ABSTRACT
A process or method is described to successfully contain tropical cyclones by cloud-seeding technique using hygroscopic anhydrous alkaline earth salts that are affordable, abundant, non-toxic and environmentally benign unlike the expensive silver iodide which is toxic, water insoluble and hazardous. It recommends striking the cyclone near to but not into the eye-wall while it is at its infantile stages and is at the suitable height of 6000 to 9000 meters above the sea level. The cloud seeding may be done conventionally from the ground with rockets or with aircrafts.
INTRODUCTION
Tropical cyclones have been the scourge of the planet since time immemorial. They are characterized by the most intense storms on Earth. They cause huge damage to property, vegetation and loss of human life. Cyclones are also referred to as typhoons, tornados and hurricanes depending on the geographical area. In India the general term used is cyclone. Various natural factors such as solar energy, winds and the rotation of the earth cause cyclones.
A cyclone is a system of winds rotating counterclockwise in the Northern Hemisphere around a low pressure center. The swirling air rises and cools thereby creating clouds and precipitation. Contrary to this, the rotation of the cyclone in the Southern Hemisphere is clock-wise caused by the rotation of the earth. When a cyclone reaches landfall, it unleashes its
Typical tropical cyclone (Photo Credit: NOAA AOML)
Fury causing death and destruction in its trail. The recent cyclone Amphan (May-June 2020) that ravaged parts of Eastern India and Bangladesh which was shortly followed by another one Nisarga on the west coast of India, caused loss of life, livestock, heavy destruction of villages, towns, forests and fields. Monetary losses have been in billions in any currency.
In meteorology, a cyclone is a large scale air mass that rotates around a center of low atmospheric pressure and works like a tall chimney where all the energy comes into the base and gets sucked up to the top. A huge amount of energy gathers in one place which results in thunderstorms. The energy created from the thunderstorms can then add to the whole system. On an average about 300 tropical cyclones rage through the planet annually and with the current rate of increase in global warming this is likely to go up numerically as well as in intensity. Countries between the equator and the tropics having a coastline face the highest incidences of tropical cyclones.
Cyclone Nisarga over the west coast of India [Gujarat & Maharashtra] May-June 2020 (courtesy Indian Met Dept)
Cyclones generally start between 300 to 450 kms away from the equator (north or south) and move towards landfall. Sea temperatures of 26.5 degrees Celsius and more along with a group of other pre-existing conditions such as strong winds or thunder storms create ideal weather conditions for formation of a cyclone. For instance Hurricane Katrina, which hit New Orleans in 2005 was formed at sea surface temperatures around 32 0C. Temperatures of 310 to 32 0C were found off the West Australian coast in early 2011 when the cyclone Rusty was formed.
A new high pressure cell with a new surge of winds can also contribute to the formation of tropical cyclones.
In short, tropical cyclones can be caused above the tropical oceans by a combination of several natural factors such as;
(i) Sea surface temperature of 26.5 0C and above;
(ii) Pre-existing thunder storms and
(iii) Strong winds speeds due to the Coriolis Effect.
CYCLONE CONTAINING MEASURES
For over 200 years, numerous attempts have been made to devise measures to contain cyclones based on hypotheses and rationales postulated as scientific knowledge on their characteristics had advanced. The net result can however be summed up to say that every effort was a “scientific success and a practical failure”.
The most significant work in this field has been that of the Chemistry Nobel Laureate Prof Irving Langmuir and his associates. Their contributions on weather modifications by cloud seeding are outstanding. They had hypothesized that on cloud-seeding with silver iodide some latent heat would be released in the area around the eye-wall of the hurricane. This would promote formation of a new eye-wall which would be larger in size than the old eye-wall. Consequently the winds of the cyclone would be weakened due to a reduction in the pressure gradient which would decrease the wind speed. Since the damage potential of a storm increases with the square of the wind speed, even a small reduction in the speed of a cyclonic wind would have a significant reduction in its destructive power. As the results of the experimental attempts were inconclusive the hypothesis was abandoned discredited as a “scientific fiction”.
The above picture is from the NCRMP and shows the storm path of two cyclones that hit India’s coastline recently. (Courtesy NCRMP)
It is pertinent to mention that in these experiments silver iodide was used as the seeding agent the crystal structure of which is similar to that of ice. It was postulated that silver iodide would thus attract super-cooled water in the eye-wall of the cyclone and the adsorption being exothermic in nature it would facilitate precipitation. Subsequently, several studies have been done on the physical characteristics of cyclones and also on cloud-seeding numerous agents as an alternative to the silver iodide which is an expensive material and insoluble in water thus environmentally undesirable.
The present method to successfully contain tropical cyclones employs essentially the same concept i.e. the technique of cloud-seeding. It does not use silver iodide as the seeding agent but employs highly hygroscopic anhydrous alkaline-earth salts such as anhydrous calcium chloride and sodium chloride or a mixture thereof. It is significant to mention that such seeding agents are water soluble, non-toxic and environmentally benign.
The present proposal further recommends striking the cyclone from the ground or aircrafts with rockets near to but not into its eye-wall and while the cyclone is at the infantile stages and at 6000 to 9000 meters above the sea surface.
