It helps in your agricultural planning to know when, on average, you can expect the first heavy frost to occur, when the last frost may be expected and what the variability of frost occurrences is.
To predict when the frosts can be expected weathermen consider five factors:
- Altitude ~ in general the higher the altitude the lower the temperatures will be and hence a greater likelihood of frost occurrence;
- Latitude ~ the further from the equator the lower the temperatures;
- Distance from sea ~ which assumes that the further a location is from the moderating influence of the ocean the more likely it is that freezing temperatures will be reached;
- A topographic valley index ~ denser cold air to “sinks” into valleys, with the intensity of valley frost occurrence depending on the steepness of the surrounding topography, and,
- General minimum temperature conditions ~ ie, those areas which experience low daily minimum temperatures, for example below 6O C, will also be more prone to frost formation.
Based on these factors and using data from 216 climate stations in southern Africa, meteorologists calculate that on average the first heavy frost occurs in May over most of the interior of South Africa.
For example, in Gauteng 80% of the province will have frost by 1 June.
According to the CSIR, most of the interior experiences its last frosts towards the end of August and early September, but in the high Drakensberg frosts are predicted to persist into early December. These are of course averages and used as a guideline, for frost is notoriously erratic.
What is frost?
What actually happens to cause frost? During the night both the soil surface and the air cool, until water vapour in the air condenses on exposed surfaces, such as foliage and lawn, forming dew. When temperatures drop below freezing, the water vapour changes into the ice crystals ~ this is known as frost.
Frost is most likely to occur on wind-free, cloudless nights as both wind and cloud cover can keep the air temperature warmer.
In some regions the air is too dry for ice crystals to form. Nevertheless the below-freezing temperatures damage plant structure and such frosts are known as black frosts, so-called because of the colour of the damaged plant foliage and stems.
Frost damage occurs when the cell sap in plants freezes. This causes the sap to expand and the cell walls rupture, thereby destroying the plant.
Some plants cannot survive frost ~ these are commonly known as frost-tender plants. Others can tolerate mild frost (0°C to -3°C) ~ these are called half-hardy plants.
The range of below freezing temperatures tolerated by plants that are commonly known as frost-hardy varies enormously. Some plants are killed outright by frost while others suffer damage, such as complete defoliation, from which they can recover.
Check back tomorrow to read about how to protect your vegetable garden from frost.