Zero may be nothing in the world of math, but in weather the significance of this integer is amazing. October is a good time to start talking about zero, as we recently saw the results of a few days of below freezing weather in Whitehorse and the southern Yukon.
I will be making much ado about nothing in terms of the daily mean, the monthly mean and the annual mean temperatures as they approach 0 C. Zero is very significant to all Yukoners, as it relates to growing food, preparing for outdoor activity, living with permafrost, frozen water pipes, slippery sidewalks, planning for the future and even today’s drive to work.
As of Oct. 19 the average daily temperature in Whitehorse will be 0 C. The daily average will remain below zero for about six months, climbing back to zero on April 11. That is not a forecast. It is a statistic based on the last 30 years of temperature records for Whitehorse. The daily mean is calculated by adding the maximum and minimum for the day and dividing by two.
Elsewhere in the Yukon, the daily mean of 0 C arrives earlier in the fall. Dawson City crossed the line on Oct. 4 and Old Crow on Sept. 25. Dawson City’s mean on Oct. 4 this year matched the long-term average and last year it was within one day. Dawsonites must have felt warm in the fall of 2017 as the mean remained above freezing until Oct. 11. In Old Crow this fall, the daily mean went below 0 C on Oct. 6, almost 10 days later than the average. This point illustrates that even though the daily mean of zero is consistently close to the average, there are variations from year to year that can’t be predicted.
Since half of the year in Whitehorse is below 0 C and the other half is above 0, it is safe to say that our annual average is around 0. In fact, our annual mean temperature is about -0.6. If you aren’t into number-crunching, it is easy enough to look out the window and notice that, in general, the small ponds in the Whitehorse area are freezing up after the middle of October and thawing by the end of April. Snowfalls after mid-October tend to stay because the ground is usually frozen by then. We can be quite certain that the daily mean of zero will arrive each October and will not leave until April.
The annual average temperature for Whitehorse deserves a whole article, but I will try to cover it in a few paragraphs. The most recent data that I have available, shows that Whitehorse had an annual average temperature of -0.6 C between 1942 and 2008. The annual average is calculated by using the daily high and daily low for 365 days. There are a few interesting points to note for Whitehorse.
The average difference between the annual mean and the 66-year mean was 0.2 degrees. (another 0!) The biggest difference was in 1972 when the annual average was -3.5, which was 2.9 degrees colder than the mean. In other words, 1972 was the coldest year since 1942 in terms of the annual mean temperature. The warmest years, on average, were 1944 and 1987 with means of 1.5 degrees. This is a mere 2.1 C warmer than the 66-year mean.
The amazing point regarding the annual mean is that we are so consistently close to zero with only a few degrees of variation. This may help to illustrate why small increments in the global average temperature are so significant and why people are concerned about the global climate warming by only 1.5 or 2 degrees.
Changes of 1 or 2 degrees, when the annual mean temperature is zero or colder have an effect on permafrost. Most northerners know about permafrost and some are acutely aware of its significance. Geologists define permafrost as any ground, including rock and soil, with a temperature that remains at or below the freezing point of water (0 C) for two or more years. Weather people probably agree with this definition. Most permafrost is located north of the 60th parallel, but at lower latitudes, alpine permafrost may occur in the high mountains.
The annual mean temperature at Whitehorse being near zero is an exception, even for the Yukon. Most other southern locations average -2 to -4 C annually. The southern Yukon, especially Whitehorse, is affected by its proximity to the Pacific Ocean, and its moderating storms that bring cloud cover and mild air on a regular basis. Permafrost in the southern Yukon is patchy.
Dawson City is in a zone of discontinuous permafrost, and Old Crow sits on ground that is permanently frozen. The permafrost correlates with Dawson’s mean annual temperature of -5 C and Old Crow’s at -10 C. The surface thaws each year, but remains frozen below a certain depth. A 10-degree difference in the mean annual temperature between Whitehorse and Old Crow is the main factor determining the existence of permafrost. A one-degree rise in the mean for Old Crow will have a noticeable impact on permafrost, but will not be noticed in Whitehorse.
Permafrost has been an important factor for building and road construction in the north and for arctic ecology. The melting of permafrost over time will have huge effects on these aspects of the life and landscape of Old Crow and many other northern communities.
Gardeners make much ado about zero when the daily mean minimum drops to freezing. The growing season depends on a frost-free period. This past summer, Whitehorse had at least 120 days between the last spring frost and the first fall frost. That is significantly higher than the average of about 90 days. Dawson’s average frost-free period is over a hundred days, but gardeners there worry about zero too.
The higher elevation and lower humidity makes Whitehorse more susceptible to frost. Conversely, Dawson’s lower elevation and longer sunshine hours provides a longer frost-free period. It is incredible how much food can be grown in the short growing season, but there is always a risk that a below zero night will wipe out some, or all, of the crop. Over the last 75 years in Whitehorse, temperatures of 0 or colder have been recorded at least four times in July, and on all but 13 days in June and August.
Climate warming of 1 to 3 degrees will probably have an effect on the average frost-free period, but it will not provide guarantees against frost in June and August. It’s not likely that corn and tomatoes will be grown in the Yukon, without a greenhouse.
A one-degree difference in temperature on a very cold or very hot day, is not really noticeable. However, when the temperature is at or near 0 C, small changes of even one degree are noticeable. Drivers won’t have to scrape the windshield when the morning temperature is +1, but it will be necessary at -1, depending on other factors, such as humidity and wind. Skiers are very aware of the difference in waxing when the temperature is -1 or +1 as the properties of snow change considerably within a couple of degrees of zero. Even walking and driving are impacted by this small change in temperature as it will affect ice formation on roads and sidewalks. Emergency hospital visits increase dramatically as a result of this small change.
Small changes in the monthly mean temperatures are not usually noticeable and can vary widely year to year. We know that 5 or even 10 degrees of change for monthly means are common and they are much less predictable. We can have a hot May one year and a cool one the next. We can have two consecutive warmer than average months, but that does not mean that the following month will be warmer than average. This makes it difficult to determine trends in temperature over short time periods. The differences in monthly means may be most noticeable in October and November when there may or may not be snow on the ground.
Very small changes in the annual mean temperature are significant on a global scale. They are more visible in areas where the mean annual temperature is close to 0 C. A change of 1 degree will affect permafrost and glacial melting. Less noticeable, but equally important, small changes in the mean annual temperature are related to a measurable rise in sea temperature.
Thanks to the Celsius system for making zero so important to our weather story. There is nothing interesting about 32 degrees fahrenheit!