A study for the effectiveness of cloud seeding in enhancing rainfall has been recently conducted under CAIPEEX phase-4 by the Indian Institute of Tropical Meteorology (IITM).
Key findings of the study:
- All cumulus clouds do not produce rainfall when cloud seeding is done.
- Cumulus clouds are detached, individual, cauliflower-shaped clouds usually spotted in fair weather conditions.
- 20-25% of cumulus clouds produce rainfall if cloud seeding is done correctly.
- The relative enhancement of rainfall was 46% as measured by automatic rain gauges.
- However, the actual increase in rainfall was only 18%.
- The cost of producing water through cloud seeding will drop by more than 50% if indigenous seeding aircraft are used.
About Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX phase-4):
- CAIPEEX phase-4 experiments were conducted by the Indian Institute of Tropical Meteorology (IITM), Pune.
- IITM is an autonomous institute under the Ministry of Earth Sciences (MoES).
- Objective: To investigate the efficacy of hygroscopic seeding in deep convective clouds and develop a cloud seeding protocol.
- Convective clouds are formed by convection, which is the process of warmer air rising since it is less dense than the surrounding atmosphere.
- It aims to understand the complexities of cloud behaviour, aerosol interactions, and precipitation enhancement.
- Tools employed: Two aircraft for cloud parameter study and seeding.
- Specific characteristics within a cloud, such as its liquid water content, vertical motion (indicative of its growth), and cloud depth can give an idea if it will rain or not.
- Convective clouds over 1 kilometer deep, are likely to evolve into deep cumulus clouds.
About Cloud seeding:
- Cloud seeding is a technique in which cloud-forming particles are used to increase rainfall.
- It is of two types: Hygroscopic and Glaciogenic cloud seeding.
| Hygroscopic Seeding | Glaciogenic Seeding | |
| Background | This seeding is done in warmconvective clouds with a cloudbase height of >0 degrees Celsius.It uses hygroscopic flares (e.g.,Calcium Chloride (CaCl2) particles)released at the convective cloudbase. | This seeding is done in cold cloudshaving both ice and water.It uses ice-nucleating silver iodide(AgI) particles inside clouds toenhance ice particle production andincrease rain from the cold part ofthe cloud. |
| Suitability | Hygroscopic seeding is suitable for the base of warm clouds with vertical velocity >1.5 ms-1 and liquid water content >0.5 gm-3 without rainfall.A warm cloud depth of 1kilometres or more duringseeding.Relative humidity in the 2-6 km layer should be > 60 %. | Glaciogenic seeding is suitable in the deep cumulus andtropospheric stratus clouds over a region, where water drops are present below 0 degrees Celsius.Supercooled liquid water content >0.05 gm-3 and cloud top vertical motions are present. |
| Flares | Four flares are burnt at the cloud base containing Calcium Chloride (CaCl2), encased in 12 cm long and 7 cm wide tubes, which produce alarge concentration of CaCl2aerosols near the cloud base.These particles have a highcapability to form cloud droplets. | One flare per cloud is burned at the cloud top containing silver iodide (AgI) particles encased in thin tubes released within stratiform clouds or ejected inside convectiveclouds. |
| Seed | The seed particles containingCaCl2 is released by burningflares by the aircraft directly at the cloud base. | The ejectable flares containing AgI are dropped from the cloud tops by aircraft. |
About Hygroscopic Seeding:
[Ref – The Times of India]
- Hygroscopic seeding is a weather modification technique that involves dispersing hygroscopic (water-absorbing) materials into clouds or the atmosphere.
- Hygroscopic materials include salts or compounds that encourage the formation of rain or snow.
- The seeding agents used in hygroscopic cloud seeding serve as efficient cloud condensation nuclei (CCN) or GCCN.
- They play a crucial role in strengthening the condensation and collision–coalescence process, thereby widening the droplet size distribution (DSD) and increasing the precipitation efficiency.
- Calcium chloride flare is used for seeding at the base of warm convective clouds during their growing stage.
- Convective cloud bases vary from 500-1,500 meters in the summer-monsoon season to 2,000 meters or higher during monsoon breaks, depending on lower atmospheric moisture.
Ref: Source
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