Q: What is the lowest temperature I can protect with frost fans?

A: There are many factors that make up the answer to this question, but some important points are: 

• frost susceptibility of your crop variety
• development of the crop and time of year
• conditions of your site including minimum temp at crop height and strength of inversion during frost events (inversion strength is the temperature difference between crop height and 20m above the ground)

Orchard-Rite research development and dealer support Ben Daking said some fruit is naturally more frost tolerant than others.

“Citrus will withstand lower temperatures than stone fruit, which withstands lower temperatures than grapes. “Within a single crop type there may also be differences - lemons are less resilient than oranges, some oranges might be more sensitive than mandarins - so accurately understanding the frost tolerance of your variety is important,” Mr Daking said.

Crop development and timing also play a role especially during colder months when growth is slow, plants naturally develop cold hardiness - slow water uptake and growth increase solute concentrations within the plant tissue which in turn retards ice formation.

Ambient conditions play a large role in fan effectiveness particularly the strength of the inversion. 

A strong inversion will generally allow a fan to cover more area and best results are achieved with frost fans that produces a wind-stream with great momentum, the product of air volume and speed.  

“Understanding your crop, measuring the conditions of your site, then overlaying measured in-field fan performance will produce the best frost fighting outcome,” he said.

Q: Does the slope of my ground affect my fan coverage

A: Without a doubt it does.

Frost fan fundamentals are generally well understood. The fan grabs warm air from the inversion and draws it down into the crop zone, raising temperature and displacing cold air to protect your crop.

“Some obvious factors affect the pattern of coverage including strength of nocturnal drift, obstructions to airflow, orientation of rows, size and density of canopy but most commonly overlooked, are ground contours surrounding the fan” Mr Daking said.

During a radiation frost, air from the inversion will be warmer than the air surrounding the crop. This means inversion air will be lighter or more buoyant than the air at ground level.

The frost fan produces a strong wind-stream to blow the warm buoyant air down to our crop, and the air stays down whilst the strength of the wind-stream is strong enough to overcome the effect of buoyancy. 

As the air moves further from the fan, the wind-stream naturally loses power until it isn’t enough to overcome the tendency for warm air to rise up and above the crop.

Universally frost fans have a blade mounted at 6ᵒ off the vertical plane. 

This angle has proven to provide the best air movement over distance into a crop across flat terrain. As a result, air from a frost fan will struggle reaching the crop if the ground slopes at a gradient greater than 6ᵒ.

“This is an important consideration across much of Australia. Many orchards have topography that exceed 6ᵒ which simply sees frost fan air wasted blowing straight over the crop or into the side of a hill. 

“Some sites see less than 50 per cent of the fan air reaching the crop, or fans placed in compromised/less than ideal positions. This can double the cost of fan protection per hectare.”

A number of manufacturers provide solutions. 

“Tilt heads” can be installed to follow topography with a constant gradient, “contour heads” follow complex ground shapes and are ideal for use when both sides of a sand dune or ridge require protection, “dog-leg towers” add even more angle for extreme ground shapes. 

Orchard-Rite is one manufacturer who has developed Tilt, Contour and Dog-leg towers to make sure every fan is providing a grower the best protection it can.

Make sure you get the most value from your frost fans and protect all of your crops by calling on your Orchard-Rite dealer technician today.