A Climatological Assessment of the Utility of Wind Machines for Freeze Protection in Mountain Valleys

Abstract

The use of wind machines for frost protection is common in several large United States fruit producing areas. However, their potential usefulness in western Colorado's high elevation orchards has been uncertain due to the existence of terrain-generated prevailing nocturnal winds. To investigate this problem, wind speeds and temperature inversions were measured in an orchard area of western Colorado during the critical spring period 1982?1986. Results showed that temperature inversions strong enough to be beneficial in the use of wind machines at the time of the lowest temperature occurred on 4 1% of all nights sampled, on 58% of nights with below freezing temperatures and on 73% of nights with damaging freezes. A weather typing scheme was then employed to separate objectively freeze events that were primarily local in nature (good candidates for mechanical frost protection) from the more widespread advective freezes (difficult to combat with wind machines). Results showed that undisturbed weather patterns accompanied 54% of all nights but 79% of all freeze episodes. This suggests that freezes are predominantly controlled by local factors. An hour by hour computation of the likely fan effect during all 15 damaging freeze events during the experiment showed that orchard warming would occur during at least part of the night on 93% of the nights. It is now concluded that wind machines are likely to be very beneficial in western Colorado's commercial fruit growing areas.