The effectiveness of all frost protection depends primarily on the temperature inversion—ceiling—but adequate measurements had not been made until an extensive field program was undertaken in northern California in the spring of 1956. Six thermographs equipped with 50' extensions were installed with other spot-climate recording stations in selected deciduous orchards. Several growers contributed to the project by keeping the thermographs in operation.

Harry Hansen, United States Frost Warning Service, members of the university of California Agricultural Extension Service, and individual growers co-operated in the studies reported in the above progress report.