Every year around this time the weather discussion begins to centre around the first fall frost.
Some years, like when we see an early-fall frost, this topic is at the forefront of conversations. This year, if you live in Alberta or Saskatchewan then there is a good chance that you’ve already seen your first fall frost and those conversations are taking place. In Manitoba, only far northwestern regions have seen frost, so for most areas of that province, the topic of frost may not be front and centre quite yet.
The first question I am usually asked about fall frost is when the different areas of the Prairies 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 0.0 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 happens for a number of reasons, and largely depends 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, especially for low-lying areas.
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 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 below -2.0 C to kill off most crops.
For these reasons we will often 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.
The trouble with using these three temperatures is that most frost tables of averages only include 0.0 C, which then makes it difficult to compare the other values. For this reason I am going to determine the dates of the first and last fall frosts simply on the first and final times each of our sites fell below 0.0 C.
The accompanying table (see below) has this year’s data showing the dates for the last spring frost (LSF), first fall frost (FFF), and the length of the frost-free season (FFS). I have also included the 1981-2010 averages. (The final numbers for the Manitoba locations are not yet in as those locations have not yet experienced their first fall frost.)
Looking at the data you can see that the well-above-average May resulted in earlier-than-average last spring frosts across all three Prairie provinces. Calgary had the earliest last spring frost while Regina and Winnipeg saw the latest.
Moving on to the first fall frosts, all of the main locations in Alberta and Saskatchewan saw either earlier-than-average first fall frosts or were right around average. In Manitoba, the cold arctic air that brought the early frosts to the other two provinces was kept at bay by several fairly strong areas of low pressure. This means (at the time of writing) that these areas have yet to see any frost, which will result in a later-than-average date for the first fall frost.
Put these numbers together and we can calculate the length of the frost-free season. Last year, most parts of the Prairies saw frost-free seasons that were a little bit longer than the long-term average. Winnipeg and Brandon both ended up having the longest frost-free season, with a respectable 134 days, which put Brandon in the top 10 per cent for length of the frost-free season.
For the second year in a row, Regina was one of the only locations that had a shorter-than-average frost-free season. But thanks to the warm spring, all other locations had a longer-than-average season. It will be interesting to see what happens across southern Manitoba. If this region can dodge frost until the end of September, the weather models are hinting at a return to well-above-average temperatures for early October which would result in some extremely long frost-free seasons. Who knows, Manitoba may even break a few records.