Does this winter feel colder to you than last year’s? So far this winter, crops in parts of the country have recorded more chill units than in the winter of 2024.
By Alani Janeke
At the start of the second week of July, some mountain peaks in the country are covered in snow, while minimum temperatures have dropped to as low as -7°C in places across the Eastern Free State on Monday.
According to Johan van den Berg, independent agricultural meteorologist, quite a lot more chill units have already been reported this year than last year.
In the winter rainfall areas, such as Ceres, Vredendal and the Hex River Valley, approximately 25% more chill units were recorded between 1 May and 7 July this year than in the corresponding period last year. “However, this is still about 10% less than in this period in 2023.”
This year, already 40% more chill units have been accumulated in parts of the Northern Cape than at the same time last year. However, this year’s chill units in that province are 5% less than recorded in the same period in 2023.
“The large number of chill units accumulated this year is very favourable for crop production. It looks like the rest of July and the first part of August will also be favourable for the further accumulation of chill units, although daytime temperatures in the Northern Cape will start to rise.”
When are chill units accumulated?
Van den Berg explains that temperatures above approximately 16°C are detrimental to the accumulation of chill units and can destroy those that have already accumulated.
The optimal range of temperatures for the build-up of chill units is between 1.5°C and 12.5°C. “Take into account that chill units are calculated on hourly information and that the number of hours in a day with good accumulation should be more than the number of hours with unfavourable conditions. The net effect should be that reasonable amounts of chill units can be accumulated in the coming weeks.”
Lake effect for snow
Van den Berg explains that the higher moisture content on the ground is likely contributing to the regular occurrence of snow this season. “Interestingly, a phenomenon called the ‘lake effect’ is developing in North America. It results when colder air passes over the warmer waters of lakes and absorbs more moisture and heat. This causes air to rise, forming clouds and snow downwind from the lakes.
“So, the warmer the surface water gets, the more snow (and rain) can occur when cold air passes over it.”
So, after the heavy late rains toward the end of summer, there are more water surfaces this year that can contribute to snow conditions.
