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The War Against Hail
June 1947 Popular Science

June 1941 Popular Science
June 1941 Science Popular Science - RF Cafe[Table of Contents]

Wax nostalgic over early technology. See articles from Popular Science, published 1872 - 2021. All copyrights are hereby acknowledged.


The War Against Hail

Advancing from man-made snow to man-made weather, science and the AAF are ready to battle costly storms that batter U.S.

Painting by Eric Sloane

By Martin Mann

The morning of Aug. 18, 1925, was hot and muggy in the flat farm lands of southeastern Iowa. The green, tall corn, almost ready to husk, grew so fast you could almost hear it. Off to the west, heavy thunderclouds promised a cool evening. In-stead they brought disaster that even a city man could feel in his heart and pocket.

Midmorning, it hailed.

A half hour later the corn was only a worthless mush pounded into the ground. Poultry and livestock had been killed, windows broken, roofs smashed, automobiles damaged. Those terrible 30 minutes cost the local farmers $2,500,000.

Hailstones as big as this 1940 Oklahoma sample, riding 100-mile-an-hour winds, can kill cattle.

This one storm was not the worst. Between June 7 and 20, 1928, hailstorm after hailstorm bombarded Kansas. The loss: $11,200,000. Only last year, on May 16, a four-minute blast of hail swept San Antonio. That evening's bill was $5,030,000.

But soon the tribute paid to hail - totaling almost $60,000,000 in 1944 - may be saved for better purposes. The war against hail has been declared. This summer, the Army and Navy will try to convert brutal hail-storms into gentle showers of rain.

Airplanes of the AAF Weather Equipment Flight Detachment will scatter silver iodide smoke or dry-ice pellets through the huge cumulo-nimbus thunderclouds that often produce hail. This "seeding" is expected to change the clouds to snow, which will melt to rain before reaching the ground.

According to Vincent J. Schaefer, the General Electric Company scientist who was the first man to start a snowstorm (PSM, Jan. '47, p. 78), the seeding might also be done by sending up rockets or drone planes equipped with automatic smoke or dry-ice dispensers, or setting up portable smoke generators on the ground so that the strong updrafts generally accompanying hailstorms would suck the smoke seeds into the clouds.

Set for take-off on a weather-control mission, Capt. C. N. Chamberlain, Jr., waves from his specially equipped B-25. He is a pilot in the Weather Equipment Flight Detachment of the AAF.

Aerographs on the side of this weather plane measure temperature and humidity, recording them automatically. The wet- and dry-bulb thermometers are used to calibrate the aerograph readings.

Electric aerograph's thermistor measures temperature; the lithium chloride unit, humidity. The screen filters out dust and liquid moisture. A mechanical aerograph has a human hair to tell humidity, a bimetal strip for temperature.

On a test flight, the weather observer must record altitude, airspeed, course and other information. The radar altimeter provides a reading of actual height above the earth for comparison with altitude shown by the pressure altimeter.

The clouds from which hail falls are tall, turbulent pillars of supercooled water vapor (tiny drops of water that are colder than the freezing temperature, 32° F., but have not yet frozen). The droplets are caught in the strong updrafts and thrown to cold, high altitudes. There they freeze, then plunge down, and grow in size as more moisture freezes on them. The updrafts may whip them up repeatedly and cause them to acquire other shells of ice as they fall again. A hailstone 17 inches in circumference fell at Potter, Nebr., on July 6, 1928!

Such a natural ferris wheel could be stopped by removing all the water at once. Silver iodide crystals trick droplets into freezing on them; iodide crystals look just like ice crystals, and water vapor can't tell them apart. It freezes around them just as it does around an ice crystal. Dry ice works differently. It produces huge quantities of submicroscopic ice nuclei, each containing only a few molecules. This crystallization process releases heat that causes turbulence, spreading the ice crystals throughout the cloud. They then act as natural seeds and change the entire cloud to snow.

In both cases all of the cloud is converted into snow particles, leaving no supercooled moisture to form huge hailstones. Either method is cheap: one pea-sized pellet of dry ice could conceivably produce 300,000 tons of snow, and 200 pounds of silver iodide would seed the whole United States.

Army Seeks Ice-free Air

Basically, seeding "triggers" a supercooled cloud, making it give up moisture before it naturally would. Since hailstorms are not the only kind of weather born in supercooled clouds, this method has other valuable uses.

It could eliminate the supercooled clouds that cause icing over airfields. This is what the Army is now trying to do.

It could stop the snowstorms that come in from the Great Lakes every winter, paralyzing Buffalo and choking its vital rail yards. The snow clouds could be triggered while still over the lakes.

It might make rain in some deserts - possibly convert them to fertile farm lands. Supercooled clouds frequently pass over desert areas only to evaporate into dry air.

Its military possibilities are almost fantastic. Snow and rain forced at strategic points could cripple communications. Rain clouds might be triggered before they reached "breadbasket" regions, and drought and famine would soon cripple the foe.

Dry ice is poured into this distributing device, mounted in an airplane's floor, to seed a cloud.

Schaefer, the first man to make snow, dashed off this rough sketch of his dry-ice dispenser for PSM. Even this gadget isn't really necessary, according to Dr. Langmuir. He says the dry ice could be sprinkled out of an ice-cream box.

