In short:
• The risk of frost damage remains high until October.
• There are still many similarities between 2019 and the current El Niño system.
• In 2019, the risk of frost increased due to very high maximum temperatures, which occurred as early as mid-September and the beginning of October.
Sharp rises in daytime temperatures, followed by sudden drops, are normal for this time of year as the season begins to change and cold fronts continue to affect the country.
Maximum temperatures have quickly started to rise since last week, with temperatures above 30 °C recorded in some areas.
“This is typical for this time of year when cold fronts move fairly quickly over the country and are preceded by warm and windy conditions,” says Johan van den Berg, independent agricultural meteorologist. “This can mistakenly give the impression that colder conditions will no longer occur.”
However, synoptic maps indicate that several cold fronts are on the way in August and September. Van den Berg says the fluctuation in temperature between warm conditions and sudden sharp drops usually causes the most frost damage.
“Plants receive the signal to grow due to higher temperatures and more sunshine, but then the new growth and buds suffer the most frost damage when temperatures drop.”
He says it’s interesting to note that growing plants do not experience the same temperatures as those measured by a thermometer at a weather station. “The thermometer at a weather station measures the air temperature, with louvres or partitions that allow airflow without the emission of heat.”
Read more: Black frost deals a heavy blow to Limpopo farmers
In particular, growing plants’ leaves and stems are directly exposed to the atmosphere, and heat radiation takes place back to the atmosphere. “Literature has shown that temperatures up to 5 °C measured at a nearby weather station can already cause light frost on plants.”
Temperatures during the 2019 frost
Moderate frost occurs at around 2,2 °C and severe frost at 0 °C. During the great frost over the southern and western interior on 30 October 2019, the lowest minimum temperatures ranged from about 1,5 °C to 3,5 °C.
Vulnerability was increased in 2019 due to very high maximum temperatures as early as the second part of September and the first part of October. Prieska in the Northern Cape experienced maximum temperatures of 35 °C and higher from 17-20 September. From 11-22 October, temperatures in this area ranged between 31 °C and 37,5 °C, representing summer temperatures.
On 28 October, the maximum temperature dropped to below 20 °C, and the next morning a minimum temperature of below 2 °C caused significant frost damage.
Also that year, rainfall continued over the winter rainfall area until the first half of November due to frontal systems. “There are many similarities between 2019 and the current El Niño system, which transitioned to neutral conditions and is expected to transition to a La Niña system in the spring and early summer,” says Van den Berg.
“This also corresponds with current predictions of further rain over the winter rainfall area and warm conditions already occurring over the summer rainfall area.
“The risk of frost damage remains very high in the coming weeks until October.”
The La Niña system
The Australian Bureau of Meteorology announced that the El Niño-Southern Oscillation (ENSO) remains neutral in the third week of August. “Sea surface temperatures in the central equatorial Pacific Ocean are currently ENSO-neutral, while atmospheric patterns, including clouds and trade winds, are ENSO-neutral.
“Three of the seven climate models monitored by the bureau suggest that sea surface temperatures in the tropical Pacific Ocean will exceed the La Niña threshold (of below -0,8 °C) from October, with a fourth expected to briefly reach the threshold.”
“The ENSO outlook remains at a La Niña watch. A La Niña watch does not guarantee a La Niña development but indicates that there is an equal chance that ENSO conditions will remain neutral or that a La Niña system will develop during the rest of 2024.” (ENSO is one of the broad indicators used for expected climate.)
According to the bureau, global sea surface temperatures were the warmest on record for every month between April 2023 and June 2024. Last August was the warmest month on record, and August this year (up to 20 August) the second warmest. The possibility is strong that it could end up as the warmest.
