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How do we define a fall frost?

Temperatures at thermometer level may vary from what's happening on the ground

Around the same time every year, if you are into agriculture at any level, thoughts start to turn to fall frost. Will it be earlier or later than average? I love overhearing weather conversations, because it often makes me realize I need to keep revisiting certain topics, and fall frost is one of them. When it comes to memories, it is interesting how our minds work, and especially interesting how we recall weather events. We often remember the extreme events and this makes sense, but what often happens is that over time, these extreme events seem to slowly morph into what we think is the normal weather we should expect. For example, someone may remember a year or two that had really late frosts, and over time, when they start to think back, their minds start deciding that those late fall frosts were what happened most of the time. They forget about all of the earlier fall frosts. So after hearing several of these types of conversations over the last week or so, combined with a chilly forecast, I thought I should once again visit this topic.

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To analyze when an area first sees a fall frost, we must first determine how frost is to be measured or recorded — that is, just what defines a fall frost. 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. As most of us already know, frost can occur even when the thermometer shows 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 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 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 below –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 data for several sites around southern Manitoba, we obtain the results in the table shown below. The dates are the average dates when 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.

If we do end up seeing widespread frost sometime during this current forecast period, that would place us on the early side of things. I don’t know about you, but for some reason, the way the weather has been going this year, an early-fall frost wouldn’t really surprise me. If we can dodge this early-fall frost, it looks like it will be a couple of weeks before we see the next good chance of having frost again. This will place us right around average. If you’re hoping not to see an early frost this year, keep your fingers crossed that we’ll see plenty of clouds, especially at night, over the next few days.

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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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