I am a little reluctant to write about fall frost this year because I have been given a little flak over the last few months about how, every time I write about something, it seems to happen.
What can I say, other than “I wish I had that kind of power over the weather!”
When you think about it, writing about frost in the middle of September is not that risky since most of us are not too upset if we see frost during the second half of the month. Looking back over the years, I note that I tend to discuss this topic almost every year, but as I point out nearly every year, I either get questions about it or I hear incorrect information being argued. While a fall frost is usually not as impactful as a spring frost can be, it is still an important event on the Prairies.
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The first question I am usually asked is, when should the different areas of agricultural Manitoba expect to receive their first fall frost? To analyze this, we must first determine how frost is measured or recorded.
The typical measurement we use is when the temperature recorded by a thermometer hits or falls below zero C. For most avid followers of the weather and who grow things, we know frost can occur even when the thermometer shows temperatures above the freezing mark. In fact, research has shown that ground-level frost can occur at readings as high as 2 C, and in some cases, as high as 5 C.
I have seen spotty frosts in my area when my weather station only recorded lows in the 2 to 3 C range. This can occur for a number of reasons, which largely depend on location of the thermometer. Because most are placed above the ground, they record air temperature several feet above the ground and may not reflect actual ground temperature.
If you remember a previous discussion about frost and the general movement of warm and cold air, you may recall that air near the ground can cool to a greater degree than the air several feet above. The reason is that cold air is denser than warm air, so it tends to settle or flow to the lowest points.
If the area is relatively flat, the coldest air settles around the ground, resulting in ground-level temperatures cooler than the air several feet above. While this is the norm, there are occasions when temperatures measured above the ground, at the level of the thermometer, are actually cooler than those recorded at ground or crop level.
Also, a frost with temperatures near the freezing mark may not severely damage or kill a crop, making it difficult to know if an area did or did not receive frost. It will often take temperatures of less than –2 C to kill off most crops.
For these reasons we will look at a few different temperatures; namely, 2 C, 0 C, and –2.0 C, to determine when we may expect the first fall frost.
Looking at the data for several sites around southern Manitoba, we obtain the results in the table. These are the average dates that these temperatures may be anticipated, based on the entire record of climate data for each location.
We must remember that this is the average date and the standard deviation is somewhere around three to five days, depending on location. While most locations should not see any frost until early to mid-September, it would not be unusual to see a frost in early September at most locations.
If we look at the date of the first fall frost and the date of the last spring frost, we can determine the length of this year’s frost-free season. We will look at the frost-free season in more detail later in September or early October, depending on when we finally see frost, but given the dates of the latest spring frosts in both Winnipeg and Brandon, it doesn’t look like most regions will have a longer-than-average frost-free season across Manitoba this year. Winnipeg recorded its last spring frost on May 22, while Brandon saw its last light frost June 3. Dauphin, with a last spring frost on May 16, was the earliest, but both Dauphin and Winnipeg came close to seeing frost on June 3.
If we add the frost-free days up to Sept. 10, Dauphin has seen 116 frost-free days, Winnipeg 112 and Brandon 99. With the long-term average coming in around 115 days, it looks like both Dauphin and Winnipeg will see an average to an above-average frost-free season while Brandon will have to see a fairly late first fall frost to avoid coming in below average.
In the next issue, if time permits, we will re-examine the work I did in my university days: trends in the length of the frost-free season across the Prairies.
