Based upon the rainfall records available all over the world, a list of world's greatest recorded rainfalls of various duration can be assembled. 
When this data is plotted upon a log-log paper, an enveloping straight line drawn to the plotted points obeys the equation
                          Pm = 42.16 D^0.475
where, Pm = extreme rainfall depth in cm and

D = duration in hours. 

The values obtained from this equation are of use in probable maximum precipitation (PMP) estimations.