(US Navy)
On the afternoon of October 13, 1947, an Air Force B-17 aircraft penetrated a hurricane 415 miles (667 km) east of Jacksonville and dumped several pounds of crushed dry ice into the storm, just to see what would happen. This was the first attempt to modify a tropical cyclone by seeding it with freezing nuclei. It was almost the last.
The previous year, Vincent Schaefer working at General Electric (GE) Laboratories discovered that by introducing dry ice (solid carbon dioxide) into an environment with supercooled water (water that was colder than 32°F [0°C] but had not yet frozen) he could induce the water to freeze into ice. He realized that this could be done in the free atmosphere and could spur microphysical reactions that would alter clouds and precipitation. At his urging, GE Labs reached an agreement with the the Naval Research Laboratory and Army Signal Corps (dubbed Project Cirrus) to carry out experiments where Air Force aircraft would ‘seed’ clouds and fog with dry ice to see what changes occurred. It wasn’t long before plans were made to try a seeding experiment in a hurricane.
However, it wasn’t until late in the following hurricane season that Air Force planes and Navy personnel became available to carry out the mission. On Oct. 12th, a hurricane (designated “King” by the Air Force Hurricane Office) moved northeastward over Florida, dumping tropical downpours on the southern end which caused damaging flooding that persisted for days. The next day the storm moved off the coast and continued out to sea. This seemed an ideal case for Project Cirrus, as the hurricane was no longer interacting with land and should not affect anyone after the experiment. That afternoon, two B-17s and a B-29 left Mobile, AL and flew eastward to intercept the hurricane. The first bomber flew at cloud top level and did the seeding. Large chunks of dry ice were fed into a crusher on the plane and dumped from the belly into the clouds below. The second B-17 followed a half-mile behind and 3000 feet (900 m) above to document the cloud changes. The trailing B-26 was the control aircraft and carried Schaefer to monitor the changes and direct the other two planes. The aircraft lacked sophisticated homing gear, so it was decided not to penetrate the eye or heavy rainbands of the hurricane, but seed the outer clouds.
They first made a half-hour run over 100 miles (175 km) long dumping 80 pounds (36 kg) of ice along the way. They backtracked then to see what the clouds had done. Next they did two mass droppings of 50 pounds (23 kg) each into one large cumulus top and orbited the cloud to see any changes. They noticed that after the first run, the cloud deck below began to break up. After the second test, the cloud top continued to grow. Satisfied with their effort, the airplanes returned to base.
The scientists were eager to examine the storm the following day. However, when they flew to the predicted storm location, they had trouble locating the eye. After some hunting around, they found the hurricane center nearly 100 miles (160 km) west of where they expected it to be. To their astonishment, the hurricane had made a 135 degree left turn and was now moving due west. On top of that, it was strengthening! By the afternoon of the 15th, Hurricane King struck Savannah, GA. One person died in the storm surge and US$2 million in damage was done to Georgia and South Carolina.
The public was outraged that the scientists had caused the storm to swerve into Georgia and threats of lawsuits were thrown about. GE’s case was not helped when the head of its Laboratories, Dr. Irving Langmuir, issued a statement that he was “99% sure” the storm had changed course due to the seeding. Chief of the Weather Bureau, Dr. Francis Reichelderfer, thought differently and appointed three of his weathermen to find a case where a hurricane had followed a similar track but had not been seeded. The case was published, demonstrating that hurricanes could swerve like that without the used of dry ice, and the threats of lawsuits eventually evaporated.
But the public’s early enthusiasm for weather modification slackened. In an era when many science fiction movies featured mad scientists threatening world destruction (or worse) from their hubris, this event seemed to fit the trope. For many years after, no scientist dared mention ‘weather modification’ and ‘hurricane’ in the same sentence. Eleven years later, the National Hurricane Research Project carried out very modest seeding equipment tests in a hurricane, but kept things on the “down low” until they were sure the storm wasn’t going to pull a swerve on them. It wasn’t until 1962 that the U.S. Weather Bureau and Department of Defense reached a formal agreement to carry out Project STORMFURY, and attempted to seed hurricanes again.
It was an ambitious experimental program of research on hurricane modification carried out between 1962 and 1983. The proposed modification technique involved artificial stimulation of convection outside the eyewall through seeding with silver iodide. The invigorated convection, it was argued, would compete with the original eyewall, lead to the reformation of the eyewall at larger radius, and thus, through partial conservation of angular momentum, produce a decrease in the strongest winds.
