By analysing past rainfall records, it is possible to make an estimate of the probability of any particular rate occurring. The more severe the rainstorm (i.e. the higher the rate of rainfall), the lower the probability of it occurring. This probability is usually expressed as a “return period”. A rainstorm with a probability of 1 in 20 of occurring in any particular year is said to have a return period of 20 years for instance, and is called a 20-year storm. This does not mean that it occurs exactly every 20 years but five times a century on average.