Summers are warm and relatively wind free. Expect temperatures averaging around 30°C and balmy evenings that rarely go below 20°C. Summer is also characterised by afternoon thunderstorms with torrential rain and lightning for a brief period before it clears up again.
Winters are crisp during the day but can still be quite warm often averaging between 18 and 20°C. Nights are considerably chillier with temperatures dropping quickly as the sun sets. Winter is the dry season so you can expect almost zero rainfall.
Located on variable altitudes there is a difference in the climate of Johannesburg and Pretoria. Johannesburg, at a higher altitude, tends to have lower temperatures than Pretoria. Keep this in mind when travelling between the two cities.