Sunshine duration is recorded in most countries by the Campbell-Stokes burning glass (CS) but in the United States by a thermometric type (US).
CS and US instruments were exposed together and compared at Blue Hill Observatory. The average values of insolation at which the CS and US begin and cease to register were estimated from Blue Hill pyrheliometer records to be 0.33 cal cm−2min&minus:1 (CS) and 0.37 cal cm−2min−1 (US) for the two instruments tested. However, the US usually exceeds the CS in summer because it responds to diffuse radiation while the CS does not.
Low-sun corrections are added to the US record by the U. S. Weather Bureau before tabulation. The US corrected record is longer than the CS record, which is not increased by a low-sun correction.
Two sets of conversion values were obtained: (1) from the monthly low-sun corrections at various stations and the direct comparison of the instruments at Blue Hill, (2) from comparison of long CS and US (corrected) records of paired Canadian and United States stations and the Blue Hill and Boston pair.
The value to be added to convert CS to US (corrected) is greatest in summer and least in winter. The annual conversion value is a percentage of sunshine duration increasing from 11 per cent in the northern United States to 14 per cent in the south.