South African solar radiation survey 1937-38

Abstract

The results of the physical measurement of the total amount of sun and sky radiation and the cooling temperature at 6 stations in the Union may be summarized as follows: A. Radiation. 1. How does the amount of radiation at the inland stations compare with that at the coastal stations? The amount of radiation obtained at the inland stations exceeded the amount at the coastal stations all the year round. 2. Which station received the greatest, and which the smallest amount of radiation during the course of the year? During nine out of twelve months the greatest amount of radiation was obtained in the climatic zone of the high veld. During the remaining three months (December, January and February) the greatest amount was received at Cape Town. The smallest amount of radiation was obtained during all twelve months at the coastal stations. In five of the twelve months Cape Town recorded the least, during five other months Durban received the smallest amount. 3. During which month was the greatest and during which the smallest amount of radiation obtained at each station? At each station the greatest amount of radiation occurred during the months November or December, 1937. The smallest amounts were recorded during the winter months of July, 1937, and June, 1938. 4. What was the ratio between the greatest and the smallest amount of radiation at each station? Johannesburg registered during the month of maximum radiation twice as much solar energy as it did during the month of minimum radiation. Comparable ratios were: 3.4:1 for Cape Town; nearly 3:1 for Port Elizabeth and 2:1 for Durban. 5. How did the amount of radiation at each station vary during the course of the year? The amount of radiation at Johannesburg during the course of the year was rather irregular. At Bloemfontein and Nelspoort the increase of the monthly amount with the increase of the altitude of the sun was progressive. At Durban great variations in the amount of radiation was registered from month to month. Port Elizabeth showed a fairly regular distribution over the year. In Cape Town the amount also increased and decreased steadily in accordance with the sun's altitude and showed particularly high readings in December and January. 6. How do the total amounts of radiation recorded at the stations during a period of six months compare with each other? During the first half year under investigation the total amount of sun and sky radiation was largest at Bloemfontein, next in amount was that at Johannesburg and at Nelspoort Sanatorium. At the coast, Cape Town and Port Elizabeth had an approximately equal amount of radiation. Durban's amount was distinctly smaller. Durban, being the station with the smallest amount of radiation during the first half year, recorded 22 per cent. less than Bloemfontein which had the greatest amount. The second half year showed a greater amount of radiation at Johannesburg than at the coastal stations. These recorded fairly similar amounts during the period. The total for the year was distinctly larger at Johannesburg than at the three coastal stations. 7. How does the amount of radiation obtained in South Africa compare with the amount in Nairobi (Kenya), Davos (Switzerland) and Bad Nauheim (Germany)? In winter the amount of sun and sky radiation at Johannesburg was distinctly larger than at Davos situated at a similar altitude above sea-level and it was 5 to 10 times larger than at Bad Nauheim at a low altitude. During two months in midsummer the amount at Davos was slightly larger than at Johannesburg during the comparable period. Nairobi experienced a greater amount of radiation during the six months under investigation than any of the other places concerned in this comparison. The ratio between the highest summer and lowest winter reading per month was 2.0:1 at Johannesburg, 4.5:1 at Davos and 10.4:1 at Bad Nauheim. The yearly total amount at Johannesburg was 122 per cent. of what was received in Davos; Bad Nauheim obtained only 69 per cent. of that amount. B. Cooling temperature. 1. During winter the mean values of the cooling temperature showed the greatest extremes at the inland stations, particularly at Johannesburg. Bloemfontein recorded higher readings at night, and Nelspoort distinctly lower recordings during day-time than Johannesburg. The average conditions at the three coastal stations showed no very distinct differences. Throughout the year Durban's mean cooling temperature values were the highest. Next in order was Port Elizabeth, followed by Cape Town. 2. The rapid decrease of cooling temperature in the afternoon continued after sunset at the inland stations but not at the coastal stations. In the latter the rapid decrease only lasted until sunset, immediately after which the drop became much more gradual. 3. The average increase of the mean cooling temperature from winter to summer was great at the inland stations, but small at the coastal stations. 4. In winter the monthly average cooling temperature was distinctly lower at the inland than at the coastal stations, in summer this difference was not pronounced. 5. The mean maximal cooling temperatures showed no regular difference between the interior and the coast, whereas the mean minimal readings were distinctly lower at the inland stations. 6. The absolute highest cooling temperature of 52.0° was registered at Port Elizabeth in August and in January, the absolute lowest reading of 4.0° was recorded at Johannesburg in September, 1937.