NCDC Frequently Asked Questions
Is it appropriate to use normals for predictive purposes?
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Normals are best used as a base against which climate during the following decade can be measured. Comparison of normals from one 30-year period to normals from another 30-year period may lead to erroneous conclusions about climatic change. This is due to changes over the decades in station location, in the instrumentation used, in how weather observations were made, and in how the various normals were computed. The differences between normals due to these non-climatic changes may be larger than the differences due to a true change in climate.