WRAL WeatherCenter Blog

What is "normal" anyway?

Posted October 13, 2009

So just what does it mean to be "normal", anyway?

Most nights, we'll show a recap of the day's high and low temperatures, and most often, we'll include information about the "normal" high and low for the day.  But what does that really mean?

Without getting terribly technical, the "normals" we show are calculations based on a 30-year period that changes every 10 years.  Currently, we're using numbers from the 1971-2000 period, and in 2011, we'll slide that window forward ten years.  The calculation involves some statistical manipulation, but it comes close to being an average of all the high and low temperatures on a given date for the 30-year period.  (Mike Moss has a deeper look at the technical side of how normals are calculated.)

Problem is, those "normals" don't always reflect what is "typical".  While the normal high for a day in mid-October is 73, it's rarely exactly 73, but that doesn't mean it's really "not normal", right?  For most of us, we'd figure anything "within the ballpark" of that normal high would be typical.

To help visualize what that typical "ballpark" might look like, the North Carolina State Climate Office has developed a tool to calculate and display a typical range of temperatures for a given day.  In the example, you can see the "normal" high temperatures (black), with the upper (red) and lower (blue) range of what would be "typical" for the next couple of weeks.  You can also see how we did last year (green).  Other than a warm spell in mid-October, we hung around "typical" for much of the rest of the month.

By the way — no matter how you slice it, the weather tomorrow will be VERY different than today and quite a-typical, too.  Don't miss the forecast!

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