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Explanation of Precipitation Quintile
    Precipitation quintile is arrived at as follows: For any month the 30 monthly values of precipitation recorded during the thirty-year normal period are listed in ascending order. The list is then divided into five groups of quintiles of six values each. The first quintile contains the lowest six values for the month in question that have been observed during the thirty-year period, and the fifth quintile contains the highest six values.

    The boundary between two adjacent quintiles is set half-way between the top value of the lower quintile and the first value of the upper quintile. Monthly precipitation quintile is reported with the number of the quintile prepared above. The following special rules are applicable in deciding quintile for the monthly precipitation:

(a) If the precipitation is 0: Quintile is reported with 0 if this has not occurred for the month concerned during the reference period; reported with 1 if it has occurred at least once but not more than 6 times; 2 if it has occurred 7-12 times; 3 if it has occurred 13-18 times, etc.
(b) If the precipitation is less than any value during the reference period: Quintile is reported with 0 (whether or not the precipitation is 0).
(c) If the precipitation is greater than any value during the reference period: Quintile is reported with 6.

    Two examples are given to illustrate the preparation and use of quintile tables. The first of these examples is straightforward, while the second deals with a special case.

Example1 Amount of precipitation Quintile Upper and Lower limit of Quintile     4.9 5 First Quintile 5.0-62.5 18 38 48 56 61 64 Second Quintile 62.6-121.5 69 86 104 105 119 124 Third Quintile 121.6-213.5 155 163 164 175 203 224 Fourth Quintile 213.6-255.5 236 236 239 249 254 257 Fifth Quintile 255.6-411.0 293 335 344 349 411     411.1

If monthly precipitation is less(more) than 5.0mm(411.0mm), the quintile is reported with 0(6).

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Example2 Amount of precipitation Quintile Upper and Lower limit of Quintile       0 First Quintile 0 0 0 0 0 0 0 Second Quintile 0 0 0 0 0 0 0 Third Quintile 0-4.0 0 0 2 3 3 5 Fourth Quintile 4.9-9.0 5 6 8 8 9 9 Fifth Quintile 9.1-28.0 14 19 20 21 28     28.1

If the monthly precipitation is 0mm, the quintile is report with 3.

Reference:
Guide Climatological Practices Second Edition (WMO-No.100)
Modification to Traditional Alphanumeric Codes (WMO, Meeting of Expert Team on Data Representation and Codes 28. III. 2002)

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