When the atmosphere 40' to 50' above ground is 13°F or more warmer than it is a few feet above the soil surface, the temperature inversion condition—in frost protection research—is considered to be strong. When the temperature difference is less than about 5°F the inversion is considered weak. The weak inversions usually found in the deciduous orchards on the floor of the Sacramento Valley appear to limit the usefulness of wind machines for frost protection unless additional heat is supplied from orchard heaters.

The National Frost Protection Co.; Walter Stiles, almond grower; W. A. Gerrans, prune grower; Gerrans Manufacturing Co.; and J. Beckett, design engineer, Hiller Helicopters, cooperated in these tests.

C. E. Barbee, C. R. Miller, Fred Lory, and H. B. Schultz of the Department of Agricultural Engineering, Davis; N. W. Stice, Farm Advisor, Colusa County; R. R. Parks, Extension Agricultural Engineer, Davis, gave essential assistance in these studies.

This is the ninth annual progress report in the study of wind machines in orchards published in California Agrirulture.