Vitamin D from sunlight in South Africa

How long you need in the sun to make vitamin D, and when the sun is too low to make any at all, for 25 cities in South Africa.

City	Latitude	Elevation	Vitamin D winter
Johannesburg	26.20°S	1,767 m	none
Cape Town	33.93°S	25 m	none
Durban	29.86°S	21 m	none
Pretoria	25.74°S	1,332 m	none
Soweto	26.27°S	1,632 m	none
Gqeberha	33.96°S	61 m	none
Soshanguve	25.47°S	1,179 m	none
Pietermaritzburg	29.62°S	636 m	none
Evaton	26.53°S	1,513 m	none
Benoni	26.19°S	1,641 m	none
Bloemfontein	29.12°S	1,396 m	none
Thembisa	26.00°S	1,577 m	none
East London	33.02°S	48 m	none
Vereeniging	26.67°S	1,445 m	none
Boksburg	26.21°S	1,624 m	none
Welkom	27.98°S	1,376 m	none
Newcastle	27.76°S	1,195 m	none
Krugersdorp	26.09°S	1,724 m	none
Rustenburg	25.67°S	1,171 m	none
Emalahleni	25.87°S	1,629 m	none
Randburg	26.09°S	1,593 m	none
Roodepoort	26.16°S	1,738 m	none
Botshabelo	29.27°S	1,448 m	none
Brakpan	26.24°S	1,631 m	none
Kariega	33.76°S	122 m	none