Although changing a cloud into snow is fairly simple (on p. 88 you can see how to do it at home), it took lots of hard work and brilliant reasoning to find that out. The research can be traced back more than 20 years to the famous small-particle theory of Dr. Irving Langmuir, a Nobel prize winner who is associate director of the GE Research Laboratory and Schaefer's boss.

From this supercooled alto-stratus cloud, dry-ice pellets produced the first man-made snow.

For almost as long, Schaefer has been interested in snow. Although he left school at 16 and has spent practically every spare minute since then educating himself into a top-ranking scientist, he still found time to go skiing. That led to a study of snowflakes.

Schaefer discovered that supercooled water vapor suddenly changes to snow at -31°F., and, still more interesting, that only part of the vapor need be cooled that much to turn a whole cloud to snow. A little dry ice (temperature: -110°F.) chills a cloud enough to start snow.

On Nov. 13, 1946, he tried dry ice on a real cloud - a three-mile alto-stratus 14,000 feet over Greylock Mountain, Mass. That three-mile cloud turned completely into snow!

Snow-makers were quickly developed. The dry-ice dispenser is basically no more than a funnel, and the generator for silver iodide smoke consists only of an oxyacetylene torch, an electric motor and a jet of cold air. The motor slowly feeds string impregnated with silver iodide into the torch flame. The cold-air jet then condenses the silver iodide vapor into tiny particles the right size for snow seeds.

Opening a pop bottle (left) or pricking a tiny balloon (right) in a supercooled cloud allows gas to expand, lowers temperature and creates a snowstorm. Dr. Langmuir (center) produces the necessary cloud by blowing into a cold chamber as Schaefer and two Army observers look on.

Curtains of snow begin to fall as ice germs grow at the expense of minute water droplets.

Schaefer's seeding technique works on only a single kind of cloud: one made of supercooled water vapor. But if there is a trigger that eliminates supercooled clouds, there must be other triggers for other clouds. With enough triggers, weather could be made to order!

Seeking the "Why"

Already scientists are looking for those other triggers. At the Institute for Advanced Study in Princeton, N. J., Dr. John von Neumann hopes to find the "why" of weather - the foundation on which weather control must be built.  

Right now we know precious little about weather, but meteorologists have learned that weather changes are caused by such things as the varying amounts of heat received from the sun, the rotation of the earth, and the presence of oceans and mountains. These causes have even been reduced to mathematical formulas. Unfortunately, the formulas are so complicated that a whole crew of mathematicians would have to work until next week to figure out tomorrow's weather!

This hurdle seemed insurmountable until the war brought amazingly fast automatic computers. Now a new type, employing a tube that can "remember" thousands of numbers (see p. 144), is being built by the Institute for Advanced Study and the RCA Laboratories. In minutes, it can solve problems that would take years by ordinary methods.

With the new computer, Dr. von Neumann plans to test the weather equations, comparing their predictions to the actual weather, then correcting the equations until reliable long-range forecasts can be made.

Once the weather equations are perfected, Dr. von Neumann and Dr. V. K. Zworykin, vice-president of RCA Laboratories, foresee their use to control weather through "model experiments." Figures corresponding to imaginary weather conditions will be fed into the computer. It will then forecast what weather would have resulted. For example, the machine would show how a higher temperature over the Caribbean Sea would have affected the weather in Miami.

Imitating an Island

Only small temperature difference at one critical location can produce radical weather changes, says Dr. Zworykin, citing small ocean islands as an example. In the daytime, the temperature above the island is a few degrees higher than the temperature above the surrounding water because the island surface absorbs more heat from the sky than the water does. This is enough to produce updrafts that cause clouds to rise high above the island, being cooled in the process so that they turn into rain.

Dr. Zworykin proposes to imitate this natural triggering process by creating artificial areas that could be either highly absorbing or highly reflecting. A thin layer of carbon black would absorb heat. By covering it with white smoke, it would become reflecting.

These areas would have to be placed in exactly the right spots and used at exactly the right times. Haphazard changes in the heat put into the air could make no change in the weather - the forces involved are far too great to be influenced directly. Yet the energy absorbed by a few square miles of carbon black on a South Atlantic island might save Florida from a hurricane, because it acts as a trigger. It does not try to stand in the way of the hurricane forces; it merely upsets their already shaky balance.

Finding out where and when a small amount of energy will work as a trigger is the big problem, and one that cannot be solved today. It won't be solved tomorrow, either, for the electronic computer able to furnish the answer is not expected to be ready until 1949. Hail prevention appears much closer.

How to Make Snow at Home

All you need to make your own snow are a book of safety matches, a silver coin, a drop of iodine and some cold air in an enclosed space. A black background and a flashlight help, however, by making the tiny flakes visible. The procedure is shown below. For cold air, try any of the following: (1) the inside of a home freezer or a frozen-food cabinet; (2) a big empty jar, chilled by packing dry ice around it in a bucket (but don't touch the dry ice with bare hands!); ( 3) the inside of a closed car, or a similar space, on a cold winter night. Blowing into the ice-cube section of a refrigerator won't work, because all the cold air spills out of it when the door is opened.

1. Put a drop of iodine on a match head and rub it on a quarter. Some silver will adhere to it.

2. Strike the match. When it burns, its smoke will contain microscopic silver iodide crystals.

3. Blow it out in cold-air space (below 10°F.). Water vapor in your breath becomes supercooled.

4. Vapor freezes upon the silver iodide seeds, and a flashlight ray reveals shining snowflakes.




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