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Retrieved from: https://ujdigispace.uj.ac.za (Accessed: Date). {\Al0 Ol\\J HAIL IN THETRANSVAAL SOME GEOGRAPHICAL AND CLIMATOLOGICAL ASPECTS by lANA OLIVIER THESIS submitted in partial fulfilment of the requirements for the degree DOCTOR IN THE NATURAL SCIENCES III GEOGRAPHY in the FACULTY OF SCIENCE at the RAND AFRIKAANS UNIVERSITY PROMOTOR: PROF. P.AJ. VAN RENSBURG MAY 1990 111111111111 3 00539 71.72 RAU BIB DECLARATION I declare that this thesis is my own, unaided work, and that it has not been submitted previously asa dissertation or thesis for any degree at any other university. ;.~~~~~ . May 1990 SUMMARY Hailstorms ~e 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 viceversa. 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. ii Hail damage patterns with regard to maize in the Transvaal exhibit temporal and spatial variations. The hail damage season is necessarily restricted to the period between crop emergence and harvest and hence differs ~om the true hail season..Most hail damage occurs during February and March when the maize plant reaches its flowering and seed-filling stages. Various indices were devised to reflect crop damage. These showed that most hail damage occurs, and hail risk to maize is highest, in the central, southern and south eastern Transvaal. iii OPSOMMING Haelvoorkoms is 'n welbekende verskynsel in die somerreenvalgebied van. Suidelike Afrika en is periodiek verantwoordelik vir omvangryke skade in veral die landbousektor. Hierdie proefskrif ondersoek die geografiese en klimatologiese aspekte verwant aan hael in Transvaal. Die drie hooftemas is: a) die tyd-ruimtelike eienskappe van haelvoorkomstes b) atmosferiese en reenvaltoestande wat ondervind word tydens haelvoorkomstes c) die geografie van haelskade soos van toepassing op mielieverbouing. In Transvaal vermeerder die haeldagfrekwensie (HDF) eksponensieel met toename in hoogte en breedtegraadligging. Hoogtevariasies, soos gereflekteer in die diverse fisiografie van die gebied, is grootliks verantwoordelik vir die variasie in ruimtelike en temporele eienskappe van haelvoorkoms. Op grond van onderlinge verskille in die aanvangstyd van haelneerslag, die seisoenale voorkoms daarvan en die jaarlikse HDF-patrone, kan sewe haelstreke onderskei word. Breedweg gesproke kom die meeste haelstorms gedurende November, tussen 16:00en 20:00voor. Die belangrikste bevinding LO.V. reen-hael interaksies, is dat daar 'n negatiewe verwantskap tussen reenval en HDF bestaan, m.a.w. tydens droe periodes is die HDF laer as normaal, en andersom. Afwykings in die daaglikse en seisoenale eienskappe van neerslag is ook waargeneem tydens 'hoe haeljare' (HID; droe jare) en 'lae haeljare' (LHJ; nat jare). Dit kan heel moontlik toegeskryf word aan veranderinge in Iugsirkulasiepatrone, soos gereflekteer in die oorgang vanaf barokliniese na kwasi­ barotropiese toestande, wat die seisoenaliteit van mesoskaalse konvektiewe sisteme beinvloed, Byvoorbeeld, tydens HHJ kom die piek haelseisoen effens later as normaal voor terwyl die reenvalmaksimum vervroeg. Voorts blyk dit dat, alhoewel die voorkoms van konvektiewe sisteme bykans dieselfde is in HHJ en 'normale jare', die gemiddelde oppervlak wat neerslag ontvang asook die hoeveelheid reenval, merkbaar afneem. Die Suidelike-Ossilasie Indeks (501) speel waarskynlik 'n rol in die veranderinge veral tydens November en Desember. Alhoewel die SOI-HDF verwantskap relatief swak is,is dit heelwat sterker as die tussen die SOl en reenval. Reenvalkenmerke van reendae, met en sonder hael, verskil deurdat dit op haeldae meer reen en die reenproduserende sisteme ook meer ekstensief is. Oor die algemeen is atmosferiese toestande meer onstabiel en die humiditeit hoer op haeldae. Voorts word hulle gekenrnerk deur warm noordoostelike oppervlakwinde en droe suidwestelike winde op die 600 hPa-vlak wat progressief in snelheid toeneem met hoogte. iv Haelskade t.o.v. mie1ies in Transvaal varieer ruimtelik en oor tyd. Aangesien haelskade beperk is tot die tydperk tussen die opkoms van die saailing en oes, stem die haelskadeseisoen nie ooreen met die . meteorologiese haelseisoen nie. Die meeste haelskade word gedurende Februarie en Maart aangerig tydens die blom en saadvullingstadia. Verskeie indekse is geformuleer en bereken ten einde die omvang van haelskade te weerspieel, Die het getoon dat sentraal-, suid- en suidoos-Transvaal die gebiede is waar die meeste haelskade voorkom en waar hae1-risiko m.b.t. mielieverbouing die hoogste is. v ACKNOWLEDGEMENTS I am deepl¥ indebted to many people without whom this study would not have been possible. I gratefully acknowledge the South African Weather Bureau, Sentraoes Insurance Company and the CSIR for providing the data on which the analyses are based. In particular I would like to thank Mrs. G Swart of the SAWE for her friendly assistance in supplying me so promptly with the vast quantity of weather data needed for the study. I would also like to thank the University of Stellenbosch and the RandAfrikaans University for the financial assistance which they provided. In particular, I thank Dr. G Held, Prof IH Van Heerden and Dr. AL DuPisani for their support, comments and constructive criticism; Dr. HL Zietsman for the extensive programming assistance and his advice regarding the statistical analyses and interpretation of results; Mr. AC Vlok for translating the summary and for the production of the computer-drawn maps; Dr. IH van der Merwe for aid in programming; and Mr. I Mouton for the newspaper articles included in the Appendix. My heartfelt thanks also go to: Mrs. Alta Pool, not only for the skilled cartographic work, but for her invaluable contribution in the final preparation of the thesis; Mrs. Marianne Botha, who unstintingly spent many hours preparing of the final text and tables; and Mrs. Anna van Heerden for typing the thesis. Apart from their help in such technical matters, these ladies were consistently friendly and supportive. Thank you. A very special thanks to Mr. MKR van Huyssteen for his valuable comments, advice, and language editing. Thanks also to all my other colleagues for their support, patience and sense of humour. To Professors CJ Swanevelder and WS Barnard, thank you for easing my way during the course of the research, especially during the final stages. My sincere thanks to my promotor, Prof PAI van Rensburg, for his guidance, supervision, encouragement