What does it mean when the weather is “normal?”

When meteorologists explain that the temperature or precipitation is above or below normal, what exactly are they referring to as "normal?"


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NWS Rapid City

RAPID CITY, S.D. – Normal is a tough concept to wrap your head around if you’re trying to make sense of meteorology, or really anything with dynamic properties. What’s a normal baseball game? A normal boat? A normal cow? The truth of the matter is normal is a concept and a tool that we use to frame our understanding of the world around us.

For time’s sake, let’s stick to meteorology. Let’s take a big bowl of numbers… in this case temperatures and plot them on a graph like the one pictured above. The green line represents the “average” temperature over time going left to right. If you notice, the actual measured temperature (blue line) is ALL over the place… but if you take all those ups and downs and mix them together you’ll see that Rapid Ctiy has averaged 1.5 degrees Fahrenheit warmer since 1942.

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Our average high temperature for October 13th is currently 62 degrees. If our temperatures were to stay *exactly* at 62 degrees every day, only going up and down following the averages… meteorologists wouldn’t have a job.

We have a dynamic and ever-changing environment that likes to spike in temperature, precipitation, pressure and other factors.

Averages change over time based upon data collected on previous years. If you have a stretch of particularly cold years, that’ll drag your average down. It just so happens average temperatures have been going up across the vast majority of the planet over the last 100 years or so.

Also, what set of data are we comparing the averages to? 10 years? 20 years? 50 years? the last million years? Big differences…

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Averages can be a great tool for forecast, giving us a base model to plot unusual changes to temperature, precipitation and waterways in a clean and colorful way.

The Black Hills, on its own, has a history of bucking averages in all categories… (ask Spearfish about breaking world records).

So, what’s “normal” weather for the Black Hills in October? If you compare Oct. 2022 to winter storm atlas in 2013… you’ll get vastly different answers. Best to use averages as a tool to help you forecast changes and trends, rather than a tool to dictate a particular forecast!


Categories: Local News, Weather Daily