Given its sporadic nature, long-range forecasting of frost is nearly impossible. Rather, the climate record of an area is used to determine probable dates of frost based on long-term temperature records. While this will not provide an actual frost date in a particular year, it will present the likelihood that frost may occur on a certain date.
The “Date of First Fall Frost” maps at 50%, 25%, and 10% risk show the likelihood that frost will occur on or before the dates shown within the maps. A frost would be expected to occur 1 in 2 years at the 50% risk date, 1 in 4 years at the 25% risk date, and 1 in 10 years at the 10% risk date.
- 50% Risk Map: http://www.gov.mb.ca/agriculture/weather/images/figure5.gif
- 25% Risk Map: http://www.gov.mb.ca/agriculture/weather/images/figure6.gif
- 10% Risk Map: http://www.gov.mb.ca/agriculture/weather/images/figure7.gif
The extent of frost damage to a crop will depend on several factors. The crop type, stage, and hardening of the crop, the soil type and soil moisture, the actual air temperature, the duration of freezing, and the rapidity with which freezing takes place are all important. A drop in air temperature of short duration will cause less damage than a prolonged period at the same low temperature. When the air temperature drops to 0°C, cereal and other crops may not sustain damage. Rather, damage or total loss is more common when minimum temperatures drop below -2°C, often referred to as a killing frost.
For more information on Manitoba’s Agricultural Climate, please visit our website at http://www.gov.mb.ca/agriculture/weather/agricultural-climate-of-mb.html
Submitted by: Pam de Rocquigny, Provincial Cereal Crops Specialist, Manitoba Agriculture
Information from the article “Risk of Fall Frost” by Andy Nadler, 2010