This is probably one of the most feared climate problems of farmers.
From one region to another, from one culture to another, the risk is extremely variable. Here for example, in the southwestern region of Andalusia, it does not freeze every year, and severe frosts, statistically, occur on average every 5 years. During the last quarter century, these are the years 1993, 1999, 2005, 2012 and 2016. Sometimes some alerts occur in the meantime, but nothing serious.
So this year, we are again in a problematic year. Specifically February 17, a short episode, since only one night, did terrible damage to all Spanish early areas on many crops, such as peach, apricot, almond, citrus, potatoes, etc.
This is not so much the strength of the frost which is responsible for the damage, but the great earlyness of vegetative cycles. The very mild winter (I told you recently about the lack of cold https://culturagriculture.blogspot.com.es/2016/01/65-bad-weather-4-lack-of-cold.html) caused a chaotic but extremely early vegetative start. These crops are therefore more sensitive than normal years, to face the same frost.
But February is not finished, and here historically, risks exist until March 10. It is therefore possible that other nights of frost occur.
Furthermore, each frost has unique characteristics. Concretely, this February 17, it was a polar air mass, accompanied by north wind, cold and very dry.
There are three main types of frost.
Frost by nocturnal radiation.
This phenomenon frequently occurs in temperate climates. The normal and permanent loss of energy from the soil, is counterbalanced by atmospheric radiation.
The conditions of cloud cover and air humidity will be paramount in these situations.
The more the sky is clear, and the air is dry, the more is high the frost risk.
In these situations, a thermal inversion ceiling is formed to a dozen meters above the ground. Cold air is denser, so it stays near the ground, causing damage to low crops, and in orchards, more damage below than above the trees.
Frost by evaporation.
This occurs if the sky clears late afternoon after rain, leaving wet plants, and the air is cold and dry overnight. The air causes an intense evaporation of water which is accompanied by a drop in temperature. The water contained in the plant tissues will also evaporate, causing an internal cooling.
It is thus possible to measure an internal temperature of the plant below the temperature of the air.
Frost by advection.
This type of gel is caused by the arrival of a cold and dry air mass, with a thickness of several hundred meters. It causes the evaporation of water from plants. The internal temperature of the plant is therefore very low. The sky is usually clear, which also increases the radiation.
In short, it is a combination of different types of gel, which exacerbates the negative effects.
The dew point is calculated by comparing the dry temperature with the wet temperature. As the dry temperature is higher than the wet temperature, the air continues to cool. This decline stabilizes when the air is saturated with moisture.
This is a very important factor for frost protection because it allows to know the intensity of risk and trigger temperature of protection depending on the system used and the sensitivity stage of the crop.
We often speak of white frost and black frost. A white frost is characterized by the formation of ice (frost) on plants, and shows that the air is saturated with water.
In contrast, we speak of black frost if there is no ice formation, showing that the air is not saturated with water. Black frosts are potentially more dangerous because the air causes more plant internal cooling, so more serious damage.
In short, spring frosts are a serious problem for agriculture, but there are means of protection.
Note immediately that whatever the type of protection, it represents a heavy investment that many farmers do not. Those who choose to do so, calculate it as a competitive investment. Indeed, in frost years, the overall production declines, market releases and prices go up, covering the investment.
This is the most used means in the world. It is also the means by which to fight against the most severe frosts.
It is to saturate the air with humidity, and cover the plant with a layer of protective ice constantly humidified. Its temperature stabilizes and prevents the internal temperature of the plant to drop. It stays at the temperature of ice.
The onset of protection is usually done before the stage of sensitivity of the plant because the startup will cause a general decline in temperature (hence the importance of monitoring the dew point).
This system is expensive to install, needs energy, and requires an important structure in pumping, piping, and especially a very high water availability, total and instantaneous.
Conventional systems use around 40 m3 of water per hectare per hour. A night of frost can apply a spray duration of 12 to 16 hours, sometimes more, or 500 to 650 m3 of water per hectare.
