The 1981-2010 normals were calculated using different methods than the 1971-2000 normals. As such, the two databases should not be directly compared. However, the National Climatic Data Center, which is responsible for formulating U.S. normals, re-calculated the 1981-2010 normals in a separate database using the 1971-2000 methods so that the two normals packages could be compared “apples to apples.”
The original 1981-2010 normals package, with the new methods of calculation, are still the official normals. This re-calculation of the normals using the old methods was done only for use in comparing the two databases, and should not be used for any other purpose.