Published: 2011-08-08 08:13:49
Updated: 2011-08-08 08:13:49
Posted August 8, 2011
By Mike Moss
You often hear us mention a "normal" high temperature for a date, or how our rainfall has compared to "normal" for a month. Those normal values are computed by the National Climatic Data Center for hundreds of weather stations around the country, and are based on the most recent 30-year period that ends with a "zero" year. The 30-year period was chosen as a length of time that would be short enough to provide reasonably representative values given any long-term trends, but long enough to allow for relatively stable values that are not overly sensitive to short-term variability.
For the last ten years, we've been using normals that were based on the period 1971-2000, but beginning on August 1st, we have switched to a new set of values that are calculated from data collected in the 1981-2010 time frame. For the Raleigh-Durham airport, the new temperature normals are a little warmer than the previous set, reflecting an average trend toward warmer temperatures in the past few decades. Generally, the new values are anywhere from a around a degree to a little over a degree higher than those from the 1971-2000 set. For example, our old normal monthly high for July was 89.1 degrees, while the new value is 90.2, the old normal mean temperature for January was 39.7 and the new one is 41.0 and so on.
This created a minor oddity last week, as our normal high, which starts to decrease in late July, fell to 89 degrees for a couple of days as we finished out the month, but when we moved on to August 1st and the new normals switched in, the normal high on our graphic was back up to 90 due to the new numbers being warmer. It has since fallen to 89 again and will of course continue to decrease as we finish out the summer and head into Fall.
It will take a while for the new data to completely populate all the sources where it is used in various tools, data sets, and references online, but for the moment the National Weather Service in Raleigh has a couple of sites with a lot of the new information available for the the Raleigh-Durham and Greensboro airports. I've included links to that data here. The PDF format links include many details on both a daily and monthly basis, while the html format link is a streamlined listing with just normal high and low temperature and normal daily precipitation.
Over time, there will be many more new products utilizing the new normals, because of changes and enhancements to the data set that NCDC made this time around to include more variables and greater time resolution (for example, "hourly" normals for some stations in addition to the older daily and monthly values).