Cloud seeding is a method of enabling inefficient clouds to bring rain, or enhance rainfall from a seedable cloud by introducing seeding material. The secondary effect of evaporation from falling precipitation improves the thermodynamic efficiency of the environment to form fresh convective clouds and more rain. This principle is used to enhance rainfall and reduce pollution. Two types of seeding—‘ground’, using generators and rockets, and, ‘aerial’ seeding using aircraft are possible. Based on the type of cloud to be seeded (warm or cold) to enhance precipitation or snow; hygroscopic seeding using salt as spray from the base of a cloud and cold cloud seeding using silver iodide as spray from the top of the cloud may be deployed.
Need for seeding
The increase in rainfall due to cloud seeding can depend on many factors such as cloud size, liquid water content of the cloud and prevailing atmospheric conditions. Seeding is done on individual cloud cells, and sometimes, on clusters of clouds. The availability of suitable clouds depends on weather conditions. If more suitable clouds are available, a lot more area is covered. Cloud seeding however, cannot mitigate drought conditions. But, timely rainfall produced through cloud seeding, even in small quantities can help the survival of crops in stressed conditions. Similarly, cloud seeding if carried out in a targeted manner over catchment areas have been found useful in increasing the water level in dams.
As cloud seeding involves increasing the efficiency of conversion of cloud water to raindrops within a cloud, it has no effect on adjacent clouds. Thus, there is no decrease in rainfall in the vicinity. Ground based and rocket methods for seeding warm clouds are not very effective as there is no guarantee that the particles produced will reach the updraft regions of the clouds.
The World Meteorological Organisation (WMO) recommends such experiments under the guidance of experts and a weather modification committee that formulates protocols for weather modification. However, this differs for different geographical regions. The success of the programme increases if the recommended WMO protocol, along with local protocol, is followed. In India, such programmes need to be planned for the entire monsoon season, so that there can be ample seeding opportunities and crops can get timely water for growth.
Seeding operations need a weather radar, aircraft with telemetry and a computer with storm track TITAN software. The different components of cloud seeding include:
C-band radar: It monitors clouds in the area of seeding and 100 km around.
Seeder and research aircraft: Seeder aircraft seeds the cloud base with the help of flares on racks positioned on the wings of the aircraft. These flares are ignited and the emissions produce hygroscopic particles, which are injected at regions below the cloud base where there are strong vertical motions. Once the seeded plume gets inside the cloud and grows, the research aircraft uses several instruments to measure the size, mass, number of cloud droplets and raindrops. The radar monitors the overall cloud properties and the associated dynamics of the cloud. The cloud is then tracked and all associated properties evaluated. There are additional ground based rainfall monitoring gauge networks to register the rainfall received in the area and to calibrate and validate the radar observations. The infrastructure is illustrated in figure 1.
Methodology of cloud seeding
Aerosol particles constitute the seeds for formation of cloud droplets in the lower atmosphere. This nucleation of cloud particles depends on the size and chemical characteristics of aerosol particles suspended in the atmosphere. Depending on the composition of the aerosols, such as dust/carbonaceous particles or sulphates, the hygroscopicity (the ability of water vapour in the atmosphere to condense on the aerosol particle) to form cloud condensation nuclei may change.
The ice forming capacity of aerosols depends on their crystalline nature. Cloud seeding involves using either silver iodide or other hygroscopic particles for being dispensed on top or at the cloud base. The hygroscopic particles dispensed can change the characteristics of the warm or cold (below zero degree temperature) region of the cloud. The particles thus provide cloud condensation nuclei and grow by condensation of water on a cloud droplet. Ultimately, they grow in size, by collecting more droplets to form rain drops, few millimetre size, that fall as precipitation (Fig. 2).
Several state governments, Maharashtra, Karnataka, Andhra Pradesh, in India have been conducting operational cloud seeding to address water scarcity. However, there are no set protocols available for operational seeding, and the consensus is based on results from experiments conducted by the Indian Institute of Tropical Meteorology (IITM) in 1970’s. However, particulates and their composition have changed as a result of increased anthropogenic activities. Hence, the impact on clouds and precipitation needs to be investigated using modern technologies on airborne instrumentation and newer, and more accurate protocols need to be formulated for cloud seeding.
The Maharashtra SDMA and experts from USA used C-Band Doppler radar to detect developing clouds within a range of 100 km through an installation in Aurangabad. The data was generated every 7½ minutes and stored automatically. The radar images were diagnosed with high quality software available within the system, to help meteorologist determine the seed-able section of the cloud. Two types of seeding were attempted during the course of this experiment—hygroscopic, in the warm cloud zone; and, glaciogenic, in the colder region where temperatures were around -5oC to -10oC.
Subsequently, village locations along the latitude and longitude of seeding points were marked and rainfall collected through rain gauges in the circle/tehsil headquarters the following day. The data collected was uploaded and later picked up by the Maharashtra Remote Sensing Application Centre (MRSAC), Nagpur, for contour analysis.
Results from other recent experiments
The constraint with operational cloud seeding is that it is not possible to separate the seeded precipitation with natural rainfall. This emphasises the need for scientific evaluation of any seeding experiment.
- Analysis of three years of operational seeding results (2006-08) by the Andhra Pradesh Rain Shadow Areas Development Department based on Radar studies showed initiation of drizzle/precipitation within 15 minutes of seeding in most cases followed by reinvigoration of convective clouds and heavier precipitation. Monitoring of this cloud-seeding experiment was entrusted to the JNTUH, Hyderabad.
- Similar results were obtained in the Hindustan Zinc Limited project in Rajasthan, and, Cauvery Basin project in Karnataka.
- Response to seeding is more in small to medium life cells having higher first echo depths, though precipitation contribution is more from medium life cells by virtue of longer lifetime, larger P-mass and higher frequency of occurrence.
- Approximately 16 per cent of the total rainfall in the control area under consideration is estimated as a result of seeding..
The CAIPEEX initiative
Despite the success of many of these privately undertaken projects, the scientific rigour of these experiments cannot be readily ascertained. Towards building a holistic capacity on cloud aerosols, MoES has recently launched the CAIPEEX. This is India’s first national experiment to study clouds, aerosols and precipitation processes, and seeks,
- to address the physics and dynamics of aerosol-cloud-precipitation interactions; and,
- to formulate a scientific basis for rain formation and rain enhancement using the recent cloud seeding technologies. CAIPEEX seeks to look into the basis for cloud droplets and rain drop formation in monsoon clouds and how aerosol pollution influences these processes.
The first three phases of this ongoing experiment were completed between 2009 and 2015 with a focus on the aerosol-cloud-precipitation interactions in pre-monsoon and monsoon clouds. Under CAIPEEX, vertical profiling of aerosol and cloud microphysical properties, greenhouse gases, black carbon aerosols, chemical and physical characterisation of aerosols was carried out for the first time ever over different parts of India.
Cloud seeding has the potential to provide a much needed respite for farmers and communities badly affected by climate change and unforeseen drought. However, there was no set protocol that could be followed under Indian conditions. Several states affected by water scarcity conducted independent experiments on the basis of IITM’s findings. CAIPEEX, India’s first-ever nationwide experiment in cloud seeding, is all set to bring in a welcome change, and has already gone on to prove cloud seeding as a successful geoengineering technology.