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Actions to combat hail, early frosts, fog and increased precipitation, also referred to as active atmospheric interventions, refer to intentional changes in the composition or dynamics of the atmosphere that take place over a given area and time period to achieve a specific objective.

These interventions can be local (an airport) or regional (a county or a physico-geographical unit). The time period could vary from a few days to several years. The objectives can be very diverse, including increasing rainfall, reducing and combating hailfall, dissipating fog or preventing early frosts. Certain substances, such as dry ice or silver iodide, can be used to seed clouds with aircraft, rockets or ground generators.


A modern rainfall enhancement programme using aviation technology includes several important aspects: climatological assessments, weather monitoring infrastructure, aircraft with highly trained pilots, telemetry systems and statistical and numerical modelling methods of seeding efficiency. In order to initiate a rainfall enhancement project, the frequency of potential cloud formations must first be studied, as well as specific methods to recognize and locate them in real time.

Seeding agents, such as silver iodide (AgI), stimulate precipitation formation and are frequently used as condensation nuclei for orographic clouds in winter and convective clouds in summer. Aircraft are used to release seeding agents into the clouds and are equipped with equipment mounted on the wings and lower fuselage. They can disperse hygroscopic (NaCl) or glaciogenic (such as AgI) condensation nuclei. There are two methods of dispersing condensation nuclei in clouds, namely by means of smoke cartridges or ejectable cartridges.

Each aircraft is equipped with an airborne data acquisition and telemetry system. The device on board the aircraft receives data from an integrated GPS receiver and from the firing systems of the smoke and/or ejectable pyrotechnic cartridge carriers. This system records the position of the aircraft (latitude, longitude, altitude and speed) throughout the flight. The telemetry information is transmitted to a receiving unit in a command centre and then retrieved by an application where it is visualised and interpreted together with the radar data. The most widely used application is TITAN (Thunderstorm Identification, Tracking, Analysis and Nowcasting).

Weather radars are also very important for detecting optimal seeding areas within clouds to which meteorologists can direct aircraft. Radar information also provides information on the effect of active interventions. Weather modification programmes in the US, Israel, South Africa or Australia are largely carried out by commercial companies to support the hydropower sector or snowmaking for ski resorts. China is unique in its use of weather modification technologies, with around 40,000 people involved in the sector. The Shanghai provincial administration has 37 aircraft equipped with AgI, dry ice and liquid nitrogen generators.


The concept of static seeding involves increasing the concentration of ice crystals and faster production of precipitation particles in cumulonimbus clouds. Experiments that used a combination of statistical and physical approaches are the Canadian studies by Isaac et al. (1977, 1982) and Marwitz (1981), the WMO Precipitation Enhancement Project (WMO 1986; Vali et al., 1988), the Australian experiments (Ryan and King 1997), the South African studies by Krauss et al. (1987) and others (e.g. Dye et al. 1976; Holroyd et al. 1978; Sax et al. 1979; Hobbs and Politovich 1980; Orville 1996).

Most of these experiments were performed on semi-isolated cumulus congestus clouds, clouds to be able to confirm causal relationships and fundamental effects of microphysical processes inside the cloud. However, these clouds do not contribute significantly to increasing the amount of precipitation on the ground.

Complex convective systems contribute significantly more than semi-isolated cumulus congestus clouds to surface precipitation in most regions where much of the annual precipitation is the result of convection. In the HIPLEX-1 project (experiments conducted in several US states) based on the seeding hypothesis using ice crystals to grow into peas and measurements made at -6°C in clouds showed high concentrations of aggregates (Cooper and Lawson 1984).

This was due to the short life of HIPLEX-1 clouds and the rapid drop of superheated liquid water, which is the main source of growth for peas. Aggregates, on the other hand, had much slower fall rates than pea particles and evaporated without generating an increase in precipitation.

This result indicates that not all clouds are amenable to seeding and that there are certain requirements. For the static seeding concept these requirements appear to be limited to continental areas where the temperatures of the upper part of cold clouds are approximately between -10°C and -20°C, and at times when significant amounts of superheated liquid water are available for ice core growth (Cooper and Lawson 1984) .

Israeli experiments (Gagin and Neumann 1981) have provided strong evidence that seeding continental cold clouds according to the static concept can cause significant increases in precipitation on the ground. Mather et al. (1996) reported promising results from cloud seeding experiments using dry ice in South Africa. Results from 127 storms analysed using radar data indicate that radar-measured rainfall fluxes were significantly higher in seeded clouds than in other clouds.


Seeding of orographic clouds increases precipitation under certain favourable conditions (American Meteorological Society 1992) and increases in snow cover may result.

An attempt was made to correlate the spatio-temporal evolution of the amount of liquid water in the cloud with terrain complexity. The conclusions of studies by Super and Holroyd (1989) in Arizona were that liquid water is present in all storms analyzed, but is highly variable over time. Its temporal and spatial variability causes problems in trying to seed it with reagents.

For example in the Sierra Nevada experiments (Deshler et al. 1990) positive seeding results were recorded in only 2 out of 36 experiments. The results of the CLIMAX I and CLIMAX II experiments (Grant and Mielke 1967; Mielke et al. 1981), which were the most convincing evidence in the United States for increased winter precipitation.

Reviewed by Rangno and Hobbs (1987, 1993) these experiments indicate a possible increase in rainfall of about 10%, which is significant, even if considerably less than originally reported.


Ejectable cartridges and silver iodide smoke grenades are also used to combat hail. In this type of intervention the reagent injection can be done both at the base of the concective cloud and in the upper area. Seeding at the base of the cloud is done in the updraft zone so that condensation nuclei can be picked up and carried to the cold zone. Seeding is also carried out directly in the overhead cloud area.

Interventions are carried out with one or more aircraft depending on the evolution of the storms. Weather radars monitor the state of the atmosphere 24 hours a day and when the first hail threatening cloud appears, meteorologists and air traffic controllers in the command centre make the following decisions:

  • change of crew readiness in 15 minutes;
  • launching an aircraft into the storm area;
  • identification of the optimal seeding site and seeding techniques (top or base);
  • the start of sowing is requested;
  • a seeding rate is required according to the intensity of the storm clouds (general rule of thumb: one smoke cartridge every 5 seconds and one ejectable cartridge every 4 minutes);
  • request to stop sowing;
  • request to return the aircraft to base.

Supercell cloud seen from an aircraft conducting hail control seeding in North Dakota, 07/13/2019.
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