Effectiveness of stationary and portable wind machines on temperature control under different frost conditions: a spatial analysis approach

Frosts are one of the most damaging meteorological events in agriculture making active control methods, such as wind machines, essential to reduce the impact on crops. A new portable wind machine (PM), Tow and Blow^®, has been commercialized for frost control. Although commercial information claims large benefits, neither its effectiveness nor its performance relative to stationary wind machine (SM), has been scientifically studied. Thus, the aim of this work was to evaluate the effectiveness on temperature spatial modification of SM and PM for different operational configurations and thermal inversion conditions. Measurements were undertaken during 12 frosty winter and spring nights along two seasons in a pear orchard located in central Chile. Temperature measurements were simultaneous and at regular spacing around the machines. All frost events were characterized by clear sky, low wind and moderate to strong thermal inversion; thus, optimal frost control performance was expected. PM showed lower effectiveness in frost control on stability, area and temperature increase. SM best performance configuration produced a temperature increase in 5.3 ha in contrast to 3.0 ha for PM, considering a temperature increase of 30% thermal inversion strength. These figures changed to 2.7 and 0.6 ha for a 50% temperature increase of thermal inversion strength respectively. The low coverage angle of PM (~26°) is an important limiting factor in frost control effectiveness. Thus, it must be operated with the minimum rotation time. The microclimatic conditions, intra and inter frost, were highly variable, being difficult to make recommendations about the optimal operational configuration (position and rotational speed mainly) for the PM without a previous site-specific study.