What is a
percentile?
A percentile is a value on a scale of
one hundred that indicates the percent of a distribution that is equal
to or below it. For example, on the map of daily streamflow conditions
a river discharge at the 90th percentile is equal to or greater than 90
percent of the discharge values recorded on this day of the year during
all years that measurements have been made.
In general,

a percentile greater than 75 is considered
above normal

a percentile between 25 and 75 is considered
normal

a percentile less than 25 is considered
below normal
In some hydrological studies, particularly
those related to floods, a variation of the percentile known as the "percent
exceedance" is used. It is simply obtained by subtracting the percentile
scale value from 100 percent. For example, a discharge at the
75th percentile is the same as a discharge at the 25th percent exceedance
(10075=25).