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Fall frost and the frost-free season

Temperatures at thermometer level can be warmer — or cooler — than at crop level

Date of average first fall frost.

Due to a busy start to my school year, I need a bit more time to dig deeper into last summer’s weather numbers. So, for this article I had a choice of a couple of questions that were sent to me over the last couple of weeks. The first one had to do with why hurricanes or tropical storms curve off to the east once they hit the United States — and how far north can they or have they ever made it? The second question was about fall frost. Both are good questions and are timely as we are at the peak of hurricane season and, well, the first fall frost could occur at almost any time now. I will discuss both at some point, but I think the winner for this week, due to the local impact it can have, is fall frost. I tend to discuss this topic almost every year, but I still keep getting questions about it. It’s a rather important event on the Prairies I suppose.

The first question I’m usually asked 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 that 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 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 here. These dates are the average date 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 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 or early October, depending on when we finally see frost, 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 frosts on May 27, with Brandon seeing its last light frost on May 31, which is very close to what we saw last year. If we added up the days there have been so far, we have seen only about 105 frost-free days. With the long-term average coming in around 115 days, we will have to make it until about Sept. 20 for an average length to our frost-free season this year. Looking at the current long-range forecasts, I’m not 100 per cent sure that we’ll make it, but as usual, only time will tell.

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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