When should we normally expect fall frost?

It’ll often take temperatures lower than -2.0 C to kill off most crops

It’s a little later than usual for me to talk about fall frost, as a good portion of agricultural Manitoba has already experienced some frost, but after receiving a few emails inquiring about this topic I felt there’s still time to touch on it.

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 frost has occurred is when the temperature recorded by a thermometer hits or falls below 0.0 C. As some of us have already had the unfortunate first-hand opportunity to realize, frost can occur even when the thermometer is showing temperatures above the freezing mark. In fact, research has shown ground-level frost can occur at thermometer readings as high as +2 C and, in some cases, as high as +5 C! This can occur for a number of reasons, which largely depend on where the thermometer is located. Since most thermometers are placed above the ground, they record air temperature several feet above the ground and may not accurately reflect actual ground temperature.

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If you remember back to our previous discussions about frost, 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 more dense than warm air, so it tends to settle or flow to the lowest points. If the area is relatively flat, then 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 have seen this year, a frost with temperatures near the freezing mark may not severely damage or kill a crop. It will often take temperatures lower than -2.0 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 shown in the table below, which gives the average dates that these temperatures may be anticipated, based on the entire record of climate data for each location.

Graphic: File

Now, 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.

Once again, this year the date for the first fall frost was pretty uniform across our three main regions (Dauphin, Brandon and Winnipeg). All three locations recorded the first subfreezing temperatures on either Sept. 7 or 8, which is a little earlier than the long-term average date. Brandon recorded its first fall frost on Sept. 7 with an overnight low of -1.1 C, but then dropped even colder the next night with a low of -1.9 C. Both Winnipeg and Dauphin recorded their first fall frosts on Sept. 8 with overnight lows dropping down to around -0.5 C.

If we combine the date of the first fall frost with the date of the last spring frost we get the length of this year’s frost-free season. While in other years all three stations usually have very similar lengths to the frost-free season, this year, one station had a significantly longer frost-free season. With that said, it was still a short frost-free season compared to previous years. The length of this year’s frost-free season for Winnipeg and Brandon was 101 and 102 days respectively, around five to 10 days below the long-term average. Dauphin ended up having the longest frost-free season as it missed out on the late-May frost that hit both Winnipeg and Brandon. Dauphin’s season ended up being 116 days, which is right around average.

Please continue to send in your questions. Next week I may take another look at the winter forecast as La Niña conditions have developed and are forecast to last most of the winter. So we will look at what impacts we might see from a La Niña winter.

About the author

Co-operator contributor

Daniel Bezte

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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