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When should we normally expect the first fall frost?

fall It’s just a matter of time before the mercury dips enough to end the growing season

With some of the first winter forecasts beginning to make the news, I had to resist the urge to jump on this topic. After all, it is still summer and I think we can wait at least one more week before we take a look ahead to see what this fall and winter may have in store for us.

Instead, I thought we should take our annual look at the first fall frost. I know I just said it is still summer, so why start looking at fall frost now? Well, last weekend I was relaxing at Riding Mountain National Park, enjoying my last camping trip of the year, and as usual I checked the weather before going to bed. What did the forecast call for? An overnight low of 4 C with a risk of frost!

So, while it might still be summer, and hopefully we will be able to avoid frost for at least another few weeks, we are unfortunately slowly moving into that time of the year.

The first question I am usually asked about fall frost is when the different areas of agricultural Manitoba should expect to receive their first fall frost. To analyze this we must first determine how frost is to be measured or recorded.

The typical measurement we use to record whether or not frost has occurred is when the temperature recorded by a thermometer hits or falls below 0.0 C. However, some of us have already had the unfortunate first-hand opportunity to realize that frost can occur even when the thermometer is showing temperatures above the freezing mark. In fact, research has shown that ground-level frost can occur at thermometer readings as high a 2 C, and in some cases, as high as 5 C.

This can occur for a number of reasons, most of which largely depends on where the thermometer is located. As most thermometers are placed above the ground, they record air temperature several feet above the ground and may not accurately reflect actual ground temperature.

If you can recall back to previous discussions we’ve had about how air cools, you may recall that air near the ground can cool to a greater degree than the air several feet above. The reason for this is that cold air is denser than warm air, so it tends to settle or flow to the lowest points.

Therefore, the coldest air settles around the ground, resulting in ground-level temperatures that are 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, as some of us may have already noticed, a frost with temperatures near the freezing mark may not severely damage or kill a crop. 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: 2.0 C, 0.0 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 you see in the table. These dates are the average dates that these temperatures may be anticipated, based on the entire record of climate data for each location.

We need to remember that this is the average date and the standard deviation around these dates is somewhere around three to five days, depending on the location. This means that while most locations should not see any frost until early to mid-September, it would not be that 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, but with nearly all regions seeing a late last spring frost, it is not looking like we will have a longer-than-average frost-free season across Manitoba this year.

Both Winnipeg and Dauphin recorded light frosts on June 2 with Brandon seeing its last light frost on May 27. So, if we added up the days there have been so far, we have seen only about 90 frost-free days.

With the long-term average coming in around 115 days, we will have to make it until about the middle of September for an average length to our frost-free season this year. Looking at the current long-range forecasts, I’m not so sure that we’ll make it, but as usual, only time will tell.

Table: Date of average first fall frost.

About the author

Co-operator contributor

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.

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