Previous tests have shown that 75% to 90% of the heat of combustion of the fuel oil burned in an orchard heater goes into producing the hot gases which rise as a plume vertically above the heater. As these hot gases rise, they cool by mixing with the surrounding air. Cooling is rapid until a height is reached where the temperature of the plume is near the temperature of the adjacent air. If the temperature of the air over the crop increases with height at this point—if a temperature inversion exists—continued upward movement of the gases soon brings them to a height where the surrounding air has the same temperature as that of the plume. Above this point, the plume's gases are actually colder than the adjacent air. Thus the gases in the plume are heavier than the surrounding air and their momentum is dissipated. This soon brings the upward motion to a halt. Then the gases fall part way back to the height at which they had the same temperature as the surrounding air and spread out at that level. This height at which the gases spread out and the height at which the upward motion ceases are measures of the depth of the air over the crop being heated.

William Kerth, North Sacramento, provided the ice skating rink for use as a research laboratory for the above experiments.