Hail is a natural phenomenon of meteorological origin that represents a threat to different productive sectors. However, due to its potential to affect plantations and crops, the agricultural sector is more vulnerable to this phenomenon. That is why, for several decades, efforts have been concentrated on understanding it physically and establishing actions that mitigate its impacts.

* This article was published in Geociências SURA Magazine | Edition 4 | December 2018.

How is hail formed?

There are several natural phenomena of meteorological origin that have a great potential to affect human lives and goods: storms, tornadoes, atmospheric discharges, wind gusts, torrential rain showers, hail showers, etc.

All these phenomena originate in electrical storms, which are systems with spatial scales that can vary from a few units to hundreds of kilometers and time scales that vary from a few minutes to several hours. 

Climate systems characterized by these spatial-temporal conditions are known, in meteorology, as mesoscale systems and are formed by masses of cumulonimbus clouds, popularly called “storm clouds.”

Cumulonimbus is the type of cloud with the highest altitude that can be seen in the atmosphere. They are convective clouds, dense and powerful, which differ from other types of clouds, because their formation dynamics make them present structures of great vertical development, being able to reach heights of up to 22 km above the surface of the Earth, mainly in the tropics.

Its formation occurs when there is a high temperature gradient between the terrestrial or oceanic surface and the upper layers of the atmosphere, which results in an increase in the mass of heat that increases at the height of the cloud column, generating a process known as “convection”. 

Cumulonimbus formation generally occurs in regions with high topographic reliefOnce the orographic effects are applied, we favor the convective processes that give rise to this type of system and, once formed, we can travel, depending on the speed and direction of the winds, in the direction of flat regions, focusing on coastal areas, as explained by Dr. Pablo Alberto Mercuri, director of the Natural Resources Research Center of the National Institute of Agricultural Technology of Argentina (INTA). 

It is important to note that, although the phenomenon is directly associated with cumulonimbus, the formation of these types of clouds does not necessarily imply the existence of hail formation processes. Stanley A. Changnon estimated, for example, that only 60% of thunderstorms can give rise to the phenomenon.

Also said, Just as a cloud of cumulonimbus has conditions that guarantee the formation of hail, it is possible that it never reaches the surface of the Earth.However, it can only come into contact with concentrated fields of liquid and go from a solid to a liquid state, causing precipitation to occur in the form of a chuva. This is why thunderstorms rarely produce hail in regions with cold climates. 

On the other hand, in relation to the proportion between solid and liquid precipitation that a cumulonimbus can generate, Gokhale, in 1975, estimated in his studies that the volume of hail that reaches the surface of the Earth is less than 10% of the volume of hail produced by An electrical storm whose precipitation occurs at an approximate speed of 150 km/h.

Hail precipitation occurs at a speed of approximately 150 km/h.

 

Hail as an agricultural hazard

Hail can have negative impacts on property, such as houses, cars, boats, aircraft and, occasionally, can be dangerous to livestock and, exceptionally, to human life. However, we can affirm that the sector is most vulnerable to this type of natural or agricultural phenomenon. 

The impact of hail in the agricultural sector can be verified in the case of Argentina, where the majority of agricultural insurance billing corresponds to protection against hail (95%) and, less expressively, to multirisk insurance, which is linked to the development of comprehensive risk policies, as expressed by María Fernanda Muñoz, vice-manager of agricultural risks at Seguros SURA Argentina.

Generally, the damage that hail can cause and its properties depends mainly on the characteristics of the event, such as the size of the hail, the angle of precipitation, the speed of the winds, the amount of hail precipitated per unit area, and the characteristics of the targeted area. However, specifically not in the case of the agricultural sector, crop damage is associated with two additional factors: the type and the cultivation phase. 

There are some types of crops that may be more sensitive to hail than others. For example, chai and tobacco crops can suffer from the occurrence of small hail events, while other crops, such as milho, are a little more resistant and are only affected by hail larger than 19 mm (Bal SK, 2014). 

Another determining factor is the possibility of damage caused by hail that a crop can endure in the phase in which it is affected by this phenomenon. A storm can cause minimal damage at the beginning of a specific crop, but the same storm can cause significant damage if it occurs in the intermediate phase of planting.

