An Empirical Description of Cold Protection Provided by a Wind Machine1

Abstract

Abstract The temperature field around a wind machine located in a citrus orchard was measured using a fast response thermistor network. The area of protection provided by the wind machine was kidney-shaped. Temperature at any given point was continually rising or falling and depended upon the position of the point relative to the jet. Maximum temperature rise was recorded 90 to 150° after the jet had passed the point. Air movement was not always away from the machine, but cold air inflow occurred just in advance of the turning jet. The wind machine protected a citrus orchard on radiation nights, provided a relatively stable condition existed in the lower atmosphere. The machine caused downward movement of warm air from aloft by creating divergence of cold, dense air at lower levels. The area and degree of protection were directly related to the inversion strength, strong inversions producing a greater effect than weak inversions. A larger area of protection was obtained under conditions of weak inversions at the lower elevation (5 ft) without leaves than when leaves were present. However, a greater area of protection was obtained under strong inversions when leaves were present on the tree. Thus, one might expect less area of protection in defoliated groves, i.e. groves which had been defoliated by earlier freezes and had not refoliated, or in deciduous orchards before leafing.