On 27 of October 2004, a hailstorm occurred in Potchefstroom, North West Province, situated just south-west of Johannesburg, South Africa.
24 May 2022
27 October 2004
Several houses and cars were damaged, roads were flooded and surrounding farmlands were destroyed. According to the local news papers report on the day the hailstones were as big as tennis balls in some places. This was a very busy day for emergency services working until late evening due to the number of people who were left without any shelter.
As regards the surface synoptic situation, a U-shaped surface trough of low pressure was situated over the central interior of South Africa, with Potchefstroom situated east of this trough. A surface high pressure area was situated south-east of South Africa causing strong onshore flow over the north eastern parts. As a result dew point temperatures were above 10°C from the central to the eastern part of North West Province, see dashed line on the surface synoptic analysis at 14:00 SAST (source: SAWS).
At 500hPa level, a broad upper trough was situated over the central part of South Africa with a deep layer of moisture from 700–500hPa, see 500hPa upper air chart at 14:00 SAST (source: SAWS). Some thermal perturbations at 700 and 600hPa were also visible over Potchefstroom with 250hPa divergence enhancing upward motion in the vicinity of the thunderstorm. The Meteosat-8 images also clearly show dry conditions west of the 10°C dew point line indicated by cloud free areas, while east of the line overshooting tops of thunderstorms were visible, see HRV image and RGB composite NIR1.6, VIS0.8, VIS0.6 at 13:00 UTC .
During the hour in which the storm occurred (13:00–14:00 UTC), an analysis of MSG imagery reveals the following:
- The images on the left show that Potchefstroom was covered with a thick, cold ice cloud, that developed within the moist air to the east of the dry line.
- The images on the right show that the storm tops over and around Potchefstroom were composed of relatively small ice particles, as indicated by the orange/yellow colour, respectively.
- Furthermore, using a methodology that is being developed by Prof. Daniel Rosenfeld at the Hebrew University of Jerusalem, an analysis of cloud microphysics can be performed by plotting the effective radius of the cloud particles (derived from the IR3.9 channel) against the cloud top temperature (derived from the IR10.8 channel) for all pixels within the area of interest (typically an area of several hundred pixels around the storm that contains convective clouds with elements representing all growing stages). The result for the area of Potchefstroom (see Cloud Analysis below upper right image) reveals a vertical structure that is indicative of a hail storm (slow increase of particle size with height, indicating strong updrafts).
Further information about this methodology can be found in the following papers/presentations:
Satellite based insights into precipitation formation processes in continental and maritime convective clouds (D. Rosenfeld and I. Lensky, 1998)
Understanding clouds and precipitation systems with MSG (D. Rosenfeld, 2004)
Finally, looking in detail at the upper right image (see close-up look), one can see the wet ground (with a dark blue colour) caused by the rain in the wake of the storms.