Frost protections are of little value in the event of an advection freeze when a large mass of dry, cold air suddenly moves into an orchard. The air is colder than the plants and not much can be done to protect them. When a mass of cold air moves after a warm day with no night-time cloud cover and little or no wind, the result is a radiation freeze; frost protection systems can help reduce loss of fruit during radiation freezing.
Plant fruit trees at least 50 feet above the floor of a valley in rows parallel to prevailing winds. Remove low hanging dense branches and prune the lower parts of windbreaks. Keep swales and other pathways open to eliminate pockets of frost.
Wind machines blow heavy, cold air away from trees so it can be replaced by inversion air, a layer of warmer air just above the frigid air on the ground. This will work with a radiation freeze but not an advection freeze when the air temperature is lower than the temperature of the trees. The tower machine has a blade that tilts to draw warm inversion air down to mix with cold air on the ground. Other stationary machines blow cold air from the ground upward to mix with the warmer inversion air. Mobile fans blow cold air out of the trees, allowing warmer air in; these are placed on the highest possible point. Helicopters can blow warmer air toward the ground, but they are extremely expensive to use.
Heaters are the only useful way to fruit trees during an advection freeze, but they are expensive to use. When the inversion layer is high most of the heat goes straight up and is wasted. A large number of small heaters are used to heat as much air as possible up to the inversion layer. Environmental concerns limit these heaters to those that run on liquified gas and oil-burning stack heaters.
When relatively warm water settles on trees, it gives up some heat to the colder foliage. Although it seems counterintuitive, as the water freezes it releases more energy to the trees. The problem is that too much ice can break limbs. Placing small sprinklers under trees helps reduce limb breakage.
Sprinklers are started before the temperature drops to freezing and continue until the danger of freezing has past. If the predicted dew point--that point in which water vapor becomes water--is 5 degrees F below the expected low temperature, sprinklers will cause cooling by evaporation and make the freezing damage worse. This is especially true of advection freezing. If protection from freezing is needed several nights in a row, the water may not have a chance to drain from the soil.
Burning pruning debris during a freeze can break the inversion layer, allowing more heat to escape into the atmosphere; this draws in cold air from surrounding areas. Fruit growers once burned tires in orchards on the mistaken belief that the smoke reflected heat back toward the ground. Smoke will not trap or reflect heat. It will only show the height of the inversion layer.