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Wintry weather blankets South Africa
But did it actually snow in Johannesburg?
Last Modified: 16 Aug 2011 10:46
Heavy snowfall disrupted traffic and led to the closure of the several roads  [Reuters]

Just 24 hours after the rare sight of snow was reported in Auckland, it's now been spotted in another unlikely place, namely South Africa.

Whereas the cold spell in New Zealand was caused by a blast from the Antarctic, the chill in South Africa was caused by what's known as a 'cut-off low'.

A cut-off low, otherwise known as a 'cold pool', is essentially an area of low pressure, which has originated from the poles. Normally a cut-off low will have begun life as a trough of cold air, high up in the atmosphere. This then develops a more distinct circulation, and gets cut off from the main west-to-east flow. The system gradually extends down to the surface bringing cold, unsettled weather to the ground below.

Certainly, this recent cut-off low over South Africa has brought its fair share of cold and wet weather with it.

On Monday it was originally icy rain that started to fall in the eastern parts of the country. KwaZulu-Natal, Mpumalanga and the Free State all saw the temperatures plummet, and gradually the rain turned to snow.

Up to 10 centimetres of snow was estimated to have fallen in parts of Mpumalanga. Several roads had to be closed as the snow got heavier, including the mountainous Van Reenen’s Pass, which stretches between Johannesburg and Durban.

In Johannesburg itself, excitement grew as white flakes began to fall from the sky. This would have been the first time that it had snowed in the capital for four years, but the South Africa Weather Service declared that it was not snow. Instead the precipitation was ruled to be hail.

For many, snow and soft hail can seem like the same thing, but technically they are different. Snow is crystalline in nature; it forms in a neat hexagonal lattice, which can then joins to other crystals to make a snowflake.

Soft hail, on the other hand, forms within a thunderstorm cloud. It forms when a snowflake circulates within the thunderstorm cloud, becoming coated with layers of additional ice. It is a white opaque colour, similar in appearance to snow, as air is also trapped within the ice layers. One of the most distinguishing features of soft hail is that it can fall at much higher temperature than snow can.

The white precipitation which fell in Johannesburg at 8:30GMT (10:30 local time) fell when the temperatures were 7C, which is too warm for snow, so this would certainly be hail.

Then, in just 40 minutes, the temperature dropped to just 5C, which could possibly be low enough for snow, but the photos of the precipitation show that it is soft hail: the precipitation is slow melting and looks more 'granular'.

Whatever it was that fell from the sky in Johannesburg, it has now cleared and the weather is slowly recovering. By Saturday the city should be basking in a sunny 20C!

Source:
Al Jazeera
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