Normal. It's a word that's used frequently by meteorologists when describing temperatures and precipitation during a particular season. The Winter Outlook for 2010 / 2011 for example, breaks down each province and region with 'normal' expectations. But what does the term mean exactly?
“The term normal is used to describe an arithmetic mean or average over a period of time,” says Chris Scott, a meteorologist here at The Weather Network. “And the period of time we look at is 30 years.”
Scott adds that while looking at an average is beneficial, it isn't a complete guarantee. Especially since there are vast extremes in weather and climate in Canada.
“An average winter temperature may well describe Victoria's climate on most days, but can't describe the ups and downs in Calgary where you might trade shorts one day for a snow shovel the next,” says Scott.
Despite the limitations of average values, using 30 years of information helps to sort temperature and precipitation totals into three buckets; above normal, near normal and below normal.
“Once this is done, we can look at the data and find the thresholds between buckets for total precipitation or average temperature. This way we come up with numbers that define the boundaries of what is normal for each part of the country,” explains Scott.
Right now, normals are calculated from data taken between 1971 and 2000. Snowier and colder winters during that time have helped to impact the 30 year average, but Scott says some of that information will begin to change over the next couple of years.
“The period of time over which we calculate the normals will move to 1981-2010. Because the last decade was generally milder than the 70's for most parts of Canada, the normals will change.”
Certain weather patterns and geography can also have an affect on how normals stack up differently. Scott compares the calculations to a class average. Although it's useful to have, it can only tell you so much.
For a closer look at what you can expect in your area this winter, head to the 2010 / 2011 Winter Outlook.