The most destructive element associated with a cyclone is the storm surge. Past history indicates that significant loss of life occurs when surge magnitude is 3 meters or above and when surge magnitude exceeds 5 meters it is catastrophic. Nonetheless India’s National Cyclone Risk Mitigation Project (NCRMP) has been in existence to undertake suitable structural and non-structural measures to mitigate the after-effects of cyclones in the coastal states and UTs of India. The main objective of the NCRMP is to reduce vulnerability of coastal communities to cyclone and other related hazards.
It is pertinent to remember that seeding of warm-season tropical clouds also exploits the latent heat released by freezing. This strategy would occur favorably on absorption of water molecules by hygroscopic materials and it can be safely assumed that the additional latent heat thus generated would add to buoyancy, would strengthen updrafts, ensure more low-level convergence, and ultimately would cause rapid growth of properly selected clouds. Such cloud seeding chemicals may also be dispersed in the conventional manner by aircraft or by devices located on the ground like rockets or canisters or fired from anti-aircraft guns.
METHODOLOGY
1. Strike the cyclone in its embryonic or infantile stage when it is weak and is growing in strength since to control the destructive potential of a full blown or mature cyclone is relatively difficult.
2. The attack should be with small rockets firing just below the dense clouds where the temperature is between -20 and -40 0C.
3. The ideal height for attacking the cyclone at infantile stage can be between 6000 to 9000 meters above the sea level.
4. The rockets should be fired in the same direction as that of the cyclonic whirl or spin and close to the eye of the cyclone so that the rotation of the cyclone will help in dispersal of the seeding agent that would create greater imbalance in the forces at work and reduce the pressure drop (ΔP) and temperature drop (ΔT) causing scattered rainfall at those heights.
5. As the cyclone starts when the surface temperature of the sea at the equator is about or above 26.5 0C and the pressure is about 1010 milibars, the water vapour rises upwards and simultaneously cools causing a pressure and temperature gradient. Ideally at the seeding stage the top pressure should be about 950 milibars or less and the temperature between – 20 and – 40 0C.
6. The seeding agent alkaline-earth hygroscopic Calcium Chloride and Sodium Chloride or their mixture to melt ice is cost effective, easily available, low toxicity environmentally benign, high reactivity at the optimal temperature that will not affect the pH or salinity of sea water.
7. Other hygroscopic chlorides as cloud seeding agents may be also used especially those suitable for tropical cyclones where temperature (ΔT) and pressure gradients (ΔP) are very critical in controlling them. The hygroscopic agent in cloud seeding operations disperses salts in the lower portions of clouds. The salt crystals grow in size as water joins with them. The exothermic heat of solution of the highly hygroscopic agent (e.g. anhydrous calcium chloride) will initiate drop in pressure (ΔP) as well as in temperature (ΔT) resulting in lesser rate of rise of water vapour into the atmosphere from the sea level and in an increased rainfall from the clouds circling the cyclone.
8. Normally, the pressure drop (ΔP) from the sea level to the clouds is about 50 to 60 milibars. As soon as the hygroscopic salt starts dissolving the super cooled water, the pressures will come down. Even a small pressure drop (ΔP) of 10 to 20 milibars together with increase in the rainfall will render the cyclone into a low intensity surge storm. By the time this low intensity cyclone touches landfall, the surge will be well below 3 or even 2 thus saving the area from massive destruction, loss of life and livestock.
9. The seeding operation must be rapid and continued till it appears that there will not be any runway rise in the surge during landfall. Efforts shall be taken to prevent the cyclone from maturing. It needs to be mentioned that the process will not avert cyclones from starting but it will slow down the infantile cyclone from maturing and reduce the intensity of landfall to a damp squib.
10. The cost of cloud seeding operation is miniscule compared to the destruction and devastation such uncontrolled runway cyclones could create. This new approach of cloud seeding with hygroscopic salts/ substances like anhydrous calcium chloride will succeed where the earlier experiments of seeding with silver iodide had failed. Although elimination of tropical cyclones appears remote but these can be effectively controlled so that the landfall doesn’t exceed surge of 1 or 2 meters.
IN RETROSPECT
This new modified approach to contain cyclones by cloud seeding will succeed where the earlier efforts were inconclusive. The seeding operation needs to be tried at various sites and under various conditions as the parameters influencing a tropical cyclone are multiple and often unpredictable. Tropical cyclones can’t be eliminated but they can be effectively controlled so that the landfall doesn’t exceed surge 1 or at most surge 2. The seeding process ought to be honed to perfection in order to get the best results. The long term benefits of controlling cyclones are immense in terms of preventing large scale death and devastation.
ACKNOWLEDGMENTS
The author gratefully acknowledges useful discussions with his mentors, teachers and colleagues Dr. B.B. Singh, Dr KP Madhavan, DrSudhir M Gadgil and Dr A. L Ravimohan
REFERENCES
1 https://www.jatinverma.org/national-cyclone-risk-mitigation-project-ncrmp
2. https://en.wikipedia.org/wiki/Project_Stormfury
3 https://vlab.ncep.noaa.gov/web/nws-heritage/-/almost-science-fiction-hurricane-modification-and-project-stormfury
4. https://en.wikipedia.org/wiki/Cloud_seeding>
