfall intensity, which is determined by the rainfall rate:

• Light rain - less than 2.5 mm/h (<0.1”/hr) or <0.04 mm/min (<0.0007”/min) rain rate, which for a 0.1 mm (0.004”) resolution rain gauge is 25 full tipping buckets per hour (25 pulses/hr).

• Moderate rain - rain rate of fall is 2.6 to 7.5 mm/h (0.1 to 0.3”/hr) or 0.04 to 0.125 mm/min (0.0017 to 0.005”/hr), which for a 0.1 mm resolution rain gauge equals 26 to 75 full tipping buckets per hour (26 to 75 pulses/hr).

• Heavy rain - rain rate is greater than 7.6 to 50 mm/h (0.3 to 2”/hr) or 0.125 to 0.83 mm/min (0.005" to 0.033”/min) which for a 0.1 mm resolution rain gauge equals 76 or more full tipping buckets per hour (76+ pulses/hr).

• Violent rain: Can sometimes be used to characterize precipitation rates grater than >50 mm/hr (>2 in/hr) or >0.83 mm/min (>0.033”/min) which for a 0.1 mm resolution rain gauge equals 500 or more full tipping buckets per hour (500+ pulses/hr, 8+ pulses/minute).

For practical advice on determining and calculating precipitation intensity with the least possible error, please read the following guide:

• Practical Guide to Determining Rainfall Rate and Rain Intensity Error

DEFINITIONS

Rainfall rate quantifies the amount of rain that has fallen per unit time and is usually given in units of mm/hr or in/hr (millimeters per hour or inches per hour) of rainfall in meteorology or climatology. Calculation follows:

• Rain rate (rain intensity) = (mm³ of rain) / (mm² rain gauge opening area) / (hour) = mm/hr (mm per hour)
  Divide the amount of liquid precipitation by the catchment area (rain gauge opening size) and again divide by the measurement time.

Often, instantaneous rainfall intensity or instantaneous rain rate is given in units of mm/hr even though the measurement period was much shorter than one hour. This is done to standardize the unit of measure to enable easy comparison of rainfall intensities worldwide. It can be interpreted as the equivalent hourly rain that would have fallen if the particular short-term rainfall rate remained constant for one hour.

Calculation examples follow:

• Violent rain: If 2” of rain have fallen in 10 minutes, then the equivalent rainfall rate in terms of in/hr will be 60 min / 10 min * 2” of rain = 12 in/hr

• Heavy rain: If 15 mm of rain have fallen in 30 minutes, then the equivalent rain rate in terms of mm/hr will be 60 min / 30 min * 15 mm of rain = 30 mm/hr