What was done
The authors analyzed near-surface air temperature data from what they describe as "five very large metropolitan areas and 19 stations from non-urban locations" of South Africa for the period 1960-1990, comparing their results with those of Jones (1994) for the same time interval.

What was learned
Hughes and Balling report that the mean annual air temperature trend of the five large cities averaged 0.24°C per decade, while the mean warming rate of the 19 non-urban centers was a statistically insignificant 0.09°C per decade over the 1960-1990 period, which values are to be compared to the overall warming rate of 0.31°C per decade that was derived by Jones for the entire country. In addition, they note that the mean rate-of-warming difference between their urban and non-urban sites was driven primarily by increases in daily minimum temperatures, which rose at a mean rate of 0.07°C per decade at the non-urban stations, but at an average rate of 0.34°C per decade at the five large cities.

What it means
According to the two researchers, the "disparate trends in temperature" that they found to exist between the urban and non-urban stations they studied "suggest that urbanization has influenced the Jones (1994) records for South Africa over the 1960-1990 period of apparent rapid warming," and that their analyses suggest that "half or more of this recent warming may be related to urban growth, and not to any widespread regional temperature increase."

Reference
Jones, P.D. 1994. Hemispheric surface air temperature variations: a reanalysis and an update to 1993. Journal of Climate 7: 1794-1802.