Dado que el potencial destructivo de un huracán aumenta rápidamente a medida que sus vientos más fuertes se hacen más fuertes, una reducción tan pequeña como el 10% habría valido la pena. La modificación se intentó en cuatro huracanes en ocho días diferentes. En cuatro de esos días, los vientos disminuyeron entre un 10 y un 30%, La falta de respuesta en los otros días se interpretó como el resultado de una ejecución defectuosa de la siembra o de sujetos mal seleccionados.
Estos resultados prometedores se pusieron en duda a mediados de los años ochenta porque las observaciones en los huracanes sin modificar indicaron:
- Esa siembra de nubes tenía pocas perspectivas de éxito porque los huracanes contenían demasiado hielo natural y muy poca agua superenfriada.
- Que los resultados positivos inferidos de los experimentos de siembra en la década de 1960 provenían de la incapacidad de discriminar entre los resultados esperados de la intervención humana y el comportamiento natural de los huracanes.
Durante un par de décadas, la NOAA y su predecesor intentaron debilitar los huracanes dejando caer yoduro de plata - una sustancia que sirve como núcleo de hielo efectivo - en las bandas de lluvia de las tormentas. Durante los años de la TORMENTA, los científicos sembraron nubes en los huracanes Esther (1961), Beulah (1963), Debbie (1969) y Ginger (1971). Los experimentos tuvieron lugar sobre el Atlántico abierto, lejos de la tierra. La siembra de la TORMENTA apuntó a las nubes de convección justo fuera de la pared ocular del huracán en un intento de formar un nuevo anillo de nubes que, con suerte, competiría con la circulación natural de la tormenta y la debilitaría. La idea era que el yoduro de plata mejoraría las tormentas de una banda de lluvia haciendo que el agua superenfriada se congelara, liberando así el calor latente de fusión y ayudando a una banda de lluvia a crecer a expensas de la pared ocular. Con una convergencia debilitada en la pared ocular, los fuertes vientos del núcleo interno también se debilitarían bastante. Para que la siembra de nubes tenga éxito, las nubes deben contener suficiente agua superenfriada (agua que ha permanecido líquida a temperaturas inferiores al punto de congelación, 0°C/32°F). Una idea genial, pero al final tenía un defecto fatal. Las observaciones realizadas en el decenio de 1980 demostraron que la mayoría de los huracanes no tienen suficiente agua superenfriada para que funcione la siembra de TORMENTAS - la flotabilidad en la convección de los huracanes es bastante pequeña y las corrientes ascendentes son correspondientemente pequeñas en comparación con el tipo que se observaría en las supercélulas o multicélulas continentales de latitud media.
In addition, it was found that unseeded hurricanes form natural outer eyewalls just as the STORMFURY scientists expected seeded ones to do. This phenomenon makes it almost impossible to separate the effect (if any) of seeding from natural changes. The few times that they did seed and saw a reduction in intensity was undoubtedly due to what is now called “concentric eyewall cycles.” Thus nature accomplishes what NOAA had hoped to do artificially. No wonder the first few experiments were thought to be successes. Because the results of seeding experiments were so inconclusive, STORMFURY was discontinued. The primary focus of NOAA’s Hurricane Research Division today is better physical understanding of hurricanes and improvement of forecasts.
La mejor manera de minimizar los daños de los huracanes es aprender a coexistir con ellos. Los códigos de construcción adecuados y la comprensión de la asunción de riesgos al elegir vivir en una zona propensa a los huracanes pueden ayudar a la gente a evaluar su situación. La preparación inteligente para los huracanes y la educación pública, junto con una mejor previsión pueden ayudar cuando un huracán inevitablemente toca tierra.
For more information about hurricane mitigation efforts, visit our hurricane FAQ.
Referencias
Barrington S. Havens, History of Project Cirrus Report No. RL-756 (Schenectady, NY: Research Publication Services, July 1952), p. 62-63.
Conrad Mook, Eugene Hoover, and Robert Hoover, “An Analysis of the Movement of a Hurricane off the East Coast of the United States, October 12-14, 1947,” Monthly Weather Review (July 1957): 243-250.
P.S. Thanks to David Reade for some corrections to this post