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Global Precipitation Percentile Maps

Global anomaly maps are an essential tool when describing the current state of the climate across the globe. Precipitation anomaly maps tell us whether the precipitation observed for a specific place and time period (for example, month, season, or year) was drier or wetter than a reference value, which is usually a 30-year average, and by how much.

The August 2012 Global State of the Climate report introduces percentile maps that complement the information provided by the anomaly maps. These new maps provide additional information by placing the precipitation anomaly observed for a specific place and time period into historical perspective, showing how the most current month, season or year compares with the past.

Precipitation Climatological Ranking

In order to place the month, season, or year into historical perspective, each grid point's precipitation values for the time period of interest (for example all August values from 1900 to 2012) are sorted from driest to wettest, with ranks assigned to each value. The numeric rank represents the position of that particular value throughout the historical record. The length of record increases with each year. It is important to note that each grid point's period of record may vary, but all grid points displayed in the map have a minimum of 80 years of data. For example, considering a grid point with a period of record of 113 years, a value of "1" in the precipitation record refers to record driest, while a value of "113" refers to record wettest.

The Drier than Average, Near Average, and Wetter than Average shadings on the precipitation percentile maps represent the bottom, middle, and upper tercile (or three equal portions) of the sorted values or distribution, respectively. Much Drier than Average and Much Wetter than Average, refer to the lowest and uppermost decile (top or bottom 10 percent) of the distribution, respectively. For a 113-year period, Drier than Average (Wetter than Average) would represent one of the 38 driest (wettest) such periods on record. However, if the value ranked among the 11 driest (wettest) on record, that value would be classified as Much Drier than Average (Much Wetter than Average). Near Average would represent an average precipitation value that was in the middle third (rank of 39 to 75) on record.