Radiation transfer through an artificial fog used in frost prevention
The problem of combatting radiation frost in an orchard by using artificial fog is investigated. The basis of the technique is to compensate the loss of terrestrial infrared radiation in the atmosphere window region of the spectrum, by scattering, absorption and re-emission from the fog. A numerical model has been developed to simulate vertical infrared radiation transfer, including the full effects of fog water droplets. Multiple scattering of radiation within the fog is also allowed for. The measured droplet distribution of an actual artificial fog released into an orchard has been incorporated to compute the theoretical effect on net radiation loss, and a comparison is made of the theoretical result with that measured. This fog is found to have a low reflectivity, with the major benefit coming from the thermal emission of the water droplets. Increasing the relative humidity has only a minor effect on net radiation compared to the effect of water droplets. The optimal modal droplet radius for such a fog is completed to be 0.5 pm.