Why there was a heat wave in Cape Town last weekend

What is a heat wave?

The recent heat wave that occurred in the Western Cape (Friday 21st to Sunday 23rd January 2022) has prompted much speculation as to whether it was a normal event, or whether it was attributed to climate change. So, we may ask what temperature qualifies as a heat wave? The South African Weather Service (SAWS) defines a heat wave as prolonged periods (at least 3 or more consecutive days) in which temperatures exceed 5oC above the average maximum temperature of the hottest month. This will vary from place to place, meaning that the threshold may be higher inland than in Cape Town, the latter of which has an average maximum temperature of 27oC for January/February (the hottest months for the city).

Heat waves tend to occur annually in the Western Cape and are caused by the presence of a continental high-pressure system associated with a coastal low that draws the inland air downwards. As this continental air mass descends towards the coast and loses altitude from approximately 1 200 metres to sea-level, the temperature rises adiabatically at a rate of approximately 1oC per 100 metres of descent. This provides an almost 10oC rise in air temperature associated with the outflow of air from the interior towards the coast, causing “Berg wind” (i.e., hot wind) conditions. Should the interior air temperature already be high (likely in summer), this can exacerbate into a potential heat wave event.

While temperatures for the weekend are still being validated, SAWS recorded maximum coastal temperatures reaching 38oC and 36oC in Cape Town on Saturday and Sunday respectively, while inland the temperature soared to 43oC in Malmesbury on Saturday, and 42oC in Robertson on Sunday. The maximum temperatures of 45.2oC and 45.9oC originally reported in the news for Robertson on Saturday and Sunday respectively have been disregarded due to environmental factors causing locally induced heat, which pushed temperatures in the vicinity of the monitoring station higher than actual air temperatures. Since temperatures in the city exceeded the highest monthly average of 27oC, this weekend’s temperature spikes can with certainty be classified as a heat wave.

What role does climate change play in these heat waves?

Heat waves have always happened in the past but are becoming more extreme and frequent worldwide. With global warming the sub-tropical high-pressure systems are increasing in size, poleward extent, and strength due to the expansion of the Hadley Cell. Amplified high-pressure systems will increase the downward movement of air (air descends in a high-pressure system and ascends in a low-pressure system), causing sunnier and less cloudy days, which enhances temperature. As the high-pressure systems reach further southward, they will extend the length of heat wave events.

The environmental and socio-economic impact of these heat waves and the potential for them to become more frequent and intense is a concern for the Western Cape. The region faces increased fire risk (exasperated by wind), heat related illness and even death (particularly to vulnerable elderly and poorer communities), as well as stress to several agricultural crops. This is especially the case in the Swartland region, where agricultural crops are not indigenous to the area and will not cope with increased heat stress.

synoptic chart sunday
The synoptic chart for Sunday 23rd (the peak day of the heat wave) shows the Indian Ocean high-pressure system ridging across the continent and the red arrows indicate the anticlockwise movement of the warm air across the continent. The dotted line represents a low-pressure trough (coastal low) which assists to move the air from the interior to the south and east coasts (sucking the air southwards). The synoptic chart illustrates “textbook” conditions for a heat wave.

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