Did You Know?

Global Temperature Percentile Maps

Global anomaly maps are an essential tool when describing the current state of the climate across the globe. Temperature anomaly maps tell us whether the temperature observed for a specific place and time period (for example, month, season, or year) was warmer or cooler 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 temperature 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.

Temperature Climatological Ranking

In order to place the month, season, or year into historical perspective, each grid point's temperature values for the time period of interest (for example all August values from 1880 to 2012) are sorted from warmest to coolest, 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 the global temperature anomaly record, the data does extend back to 1880. But not all grid points have data from 1880 to present. Considering a grid point with a period of record of 133 years, a value of "1" in the temperature record refers to record warmest, while a value of "133" refers to record coldest.

The Warmer than Average, Near Average, and Cooler than Average shadings on the temperature percentile maps represent the bottom, middle, and upper tercile (or three equal portions) of the sorted values or distribution, respectively. Much Warmer than Average and Much Cooler than Average, refer to the lowest and uppermost decile (top or bottom 10 percent) of the distribution, respectively. For a 133-year period, Warmer than Average (Cooler than Average) would represent one of the 44 warmest (coolest) such periods on record. However, if the value ranked among the 13 warmest (coolest) on record, that value would be classified as Much Warmer than Average (Much Cooler than Average). Near Average would represent an average temperature value that was in the middle third (rank of 45 to 89) on record.