On August 1, 2011, NOAA's National Climatic Data Center (NCDC) will update all the National Weather Service (NWS) climate databases with the new 1981-2010 Normals. Climate normals are a 30 year average of climatological variables, such as temperature, precipitation, and degree days. Majority of this article contains information about climate Normals provided by NCDC. To find more information on this subject, visit NCDC @ http://www.ncdc.noaa.gov/oa/climate/normals/usnormals.html.
NOAA's computation of climate Normals is in accordance with the recommendation of the World Meteorological Organization (WMO), of which the United States is a member. The WMO became a specialized agency of the United Nations in 1951, but had international connections dating back to 1873. The WMO mandates each member nation to compute climate Normals every 30 years (1971 - 2000, 1981 - 2010, 1991 - 2020, etc.), and recommends a decadal update, in part to incorporate newer weather stations, change in location of a station, and instrumentation.
Methods of computing Normals have changed over the decades, none more so than the last 30 years. The most obvious advancement has been the computer. Underneath the technological advancements, calculations of the new Normals have also changed with what data is initially used, quality control procedures and statically analysis. Unlike the 1971-2000 Normals, daily data were used extensively in the computation of daily temperatures and precipitation Normals as well as heating and cooling degree day Normals, providing greater precision of intra-seasonal features. The 1971-2000 Normals, basically started with monthly data then computed daily normals with a statically analysis known as Thom method or a spline fit. NCDC found that using a harmonic fit provided better statically constrain for temperatures and precipitation than the spline fit. Figures 1 and 2 are flow charts on how the 1971-2000 and 1981-2010 Normals were calculated. For higher resolution, click on figure.
Across the United States, the new Normals are warmer than the previous version (1971-2000). This is not a surprise, since the cool decade of the 1970s were dropped and the warmer 2000s were added. The difference of actual number values between the warmer 2000s and the cooler 1970s mean temperature is +1.46F. However, this number is misleading, since both the new and old Normals used different methods. For an "apples to apples" comparison, NCDC re-compute the new 1971-2000 Normals with the new methodology for both maximum and minimum temperatures. They found on average, the new Normals were 0.5F warmer. According to the National Climatic Data Center (NCDC), this difference in the temperature is primarily due to station moves and changes in observing practices or instruments. NCDC only re-computed the old Normals for a comparison and will not release this data. The average change for all station month's maximum temperature is <+0.1F (Figure 3). The average change for all station month's minimum temperature is <+0.6F (Figure 4). For higher resolution, click on the figure.
NCDC states, "Normals were not designed to be metrics of climate change. In fact, when the widespread practice of computing Normals commenced in the 1930s, the generally-accepted notion of the climate was that underlying long-term averages of climate time series were constant. Changes from one installment of Normals to the next do, nonetheless, provide some evidence of climate change impacts. However, care must be taken when interpreting changes between one Normals period and the other. Differences between the reported 1971-2000 Normals and the 1981-2010 Normals may be due to station moves, changes in methodology, changes in instrumentation, etc. that are not reflective of real changes in the underlying climate signal."
The graphs below are a comparison of the new 1981-2010 Normals verses the old 1971-2000 Normals for NWS Blacksburg first order climate stations (NWS Blacksburg VA, Roanoke VA, Lynchburg VA, Danville VA, and Bluefield WV). The comparison between the two uses the two different methods mentioned earlier. Therefore, this is an "apples to oranges" comparison. Note: The ragged look to the 1971-2000 Normals is due to rounding to the whole number while the smoother 1981-2010 Normals are rounded to the nearest tenth.
Overall, the warming of the new Normals can be seen better in the mean temperatures. Outside of the different methods used, this warming of mean temperatures is due to warmer overnight minimum temperatures. Roanoke's minimum temperature difference is more pronounced than Blacksburg and Lynchburg, likely due to urban heating. Bluefield and Danville graphs are also showing a significant difference in both maximum and minimum temperatures. These two stations were commissioned first order climate sites in 2000. Therefore, the large differences in the new Normals are likely due to a station relocation. Spreadsheets for the 1971-2000 and 1981-2010 Normals for NWS Blacksburg first order stations can be found @ http://www.weather.gov/climate/local_data.php?wfo=rnk. For higher resolution, click on the graph.