What are “normals”?
When talking about weather and climate, it is often useful to compare the current conditions to a fixed reference point. In the US, this reference point is most commonly the climate normals. These are 30-year averages that are updated every 10 years. The current normals in use are based on daily and monthly data from 1981-2010. The 30-year time frame was selected as a period long enough to capture the variability in a region, but short enough to have a large number of stations available. Normals are produced by the National Climatic Data Center, a part of National Centers for Environmental Information. These normals are available for individual station as daily and monthly values. They are also aggregated into state and climate divisions. Below is a chart showing how the “normals” have varied over time, and compared to the long-term average.
Mary Knapp, Weather Data Library