Why the large diurnal temperature variation?
Below shows the topographic map of South Africa.
The place in question has mountains to the east and this leads to cold air drainage at night. During the day, the regions heats up like a furnace given the shape being like that of a furnace.
The air was very dry with dew points below -10°C during the day near the surface and dew points below -30°C on the mountain peaks, such dry air would have encountered cold air drainage overnight. There were clear skies and light winds.
The place also despite not being on a mountain, is on elevated ground and is around 1600 meters above sea level. On elevated ground, more infrared radiation can escape to space because there is less atmosphere and greenhouse effect for the infrared radiation to pass through. Also there is more solar radiation reaching the surface during the day.
In addition to that the atmosphere is thinner on higher ground and a thinner atmosphere can change temperature more easily because of lower thermal capacity.
The soils are also very dry and hence can change temperature easily.
Topographic map (the place in question lies to the east of Johannesburg but to the west of the mountains:
The place in question has mountains to the east and this leads to cold air drainage at night. During the day, the regions heats up like a furnace given the shape being like that of a furnace.
The air was very dry with dew points below -10°C during the day near the surface and dew points below -30°C on the mountain peaks, such dry air would have encountered cold air drainage overnight. There were clear skies and light winds.
The place also despite not being on a mountain, is on elevated ground and is around 1600 meters above sea level. On elevated ground, more infrared radiation can escape to space because there is less atmosphere and greenhouse effect for the infrared radiation to pass through. Also there is more solar radiation reaching the surface during the day.
In addition to that the atmosphere is thinner on higher ground and a thinner atmosphere can change temperature more easily because of lower thermal capacity.
The soils are also very dry and hence can change temperature easily.
Topographic map (the place in question lies to the east of Johannesburg but to the west of the mountains:
Topographic map zoomed in on the area (Notice how the place has rugged topography and also mountains to the east allowing for excellent cold air drainage):
The first map is from Megan Holloway's pinterest and the second map is from Google Maps.