Observations of atmospheric radiation

Abstract

Atmospheric radiation is the primary variable in nocturnal cooling, although wind has greater influence on the temperature of surface air. Since temperature inversions near the ground cannot be formed unless surfaces are cooled by radiation loss to the sky, a knowledge of atmospheric radiation is essential to the proper understanding of radiation frosts. The frost hazard is so serious to the citrus industry that orchard heating is practiced extensively at an average cost of more than $1,000,000 per year. In the special project on orchard heating carried on during the last four winters by members of the Agricultural Engineering Division of the University of California, several measurements of nocturnal radiation were made at Riverside, California in 1938-39. These reveal a significant relation in the change of intensity with angle of sight from the zenith to near the horizon, which is not revealed in Elsasser's radiation chart. It was also noticed that the Hottel & Mangelsdorf measurement of water-vapor radiation over short paths indicated a rate of change with path length much greater than that derived from Elsasser's chart. These differences between observation and deduction invited further observational study, with a parallel-beam radiometer, of atmospheric radiation from short paths and from the sky, which are here reported. The laboratory measurements (for path lengths up to 6 meters) were carried out with Professor H. C. Hottel in the Chemical Engineering Laboratory of the Massachusetts Institute of Technology. The nocturnal sky observations were made at the Harvard Blue Hill Observatory simultaneously with special radiosonde flights conducted by Dr. K. O. Lange. The same radiometer was used for both the laboratory and the sky observations.