By Niki Morock, Meteorologist
It's nice to have a stretch of normal fall days, isn't it? With highs in the 60s and lows in the 40s last week's weather really was normal for our area while today's temperatures are below normal, but what exactly does "normal" mean to a meteorologist? It turns out, it could mean several things now.
Traditionally, the word "normal" has actually meant the average over a specific 30 year period. For example, the current period for climate averages is from 1981-2010. When looking at a climate summary with its normal high and low temperatures and rainfall for a specific date, that data is from a period of those three decades.
In areas where the most recent decade has been warmer than the 1980s were, people wonder exactly how useful that thirty year average is for predicting future weather for a specific date or even month. Assuming that the warming trend continues, the farther we get from a past decade, the less accurate using it for a forecast appears to be.
Recently, the National Oceanic and Atmospheric Administration answered this concern by devising alternate ways of finding "normal" temperatures for reporting stations across the country. Now instead of just looking at a 30 year average, interested people can also check 5, 10, 15, and 20 year averages, as well as the Optimal Climate Normal and the Hinge Fit Normal. According to NOAA's explanation of the new definitions of "normal," Optimal Climate Normal is "a 'smart' average where the data values tell you how many years to average over," and the Hinge Fit Normal is more of a line of best fit for recent years.
Statisticians may argue that the 30 year average is the best because you need a minimum of thirty data points to get a reliable number, which is why that is still the accepted standard. However, those who want to give more weight to recent years over the more distant past now have that option with the new alternative normals.
Not a statistics major? Don't worry. Most of us aren't. What you need to know is that while there are now several ways to define "normal" when it comes to the weather, the standard 30 year average is currently still the general rule of thumb when talking about the weather among friends.
The chart to the right shows all of the options for "normal" for RDU International Airport, which is our local reporting station.