1. What is a Normal?
2. What are temperature normals based on?
3. How are sunrise and sunset is determined?
4. How are "normal" temperatures are calculated?
5. How do I find today's sunrise and sunset times for Canadian cities?
   
   
6. Back to Frequently Asked Questions

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Answers

1. What is a Normal?

   Bryn Hendricks of Delta, BC wants to know how do you determine the normal temperature for a particular date? And Yvonne Lewanski adds that the word "normal" seems to be a misnomer, especially in Calgary. A normal of temperature, precipitation or any other weather element is defined as the arithmetic average of the observed values of that element. You are right, the word "normal" is an unfortunate choice because it seems to convey the idea that a value other than the normal is abnormal or unusual. Normals are not averages of all the observations on that date, because some stations may have 10 years and others 110 years.

   There is an international agreement that averages or normals should be from a 30-year period from 1961 to 1990. To calculate the normal daytime high temperature for Winnipeg on say September 15, we would add together all the daily high temperatures on this day over the 30-year period from 1961 to 1990, divide the total by 30, to get the average or normal. Normals are supposed to be representative, not necessarily what should occur. For example, at Winnipeg daily highs on September 15th for 30 years didn't even come close to matching what the calculated normal high is for this date. In Canada, the atmosphere is rarely normal.

   
   

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2. What are temperature normals based on?

   Temperature normals are based on climatic trends over a thirty-year period which are averaged to create the temperature normals.

   
   

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3. I have always wondered how sunrise and sunset is determined?

   Is it when the sun is completely gone from the horizon? What if there are mountains in the way.
   
   The sunrise and sunset is calculated based on a smooth earth (no mountains). The measures are taken for the top of the sun, the middle of the sun and the bottom of the sun touching the horizon for three various groups (astronomical, civil and nautical). At The Weather Network, we calculate our sunrise based on the top of the sun breaking the horizon and the top of the sun going just below the horizon. You may also want to check out this site: http://aa.usno.navy.mil/AA/data/docs/RS_OneDay.html.

   
   

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4. My understanding is that "normal" temperatures are calculated as a 30 year moving average. Based on this principle, how is it possible to have 2 to 3 degree Celsius difference between daily normals? For example, the daily normal temperature for January 5th in Guelph, ON would be -2.3C, while on January 6th it could be 0C or -5C. Meanwhile, the monthly average daily maximum is -3C. Why is there such a large daily variation in the 30 year moving averages? Simple statistics would suggest a normal distribution of values would it not?

   You are correct in that climatic normals are done on a 30 year data set and are recalculated every 10 years (last one was done in 1991). There is however, a bit more background needed to understand them.

   In Canada, we calculate the daily normal high using the recorded daily high for the past 30 years. The high of the day before or the day after is exclusive and not considered in the calculation. In the averaging of the recorded highs, there may be a few extreme values in the data set that will cause there to be a high degree of variation in the "normal" temperatures, such as in your example.

   In the United States, the climatic normals are calculated differently. The high temperatures for each month are run through a statistical algorithm and the extremes mentioned are removed. After the variations are removed, the 30-year normal values are calculated.

   
   

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5. How do I find today's sunrise and sunset times for Canadian cities?

   On TheWeatherNetwork.com home page, enter a city in the City Search box or click on the Canadian Cities link. Once you have found the city you are looking for, click on "Sun and Normals" on the blue menu located on the right hand side of the page.

   
   

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