The Weather Vane is prepared by Daniel Bezte, a teacher by profession with a B. A. (Hon.) in geography, specializing in climatology, from the University of Winnipeg. Daniel has taught university-level classes in climate and weather and currently operates a computerized weather station at his home near Birds Hill Park, on 10 acres he plans to develop into a vegetable and fruit hobby farm.
Contact him with your questions and comments at [email protected]
BY DANIEL BEZTE
Besides drought, frost probably has the greatest impact on agriculture, with untimely occurrences resulting in substantial losses of crops in some years. Also, it is the length of the frost-free season that restricts agricultural opportunities in any given region. The dates of the last spring and first fall frosts vary from year to year, but overall, they are seen to be relatively constant. If this were the case, then we wouldn’t have to worry about losing a crop, or our flowers, to a late-spring frost.
There has been a considerable amount of study relating to frost and its effects on plant life and as I mentioned last week, now would be a good time for us to revisit the relationship between frost, topography and crops.
So how do you define frost? Using a climatic definition, frost occurs when the temperature of the land surface drops below 0 C, but with respect to agriculture, frost is substantially more difficult to define. Looking at cereal crops for example, a temperature of 0 C will not result in the death of the crop; in fact, the crop will rarely even be damaged at this temperature. So when it comes to agriculture, the terms killing frost or freeze become more useful. A killing frost usually refers to conditions where more than 50 per cent of the vegetation in a region has been killed off by frost, and a freeze is when ice crystals form in plant tissue resulting in permanent injury to the plant.
At what temperatures are different crops affected? There are three development stages in which crops have varying degrees of resistance to frost: germination, flowering and fruiting. The germination stage has the greatest resistance to frost, whereas flowering has the least resistance. In the germination stage, crops such as spring wheat and oats can resist temperatures as low as -8 to -10 C. Compare this to crops such as corn, cucumbers, and tomatoes, which will suffer damage when temperatures drop only slightly below zero. In the sensitive flowering stage, every crop is susceptible to frost, with critical temperatures ranging in the 0 C to -3 C range. It’s for this reason that this period of growth is what concerns growers the most when dealing with frost.
Topography, or the lay of the land, can have a significant impact on which areas receive frost. Since cold air is denser than warm air it will flow downward and accumulate in areas of lower elevation. This phenomenon has been referred to as frost hollows, cold islands, cold air pooling and cold air lakes. The important point is, no matter how you describe it, there does not have to be a huge change in elevation to have a significant impact on temperature. In one study, a change of less than one metre in elevation over 100 metres resulted in overnight temperature differences of 3 to 5 C. This is why you can experience frost even though your thermometer tells you it’s above freezing.
Now, how can you protect yourself from frost? If you have 500 acres of wheat there is not much you can do, but if you are worried about flowers, a vegetable garden or a commercial garden such as strawberries, there are a few things that may save your crops.
The first thing you need to understand is that there are two types of frost, radiative and advective.
Radiative frost is the typical frost you would experience on a clear, calm night. During this type of frost, the clear, calm conditions allow heat to escape into the atmosphere cooling down the very lowest layers. You can prevent such a frost from affecting your garden simply by covering the crop to help reduce the heat loss. Another method is to spray water onto the crop. This works because water has a high specific heat and therefore has a large amount of heat energy to release. This heat will help keep temperatures warmer. Also, water releases a large amount of heat energy when it freezes. This is why you can have ice on a plant but not have any frost damage. The third preventive measure you can take is to try and stir up the lower levels of the atmosphere. Since the coldest temperatures are at ground level and warmer air can be as close as only a few feet above the ground, pulling this air downward can prevent freezing temperatures. This can be done using fans or fires to help create convection currents.
The second type of frost, advective frost, is much more difficult to prevent. This type of frost occurs when a cold or subzero air mass moves into a region. It is usually accompanied by winds and lasts for a much longer period of time than a radiative frost. Since all levels of the atmosphere are cold, we cannot pull in warmer air, and since it lasts much longer, using water will probably not work. The only method left is to cover the crop as good as possible and hope that the sun comes out to help warm things up.
Next week we will take a look a what the final weather numbers were for March. Just how much above average will it end up being?