What are Frosts?

About Frosts

Frosts are cold weather conditions, of relatively short duration, during which organic material may be damaged by exposure to surrounding air temperatures lower than 0°C. Frost damage to agricultural crops continues to cost growers millions of dollars each year.

Heat Transfer

The earth absorbs heat from the sun during the day and releases it into the colder atmosphere at night. The heat loss is greatest just before dawn and this is usually when the maximum danger of frosts and frost damage occurs.

This nocturnal release of heat creates an 'inversion layer' of warmer air, which can be found from 10-50 metres above the ground.

During a radiation frost, a frost fan is used to draw down the warmer air in the inversion layer and blow it into the orchard or vineyard. The frost fan needs to blow as much air as it can, to the greatest distance possible. This will give the most economical coverage for the frost fan.

In order to reach the greatest distance, the fan needs to produce a strong, uniform wind. The distance the wind can penetrate into the orchard is proportional to the momentum of the wind. The momentum of the wind or wind momentum is the product of the wind flow rate, the wind speed and the air density.

Frost Fans

Frost fans have become the central element in most frost protection strategies. They use the warmer air in the inversion layer to protect a crop from frost damage. The frost fan is angled slightly downwards to pull this inversion layer down to ground level, to protect the crop from frost damage.

Frost fans are turned on before a frost occurs, usually 0.5 to 1.5°C above freezing. There is documented proof that turning on frost fans early and shutting them down late, results in considerably better crop protection performance. It is recommended that growers use this procedure, even when the temperature drops below freezing for several hours, and it may appear to be a 'lost cause'. Substantial crop salvage has been achieved with a “slow-freeze-slow-thaw” even under severe frost conditions.

In severe conditions, the use of orchard heaters in conjunction with frost fans strengthens the useable inversion layer which the frost fan then mixes and distributes over the crop, thereby providing better protection.

The initial expense of protecting a large area with frost fans can be significant, but the cost of capital, maintenance and operation is comparatively lower than other types of frost protection systems. Calculate the ROI on installing FrostBoss® C49 machine/s by running your numbers through the FrostBoss® ROI calculator.

Depending on the type of crop, geography, local climate and crop layout, a single frost fan will provide protection for about 6 - 8 hectares. Multiple frost fan installations will increase the effective coverage of individual fans.


Types of Frosts

Radiation Frost

  • only occurs at night
  • is caused by long wave radiation cooling from the earth to the sky
  • has calm, clear and dry atmospheric conditions
  • the coldest air is closest to the ground
  • temperature increases with height above the ground until the inversion cap is reached
  • the most common type of frost in spring and autumn
  • radiation frosts are far more common than advection frosts. Frost fans are an efficient and
  • effective method to protect against frost damage and cold injury during radiation frost events

Advection Frost

  • can occur during either the day or night
  • is caused by a large scale invasion of freezing air from the polar regions
  • is generally associated with moderate to strong winds
  • fully mixed atmospheric conditions
  • the air temperature decreases with height above the ground
  • the temperature of the entire air mass is well below 0°C
  • large scale protective action in horticulture is not economically feasible
  • generally only occur in winter

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06 879 8312

Mike Annand (NZ Sales):
+64 27 533 3343


New Zealand Frost Fans Limited
1429 Omahu Road
Hastings 4175
Hawke’s Bay