Uncovering constructive ways to minimize frost damage in orchards is a serious undertaking for fruit tree producers. One method that is highly effective in frost protection is the proactive approach to mitigate the damage to plants and trees caused by frost is via the implementation of helicopters. In 2006, the University of Arizona‘s College of Agriculture published a report highlighting a helicopter’s impact in frost prevention.
In 1987, the state of Arizona‘s orchards suffered catastrophic losses as the result of a killer winter storm that impacted the area. According to the study, considerations like the orchard’s site situation and an appropriate tree variety selection can serve to lessen the effects of frost damage, however, implementing an active protocol alongside the aforementioned may be warranted also.
To understand why applying an effective protocol is important, its necessary to comprehend how negatively frost damage can impact fruit harvests. During the winter, when the orchard is dormant, the risk of frost damage is diminished. When the tree starts to emerge from dormancy in Spring is when the fruit is subject to the most danger. Newly emerging leaf buds and shoot growth can suffer serious injury as a result of late winter storms, extreme cold or frost.
Each type of fruit has a different temperature at which it’s bud will ultimately be affected by frost damage. The bud’s stage in maturation and how prolonged the exposure was to colder temperatures or frost will also play a role. As there is no avoiding the occasional frost, blooming fruit trees affected during their Spring acclimation will be left to wait until the following year to be harvested. Frost prevention is key to protecting trees and harvests from damage from inclement weather with freezes.
The College of Agriculture study has intimated that one method commonly used today that has been proven effective is via the utilization of helicopters to circulate warm air down into the orchard on nights when the temperature is expected to dip.
Although there are reports that indicate helicopters have been used to this effect since the 1940s, there is little to no evidence to support how beneficial the practice was overall. In 1988, however, a study was initiated in an effort to acquire data regarding the value of helicopter applications in the mitigation of frost damage.
The study was conducted during the evening and early morning hours at an apple orchard in April 1988. Rapid response temperature sensors integrated with thermocouples were affixed to the apple trees at various heights on the tree in two separate areas in the orchard. The sensor’s output was monitored and recorded every 20 seconds using an automatic data logging device and then committed to magnetic tape.
The helicopters used throughout, Bell Jet Ranger models 206B2 or 206B3, flew across the study areas at elevations roughly 20 to 35 feet above tree level at approximate ground speeds of 15 to 20 mph. Personnel on the ground closely monitored the overflights while recording the timing and location with the temperature sensors of each pass. The helicopter flyovers were commenced at various time intervals when the orchard temperature levels approached the freezing point.
Following the study’s conclusion, the data collected in its various forms was sent to be evaluated. The information was uploaded to a computer, processed for inconsistencies and errors, and entered into a data management system for further perusal. Individual data selections were scrutinized to calculate the overall value the helicopter overflights produced.
The report’s summary indicated the following: for a helicopter to be used effectively in frost prevention an inversion must be present. For the sake of definition, an inversion is the reversal of the normal decrease of air temperature with altitude, or in other words, the temperature aloft wherein the helicopter flies must be warmer than the temperature within the orchard.
The study also indicated that temperature increases in the orchard were swift and widespread as a result of the passage of the helicopter. The temperature increases were noted to occur regardless of the precise altitude at which the overflight took place. However, while the positive implications of the temperature’s almost instantaneous rise were empirical, it’s important to note how rapidly (within 5 minutes) the temperatures returned to previous levels following the overflight.
The study went on to say that the relative causes may be the result of a combination of factors including:
- The replacement of the warm air by cold air drainage winds
- Air temperatures on either side of helicopter flight paths can be affected up to a distance of 150 feet. If cooler winds are present and are flowing perpendicular to the flight path, the strip of air modified by the helicopter’s flight could become displaced. Especially if there is little to minimal foliage present to offer wind resistance throughout the orchard. This resulted in drainage winds flowing unremembered through the orchard resulting in a rapid temperature drop.
- Radiational cooling
- High radiative cooling occurs when the air nearest the ground experiences a surface loss via radiation and as a result, the air temperature is cooled. Radiative cooling was noted as being present on the night of the study on orchard surfaces following the helicopter’s pass.
- The horizontal displacement of cold air by the helicopters themselves. Helicopters warm the air nearest the surface by mixing warm air aloft with colder air near the surface. Consequently, in the process of the air mixing, cold surface air may be displaced horizontally, which could move back over the area through which the helicopter made a previous pass.
Past research has indicated that multiple helicopter passes completed in rapid succession may be necessitated to achieve maximum warming benefits. Helicopter passes completed one right after another will gradually serve to warm pockets of cold surface air dislocated throughout the orchard.
Results of the study further iterated that helicopters can be used to beneficial effect in frost prevention. It should be noted that helicopters used for these intents obviously would not be utilized to succession with a single pass over an orchard.
Also, the results presented in this article are indicative of a smaller scale case study and that larger more advanced study parameters would need to be established to determine all of the benefits of helicopter‘s involvement in fighting the effects of Spring frosts.