As some regions experienced their first fall frost this weekend, I felt it was time to revisit this topic.
The first question I am usually asked about fall frost is “When should the different areas of agricultural Manitoba expect to receive their fist 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 C. However, some of us have already had the unfortunate firsthand opportunity to realize frost can occur even when the thermometer shows 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, most of 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 we‘ve had about how air cools, you may recall 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, 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, 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 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 results shown in the table here. These dates are the average dates for which these temperatures may be anticipated, based on the entire record of climate data for each location.
Now, we need to remember this is the average date and the standard deviation around these dates is somewhere around three to five days, depending on the location. 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.
So far this fall (up to Sept. 15), three main regions (Dauphin, Brandon and Winnipeg) have not yet seen frost. There was some scattered light frost on Monday morning (Sept. 16) but most regions stayed just above the freezing mark, with only the frost-prone areas dropping below 0 C. With the next chance of frost coming next weekend, it looks like most areas will either see an average first frost date, or even a later-than-average date.
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. Both the Brandon and Dauphin regions saw a late frost this spring, with temperatures dropping below the freezing mark in early June. Winnipeg stayed about a degree above freezing during this early June cold snap and thus had a much earlier last spring frost, with the previous frost occurring May 12. This would put the length of the frost-free season for the Winnipeg region at about 127 days, which is above the long-term average. Both Brandon and Dauphin have seen about 107 frost-free days so far this year, which puts them near average to date. If these two regions can dodge frost for another week or two, they will also have a longer-than-average growing season.