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## Rainfall Intensity

** The rainfall intensity is the rate at which it is falling**. It is expressed in cm/hr. It can be determined with the help of automatic rain gauges.

### Formula Of Rainfall Intensity

The intensity of rainfall is greater when shorter periods are considered while it is lower when large periods are considered. The intensity of rainfall (p) in mm/hr is given by the following general formula:

**p = 254a / (t+b) mm/hr** Where, **t** = Duration of storm in minutes, **a** and **b** are constants.

The following value of **a** and **b** are considered adequate

**a = 30 and b = 10 when t is 5 to 20 minutes.****a = 40 and b = 20 when t is 20 to 100 minutes.**

### Duration of Rainfall

The duration of rainfall is the time for which rain falls with given intensity.

### Frequency of Rainfall

The frequency of rainfall is the number of times it falls.

*Read Also:*

**Factors Affecting The Quantity Of Sanitary Sewage **

**Determination of Quantity of Sanitary Sewage**