Implementing a Range for NCDC Daily Normal Temperatures in North Carolina

Every ten years the National Climatic Data Center (NCDC) calculates monthly normal temperatures from thirty years of observed data for NWS Cooperative (COOP) Observing Network Stations around the country. The daily normal temperatures are then calculated from the monthly normal temperatures using a cubic spline interpolation, which produces a smooth curve used to examine day to day trends in temperature. These daily normal temperatures are often used by broadcast meteorologists for comparison to the maximum, minimum and mean temperatures for each day. However, daily and weekly temperature variations rarely follow a smooth curve over each year. Nearly every day will be somewhat warmer or cooler then the daily normal for that day. This can be misleading for many people and leads to the question of what is considered “normal.”

Previous investigations have defined a range of normals as being a meaningful answer to that question. The natural variability of temperature at a station is calculated from the standard deviation of the daily temperature values over the thirty year period for which the daily normals are calculated. Using the range calculation method defined in previous studies, a product was created that implements the normal range for COOP stations in North Carolina. Only stations that have 1971-2000 daily normals were considered in making this product. The result is more intuitive, and arguably more useful, climate normals for North Carolina.