To overcome such drawbacks, many are working to find systems that use less water, but less effective, from 12 to 15 m3 of water per hectare per hour. This is a micro-sprinkler, based on much smaller droplets. This is effective but has some drawbacks. For example, in the famous night of 17 February 2016, the frost was accompanied by icy winds, causing drifting drops and a poor quality of the protection.
The years 2005 and 2012, here, have required around twenty nights of protection.
Under these conditions, a significant risk of sprinkling was asphyxiation caused to crops. In fact, in 2012, a neighboring farm was protecting its clementine orchards with sprinkler. But the farmer was forced to abandon the protection, given the length of the period of risk, not to lose the plantation.
Another disadvantage is the risk of breakage of branches or trees under the weight of ice.
The principle is to stir the air, so the hotter air located high (12 meters from the thermal inversion ceiling) is mixed with colder air near the ground. If necessary, this system can be combined with burners.
A variant can be carried out with helicopters, flying low.
This system is really effective in case of thermal inversion.
Here in Andalusia, usually dangerous frosts are advective frosts, for which this system does not work, since there is no thermal inversion ceiling.
This system is only effective if the conditions are right. It also represents a very high energy consumption.
This is the first system antifreeze ever used in agriculture, with fuel heaters. This system was abandoned because of its cost and the pollution it causes.
Now we use paraffin-based systems, whose calorific value is known and whose smoke production is very low. Paraffin being a by-product of refining, its cost is quite acceptable.
It simply consists in producing calories to compensate for temperature drop.
This is the system we use here. The effectiveness depends on the number of lit cans. The colder it is, the more cans you light.
This system however has two disadvantages, the need for night labor, and the risk of smoke. The ignition is manual. So you have to have staff available at any hour of the night during the risk period.
The risk of smoke depends mainly on the type of paraffin, but also the conditions of the freezing night. The wind reduces the effectiveness of protection while increasing the production of smoke, which is above all a sign of poor combustion.
These systems are called active protection, that is to say the intervention of the farmer.
But there are ways of so-called passive protection, that is to say the means and techniques to reduce the risk of damage.
Each plot, depending on its configuration or environment, will be more or less sensitive. We can cite :
- A marked slope will reduce the risk at the top, but can increase it down.
- To prevent the accumulation of cold in low areas, avoid planting hedgerows, woods, bushes or the construction of buildings in the places, in short of any obstacles that may prevent the cold to flow.
- The proximity of surface water bodies (lakes, ponds, rivers), also reduces the risk, especially if placed on the side of the usual arrival of the cold.
- The vegetal ground cover. This technique, very beneficial agronomically, increases the risk of frost. So it is at least necessary to make a short mowing at the approach of the cold.
- The soil moisture condition. Dry soil releases less energy than wet soil, thus increasing the risk.
- The choice of crops. Sensitive areas will be reserved for very sensitive or late crops, to reduce the risk of damage caused by the cold.
Finally, in some countries, like Spain, farmers have the opportunity to contract an insurance against freezing, allowing them to withstand these climatic disasters that can destroy a crop completely, and therefore the income of the farmer. But it does not exist everywhere.
Moreover, it must be said that this is just a solution for survival, the contracts do not cover all the needs. But they reduce the necessary debt in case of frost, to be able to go to the next campaign.
If you want more information, you can consult the article "Frost protection in orchards and vineyards", of the USDA http://www.sidney.ars.usda.gov/personnel/pdfs/Frost%20Protection%20in%20Orchards%20and%20Vineyards.pdfor the excellent and very well illustrated brochure of the Gard House of Agriculture, France (in French), one on the best available documents about this subject http://www.gard.chambagri.fr/fileadmin/Pub/CA30/Internet_CA30/Documents_Internet_CA30/Arbo/BrochureGel-basse_def.pdf