## Saturday, July 5, 2008

### A Sound of Thunder

Most of the windows in our house look out across the little valley that separates New York's Taconic Range from the Green Mountains of Vermont. The elevation of the house, 1250 feet, gives us an eye-level view of approaching thunderstorms. On the hottest summer days, thunderheads gather on the far western horizon all afternoon, bunching higher and higher, until grey misty clouds spill over into the valley. The grey raft floats towards us, dropping a curtain of rain at its leading edge. Standing in the living room and looking out to westward, I can almost imagine I'm sailing into a squall.

Lightning occasionally strikes the marsh down below the house; the picture below shows a lightning bolt from a spectacular electrical storm just a few days ago.

This strike was about 300 yards from the house. We've had closer. A few years ago, our well pump motor was apparently fried by lightning that must have struck one of the trees on the south side of our yard. The current flowed into the ground and jumped to the wiring that leads into our mechanical room.

***

A couple of weeks ago, I was awakened during the night by a storm. The thunderclaps were loud, but it was actually the brightness of the lightning that disturbed my sleep. I lay for a time in the intermittent dark, calculating distances and velocities from the delay information, when it occurred to me that there was something peculiar about the sound of the thunder. Mixed in with the sounds of the storm, I heard a regular beat of deep booming sounds spaced a few seconds apart. The uniformity of the spacing in time marked the sounds as unnatural. It was then that I remembered an article I'd read a few days earlier in the local newspaper: Hail Cannon Stirs Complaints.

The regular booms I had heard were those of a "hail cannon." You can google around to learn more about these devices, but the point of them is to emit a loud sonic boom every few seconds, the goal being to suppress the formation of hailstones. Hail cannons were popular in Europe during the 1890's and 1900's, but they were gradually abandoned after they were perceived to be ineffective. Hail cannons have made a comeback in the last decade or two, and this year a large orchard in Bennington has purchased one and put it to use, causing dozens of complaints in Bennington and neighboring towns.

It is unlikely that hail cannons actually work. According to a 2006 paper by Dutch meteorologists Jon Wieringa and Iwan Holleman in Meteorologische Zeitschrift, the few publishable experiments that have been carried out yielded results ranging from mostly negative to inconclusive at best. And there appears to be no known meteorological mechanism that would lead one to believe in their effectiveness a priori. Wieringa and Holleman conclude that "the use of cannons or explosive rockets is waste of money and effort." The Commission for Atmospheric Sciences Management Group of the World Meteorological Association also declared in 2007 that "...hail cannons...have no physical basis and are not approved."

Nevertheless, hail cannons are growing in popularity. In one sense, this is understandable. Hail is a very destructive force, costing billions of dollars of damage in the U.S. each year. And premiums for hail insurance are apparently extremely expensive. Caught between a hailstone and a hard place, a farmer (especially a gullible one) might view a $30,000 hail cannon as a lifesaver. Of course, even a correct calculation about the risk of hail, the cost of insurance, and the cannon's own cost and effectiveness ignores another relevant factor, the property rights of the neighbors who are being rattled every time the cannon fires. In Bennington, a noise ordinance prohibits the cannon from being fired at night, but the orchard has violated the ordinance twice. The Town may be considering litigation against the orchard, and the neighbors may be doing so as well. In any event, I think it will take a judge to balance the perceived and real interests of the orchard against those of the surrounding property owners. A healthy agricultural sector is in everybody's best interest. For this reason, some residents of the Town support the hail cannon, which the farmer has said he believes in "100 percent." But how can it be good for agriculture when a farmer spends$30,000 on a machine that in all likelihood fails to protect his crops? Would we applaud the farmer if he had spent \$30,000 to have the Rite of the White Tiger performed on his property? Tibetan farmers used to believe in this anti-hail ritual 100 percent too. But belief alone didn't make it effective at preventing hail.

It can't be pleasant for farmers and car dealerships to have no good options for dealing with hail. But a noisome gimmick isn't a good option either. Perhaps Vermont and other states could consider setting up a taxpayer-subsidized hail insurance program. After all, insurance is a proven method for dealing with Acts of God. And it's also very quiet.

SpiderWomanKnits said...

Thanks for this great post. It is astonishing to me that even after SVO has decided to stop using the cannon the debate still continues on an emotional level without any inclusion of whether the device actually works or not.

That picture is amazing by the way.

danimal said...

I'm sure that the articles you cite have investigated the effect of the hail cannon objectively, and that it is not effective.

But just for interest, having worked in the area of colloids and nucleation theory for a couple of years now, I can *imagine* a mechanism where a hail cannon *might* work.

In the process of "sonication" (cf. Wikipedia entry), ultrasound, i.e. sound waves with frequencies above the range of human hearing, is used to agitate particles in solution. In some cases, it can aid in dissolution of particles, and even provide energy for chemical reactions.

Hail forms via water vapor condensing and then crystallizing in a cloud, and then growing while being buffeted about in the currents in a thundercloud. (The longer it is buffeted about, the larger the hailstones can grow.) Based on lab uses of sonication, using sound to disrupt that process is not a crazy idea.

But the lab systems are well-controlled systems under a constant and focused application of ultrasound. In the case of a hail cannon in a thunderstorm, you do not achieve a very focused application of sound (it spreads in all directions from the cannon), and apparently it isn't effective in disrupting the ice formation.

Overall, then, an interesting idea that doesn't work in practice.

I'm not sure what the environmental effects of ultrasound are (except that dogs wouldn't like it!), but perhaps you could try frequencies above human (and even dog) hearing...

JasonZimba said...

Makes good sense - and I should emphasize that when I wrote that there is apparently no known physical mechanism that would lead one to believe that the devices are effective, I was referring only to the devices as they are designed today. The orchard's hail cannon can't be emitting very much energy in the ultrasonic range of the spectrum.

Someday we may have effective hail suppression techniques. Cloud seeding is still a viable candidate, I believe. Anyway, it has apparently seen wide use in China.

It should also be said that none of the research on hail cannons, pro or con, appears to be what you'd call gold-standard. For example, one of the studies cited by Wieringa and Holleman published a 4-by-9 table of numbers suggesting a meaningful suppression effect within 400 meters. But from what I can tell, it seems that nobody thought to move the cannon around every few years, to generate case/control numbers for each region of terrain covered in the study. And the negative studies were simple comparisons of frequency of hail events in regions covered/not covered by a cannon, again with no geographical permutation or other clinical-type design.

Finally, a question I'm personally curious about is this: When hail does reach the ground, where in space did it originate? If you rewind the hail's trajectory, will you find that it formed anywhere near the site of the damage? (In other words, if you had a device that worked, where in three dimensions would you aim and focus it? And where on the ground would the benefits "rain down," as it were?)

Anonymous said...

Danimal..
Very well said.
I enjoyed your explanaton of hail.
Thank you for explaining it.

Anonymous said...

A footnote..