Abstract
M.Sc. (Geography)
Hailstorms are well-known phenomena in the summer rainfall region of southern Africa where they
cause extensive damage - especially in the agricultural sector. This thesis examines the geography and
climatology of hail in the Transvaal. It deals with three main issues, namely
a) the spatial and temporal characteristics of hail days (HDs);
b) rainfall and atmospheric conditions prevailing during hail events; and
c) the geography of hail damage as it pertains to maize.
In the Transvaal, hail day frequency (HDF) increases with altitude and latitude in a non-linear
(exponential) manner. Variations in altitude, as reflected in the diverse physiography of the area,
account for most of the spatial and temporal variations in hail occurrence. Seven hail regions can be
distinguished which differ from one another in terms of the onset times of hail, its seasonal occurrence
and annual HDF patterns. In general, most hailstorms occur during November with the peak onset
time varying between 16:00 and 20:00.
The most notable finding concerning rain - hail interrelationships, is that rainfall and HDF appear to
be inversely related, years/months with high hail incidence being dry and vice versa. Daily and seasonal
precipitation characteristics also differ between 'high hail years' (HHYs; dry) and 'low hail years'
(LHYs; wet). For instance, during HHYs, the peak hail season is delayed while the rain season peaks
earlier. Moreover, while the frequency of convective systems remains nearly the same during HHYs as
in 'normal' years, the average precipitation area and the volumetric production decreases significantly.
These anomalies appear to be the result of changes in the large-scale circulation patterns (as reflected
by the transition from baroclinic to quasi-barotropic conditions) which influence the precipitation from
mesoscale convective systems. It is likely that the Southern Oscillation plays a role in these changes,
particularly during November and December. However, although these Southern Oscillation Index
(SOl) - HDF associations are weak, they are appreciably stronger than those between the SOl and
rainfall.
Rainfall characteristics on HDs differ from those of non-hail rain days in that, on HDs, more rain falls
and the rain-bearing systems are more extensive. In general, atmospheric conditions are less stable,
and the humidity level is higher, on HDs than on other days. Furthermore, HDs are characterized by
warm north easterly winds near the surface but cold, dry south westerlies at the 600 hPa level. Above
this the south westerlies become progressively stronger