The Ingredients of “Normal” Weather
November 27, 2016
When meteorologists speak of normal temperatures, do you know how those are calculated? The ingredients which comprise our normals consist of the daily high and low temperatures from a 30-year period. With the normals updated at the end of each decade, they are now based on 1981 to 2010 data. Those who were born at least 50 years ago likely remember that the 2000s were a decade with much milder winters than the 1970s. Because of this, the winter season normals in use previous to 2010 (using the years 1971 to 2000) were noticeably colder (likely by two to three degrees in mid-winter). I will contend that “normal” temperatures are not the best guide for telling us what is possible. Record high and low temperatures are better for that purpose, because they are based on a much longer period of time. Most of the major reporting sites in the central US were established between 1850 and 1920, so the period of record is often 100 years or more. In many cases, the original reporting sites were relocated when airports were constructed early in the 20th century.
Trouble with Winter Forecasts
When considering the alternatives for wintertime air over the northern US, much of our time is either spent shivering in an Arctic air mass or basking in the relative warmth of Pacific air. It seems relatively little time is spent in the middle ground – this is, near normal. The difference between the two types of air masses can be as much as 20 to 30 degrees, with a sharp gradient often existing along the boundary which separates them. This creates trouble with winter forecasts whenever we experience sharp rises and falls in temperature. We tend to see much more variability on either side of normal in the winter than in the summer.
Averages Wash Out the Variability
To illustrate the way in which calculated averages wash out the variability seen from day to day, I now provide a simple example. I am looking at the daily high temperatures in Marshall, MN for the month of October, 2016. The average high temperature was 60.9 degrees, or -0.4 degrees below normal (though the month overall came in warmer than normal due to relatively mild nights). The individual daily high temperatures, however, ranged from 23 degrees warmer than normal to 12 degrees colder than normal. Also, more than half the month’s days were not even close to normal – with 18 of the 31 days in the month showing at least 6 degrees difference from the normal – with nine of the days six or more degrees warmer and 9 days six or more degrees colder than normal. So you see that a near normal month often contains a lot of ups and downs, and several extreme days – at least compared to normal. One conclusion is that the daily and monthly normals are a much better measuring stick for looking back at what has occurred than a predictive measure for what will happen. One thing is certain — the future weather will vary from what occurred in the past.
Next week, I will offer an opportunity for you to look at how much the forecast for your location can change from one day to the next. The prospect of a more active December weather pattern could make that a very interesting exercise!