The intensity of hail in a given geographical region can be defined as a combination of four main factors that explain the severity of the phenomenon:

Event frequency: corresponds to the number of times that hail events have been recorded in a specific geographic region. 

Average hail size: The size of hail is directly related to the damage it can cause. The larger the hydrometeor, the greater the damage caused.

Number of precipitated hydrometeors: Studies show that, in general, the more frequent the hail storms are at a given point, the greater the hydrometeors and the number of precipitated elements per unit area.

Wind speed during hail storm events: The angle of hail depends on the size of the particles and the speed of the wind. At a larger angle of view in relation to the vertical, storms can cause more damage, since they can affect surfaces that would not be affected in a different situation.

What are the protective measures against hail?

Due to the high impact that hail exerts on the agricultural sector, there is difficulty in anticipating the occurrence of this phenomenon to operate in such a way that it is possible to mitigate its effects, Various techniques have been developed to reduce the damage that hail precipitation causes to crops. 

Some of these techniques are linked to artificial modification of the atmospheric conditions that give rise to hail, while others focus on structural measures to protect crops: 

Spraying or seeding of clouds: It is based on the modification of cloud microphysics that results in the formation and precipitation of hail. It consists of introducing a substance into the storm that has already formed (by means of aircraft, ground generators or anti-hail fires), so that other crystals of ice, which also compete with excess water in clouds, are formed around these particles.

It produces smaller ice particles that, in case of precipitation, have a greater probability of melting when they come into contact with the colder air, becoming cold. Esses artificial ice cores Generally it uses carbon dioxide, but it can also be used potassium iodine or solid carbon dioxide (dry gel).

Cloud formation can also be done in the initial phase of the convective system, and is intended to force chuva to fall prematurely, so that the potential for ice formation is not exhausted.

In Argentina, this mitigation system has been used in the province of Mendoza for several years and is part of a comprehensive plan to combat hail, motivated by the large agricultural presence in this region and the high level of formation and precipitation of considerably large hail, which affects The vinhedos and prejudices to the quality of the grapes.

Anti-hail fabrics: only protective fabrics with high mechanical resistance and generally made of high-density polyethylene. These fabrics withstand the impact of the weight of precipitated hail, but, in some cases, they can modify the microclimate of the crop, a factor that must be considered to make its installation viable.

Tree planting: The planting of trees adjacent to crops is used to intercept hail and help reduce wind speed and hydrometeor impact on the surface.

What are the hail measurement and detection systems?

Satellites 

Through the use of satellites, information is obtained about the meteorological conditions in which severe storms, with the potential to produce hail, develop. These sensors allow atmospheric variations precursors of the phenomenon to be measured. 

Weather radar 

Radar measures distances using electromagnetic waves. It is based on the measurement of the time that the wave emitted by the radar itself takes, once reflected by the particle, to return. Most radars do not detect hail directly. Its presence must be inferred by techniques or criteria that use data obtained from radar and, in other cases, these measurements must be combined with data from probes or numerical models.

Disdrometers 

They are used to measure all types of precipitation, recording the size, quantity, diameter and speed of fall of the hydrometeor. From this information, it is possible to calculate the reflection factor and the precipitation rate.

Hail gauges 

Rigid foam plate where hail impacts are marked. From the information on this plate, it is possible to know the diameter, density (amount of hail that fell), impact intensity (kinetic energy) and/or total volume of ice that fell in the area.

Sources

• Juan Pablo Restrepo. Civil engineer and specialist in Hydraulic Resources for the National University of Colombia.
• Luisa Fernanda Vallejo. Civil Engineering of the Antioquia Engineering School and Master in Hydraulic Resources of the National University of Colombia.
• Maria Fernanda Munoz. An agricultural engineer for the National University of La Plata and a specialist in agribusiness for the University of San Andrés.
• Pablo Alberto Mercuri. Agricultural Production Engineer at the Argentine Catholic University, master in teledetection agricultural applications and professor in agricultural and biological engineering at Purdue University.