What are the active interventions in the atmosphere?

Actions to combat hail, early frost, fog and increase precipitation, also called active interventions in the atmosphere, refer to the intentional modification of the composition or dynamics of the atmosphere that take place over a certain area and period of time to achieve a certain objective.

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

1. Increase and uniformity of precipitation with the help of aviation

A modern rainfall enhancement program using aviation technology includes several important aspects:

  • climatological assessments
  • weather monitoring infrastructure
  • aircraft with highly trained pilots
  • telemetry systems, and
  • statistical and numerical modeling of seeding efficiency.

In order to initiate a project to increase precipitation, the frequency of potential cloud formations must first be studied, as well as specific methods for their recognition and location in real time.

Seeding agents, such as silver iodide (AgI), stimulate the formation of precipitation and are frequently used as condensation nuclei for orographic clouds in winter but also for convective clouds in summer. Aircraft are used to release cloud seeding agents and are equipped with wing and underbody mounted equipment. 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 on-board device receives data from an integrated GPS receiver and the firing systems of the smoke and/or ejectable pyrotechnic cartridge holders. 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 located in a command center and then retrieved by an application where it is viewed and interpreted along with the radar data. The most 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. Also, the information from the radars provides information about the effect of active interventions. Weather modification programs in the US, Israel, South Africa or Australia are mostly carried out by commercial companies to support the hydroelectric 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.

1.1 Convective cloud seeding

The concept of static seeding involves increasing the concentration of ice crystals and the 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 Improvement 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 Politovich1980; Orville 1996).

Most of these experiments were performed on semi-isolated congestus cumulus clouds, clouds to be able to confirm the causal relationships and fundamental effects of the microphysical processes inside the cloud. However, these clouds do not contribute significantly to the increase in 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 framework of the HIPLEX-1 project (experiments carried out in several states in the USA) starting from the hypothesis of seeding using ice crystals to develop acorns and measurements made at -6°C in clouds indicated high concentrations of aggregates (Cooper and Lawson 1984).

This was due to the short life of the HIPLEX-1 clouds and the rapid depletion of superheated liquid water, which is the main source of growth for the pea pods. The aggregates, on the other hand, had much lower fall velocities than the pea particles and evaporated without generating an increase in precipitation.

This result indicates that not all clouds can undergo seeding and that there are certain requirements. For the static seeding concept these requirements appear to be limited to continental areas where cold cloud top temperatures are roughly between -10°C and -20°C, and when significant amounts of superheated liquid water are available for growth of ice nuclei (Cooper and Lawson 1984).

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

1.2 Orographic cloud seeding in winter

Orographic cloud seeding increases precipitation under certain favorable conditions (American Meteorological Society 1992) and increases in snowpack may result.

An attempt was made to correlate the spatio-temporal evolution of the amount of liquid water in the cloud with the complexity of the terrain. The conclusions of the studies carried out by Super and Holroyd (1989) in Arizona were that liquid water is present in all the analyzed storms, but it is very variable in 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 of the 36 experiments. The results of the CLIMAX I and CLIMAX II experiments (Grant and Mielke 1967; Mielkeși et al. 1981), which were the most convincing evidence in the United States for increased winter precipitation.

Reanalyzed by Rangno and Hobbs (1987, 1993) these experiments indicate a possibility of an increase in precipitation of about 10%, which is significant, even if considerably less than that originally reported.

2. Fighting hail with the help of aviation

Ejectable and smoke cartridges with silver iodide are also used for anti-hail operations. In the case of this type of intervention, the injection of the reagent can be done both at the base of the conceptive cloud and in its upper area. Seeding at the base of the cloud is done in the updraft area so that the condensation nuclei can be picked up and carried to the cold area. Seeding is also done directly in the supercooled cloud zone.

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

  • changing the state of preparation of the crew in 15 minutes;
  • launching an aircraft into the storm area;
  • identifying the optimal seeding site and seeding techniques (top or base);
  • seeding is requested to begin;
  • a seeding rate based on storm cloud intensity is requested (rule of thumb: one smoke cartridge every 5 seconds and one ejectable cartridge every 4 minutes);
  • request to stop seeding;
  • the request for the aircraft to return to base.

Supercell cloud seen from an airplane seeding for hail control in North Dakota, 07/13/2019. Image source: