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You’re watching a local weather forecast or checking it online when you notice it says temperatures will be 'above normal or below normal.' But what does this actually mean?

Normal is Average

In most cases, the word 'normal' is used in place of 'average.'

  • Average Daily Temperature: This is calculated by adding the highest and lowest recorded temperatures and dividing by two.
  • Average Highs and Lows: For an average daily high, all the high temperatures for a date are added together and divided by the number of years used. Same goes for average lows. 

As we all know, the high and low temperatures for a particular day are never the same from one year to the next. What average temperatures do though is provide a nice reference point. To get that average we know there will be temperatures above and below it. 

So when you hear a weathercaster say temperatures will be above or below normal, just know that average temperature is many times called normal, but normal is more of a range of temperature. If it’s within 10 to 20 degrees of the daily average, it’s really still 'normal.'

What is the normal range of temperatures? 

The next question is, what is the normal range of temperatures? Let’s take a look at normal range of temperatures for Jacksonville, Ill., using the above graph of average daily temperatures, provided by Illinois State Climatologist Trent Ford.

The gray area shows 90% of highs and lows compared to the average. You can see that what is actually 'normal' can be 10 to 20 degrees away from the average temperature.

Why is the range of normal temperatures more varied in winter? 

Notice in the graph the range of 'normal' is wider in the winter than it is in the summer? One explanation for this is that we have more invasions of air from both warm and cold regions in the winter, while during the summer the jet stream moves north and hems in the colder air to more northern regions.

This also helps explain the lower graph, which shows the variation in the top 5% of extreme highs and lows compared to average temperature. In the winter, the highest and lowest 5% of temperatures are up to 25 degrees higher or lower than average. In summer the variation is only about 10 degrees.

The absolute range in winter compared to summer is even higher. If you look at record temperatures for days in January, you will see up to an 80-degree swing, while in June the difference may only be 50 degrees.

 

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MEET THE AUTHOR
Duane Friend is an energy and environmental stewardship educator with University of Illinois Extension, serving the organization in many roles since 1993. Duane provides information and educational programs to adult and youth audiences in the areas of soil quality, weather and climate, energy conservation, and disaster preparedness. These programs provide practical solutions for families, farms, and communities.  He assists families in creating a household emergency plan, farmers with the implementation of soil management and conservation practices, and local government officials and business owners with energy conservation techniques.

ABOUT THE BLOG
All About Weather is a blog that explores the environment, climate, and weather topics for Illinois. Get in-depth information about things your weather app doesn't cover from summer droughts to shifting weather patterns.