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’m 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 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. 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 remember back to 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 denser 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 which 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 C to kill off most crops.
For these reasons we will look at a few different temperatures, namely: +2 C, 0 C and -2 C, to determine when we may expect the first fall frost. Looking at the data for several sites around southern Manitoba, we obtain the following results (see Table 1). These dates are the average dates that these temperatures may be anticipated, based on the entire record of climate data for each location.
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.
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. 18 or 19. Both Brandon and Dauphin recorded overnight lows colder than -2 C, which would put this year’s first fall frost a little earlier than average. Winnipeg recorded an overnight low between 0 and -2 C – near to a little earlier than average.
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. Interestingly, the numbers for our three main stations are once again nearly identical. The length of this year’s frost-free season was around 132 days, which, according to long-term averages, is a good 10 to 25 days above average.
DATE OF AVERAGE FIRST FALL FROST