A Short History of Hydrogen Sulfide

From the sewers of Paris to physiological messenger

Roger P. Smith from http://www.americanscientist.org/issues/pub/2010/1/a-short-history-of-hydrogen-sulfide/2

Early last year, reports began to emerge in the Southeastern United States of a strange illness. Homeowners reported nosebleeds, sinus irritation and respiratory problems that appeared to be associated with corrosion of copper pipes and air conditioner coils in their houses.

The culprit seems to be drywall imported from China and possibly contaminated with strontium sulfide, an unstable salt that releases hydrogen sulfide on exposure to moisture. It was used widely in the housing boom of 2004–2007, and in the rebuilding efforts after Hurricane Katrina in 2005, when domestic suppliers could not keep up with the demand. The Consumer Products Safety Commission is now investigating whether sulfide gases given off by the drywall, including hydrogen sulfide, are to blame. The Florida Department of Health maintains a Web site with information for consumers. Lawsuits abound, and many who are able to do so have moved out of their homes. Several estimates place the number of affected houses at 100,000.

To those experiencing or investigating this phenomenon early on, it seemed bizarre. But in fact, this is just the latest chapter in the history of a chemical whose effects were first noted in the 16th century. And there is still more to learn about its role in the human body. Recent research offers insights into its biochemical actions as well as some intriguing suggestions for medical uses.

Occupational Exposures

In 1713, a remarkable Italian physician named Bernardino Ramazzini published De Morbis Artificum, or Diseases of Workers. In Chapter 14, titled “Diseases of Cleaners of Privies and Cesspits,” he describes a painful inflammation of the eyes which was common among such workers. The inflammation often led to secondary bacterial invasion, and sometimes to total blindness. Displaying amazing insight, Ramazzini hypothesized that when the cleaners disturbed the excrement in the course of their work, an unknown volatile acid was produced, which was irritating to the eyes. It was also at least partially responsible for the odor of excrement, and it is now known to be generated wherever organic matter undergoes putrefaction.

Ramazzini further postulated that that same acid was causing copper and silver coins which the workers had in their pockets to turn black on their surfaces—an eerie resonance with the phenomena recently observed by U.S. homeowners. Around 1777, a series of accidents—some of them fatal—began to occur in Paris due to a gas emanating from its sewer system. The commission appointed to study the cases made its findings public in 1785. The report described two distinct types of poisonings: a mild form involving inflammation of the eyes and mucous membranes as already described by Ramazzini, and a severe form that was characterized by a fulminating (rapidly developing) asphyxia. It is little wonder that the French Romantic writer Victor Hugo (1802–1885) referred to the Parisian sewers as “the intestine of the Leviathan.” Many years were to pass, however, before chemical analyses would establish the presence of hydrogen sulfide in the sewers and implicate it as the cause of the accidents.

The history of exposures has focused on sewers and the workplace, but the corrosive effects of hydrogen sulfide are common knowledge in Rotorua, New Zealand, which was built over centuries in a geothermally active area. The constant exposure to low, environmental levels of hydrogen sulfide produces such damage even as residents enjoy spas, indoor heating and cooking with the hot gases.

Today, the American Conference of Industrial Hygienists has set the so-called threshold limit value for its presence in the workplace at 10 parts per million (ppm) of hydrogen sulfide in air for eight hours a day, five days a week over a working lifetime. The U.S. National Institute for Occupational Safety and Health estimated in 1977—some 200 years after the Paris accidents—that 125,000 workers in at least 77 occupations, including drilling for petroleum, tanneries and the paper industry, may be at risk of exposure to hydrogen sulfide.

Chemical Discovery

Around 1750, a humble young Swede beginning his career as an apothecary was fortunate to have a series of very understanding mentors who allowed him considerable free time for reading and experimentation. His name was Carl Wilhelm Scheele, and he turned out to be a gifted chemist. Like many chemists before and after him, Scheele seems to have given little thought to the biological effects of the materials with which he worked. One day while distilling potassium ferrocyanide with sulfuric acid, he noted a “strong, peculiar and not unpleasant odor.” He brought himself to taste this gas and described it as “slightly on sweet [sic] and somewhat heating on the mouth.” Today we describe the odor as that of bitter almonds and call the gas hydrogen cyanide. Scheele may have been fortunate to have escaped with his life.

Perhaps a guardian angel was with him again on the day that he treated ferrous sulfide (pyrite, or fool’s gold) with a mineral acid. He called the rank odor that resulted Schwefelluft (sulfur air) and referred to it as stinkende (stinking or fetid). Today we refer to the odor as that of rotten eggs. His patron, the Swedish chemist and mineralogist Torbern Olof Bergman, also demonstrated its presence in some mineral springs. The publication date for these original observations was 1777—around the time of the Paris accidents. The fact that the same man discovered both hydrogen sulfide and hydrogen cyanide was the start of a long series of coincidences and discoveries about the two chemicals that would uncover their similarities.

Biological Effects

Investigations on the biological effects of hydrogen sulfide began around the turn of the 20th century. François Chaussier, François Magendie, Claude Bernard and Felix Hoppe-Seyler were among the well-known scientists of the day who labored in that vineyard.

In experiments with dogs, marked differences in the effects of hydrogen sulfide were noted with only small changes in its concentration in the air they were breathing. Fifty ppm, which was considered a minimally lethal concentration at the time, resulted in a slight progressive depression in the rate and depth of respirations. After many hours of exposure, the dogs died from a type of pulmonary edema, acute respiratory distress syndrome (ARDS). When that concentration was doubled to 100 ppm, death resulted in 15 to 20 minutes. In these cases the respiration was stimulated almost immediately, this progressed to a pronounced hyperpnea (deep breathing), and death in apnea followed. At 300 ppm, respiratory arrest occurred after a few violent gasps. The same effects on respiration, with the exception of ARDS, were known to occur with injected or inhaled hydrogen cyanide. Mice may be more resistant to the effects of hydrogen sulfide. In a 1964 experiment, they survived for 10 minutes in an atmosphere of 1,872 ppm and for 20 minutes at 985 ppm.

A more complete explanation of the respiratory stimulant effects of hydrogen cyanide and hydrogen sulfide had to await the discovery of the chemoreceptor function of the carotid body (shown at right), and the reflex effects that follow the activation of those receptors. The respiratory stimulant effects of cyanide and sulfide could be completely abolished by severing the sinus nerve and thus denervating the carotid sinus. In that case, larger doses of either chemical resulted only in respiratory depression, which was presumably mediated via the brainstem. Similarly, injections of sulfide or cyanide into the internal carotid or vertebral arteries also failed to stimulate respiration, since in those cases the chemicals reached the carotid sinus only after dilution in the general circulation. When innervation of the sinus was intact, the hyperpnea was accompanied by a fleeting rise in systemic blood pressure, and sometimes by a slowed heart rate (bradycardia). The cardiovascular effects varied with the injection site and the species and are still not adequately explained.

A Persistent False Trail

The great German biochemist Hoppe-Seyler became famous for his discovery of the abnormal form of hemoglobin known as methemoglobin, in which some or all of the heme irons have been oxidized to the ferric form. This reaction is readily mediated both in vivo and in vitro by sodium nitrite. Methemoglobin cannot reversibly combine with oxygen, and the disruption of the oxygen-transport function of the blood can result in hypoxia and death.

In 1863, Hoppe-Seyler passed a stream of pure hydrogen sulfide through a sample of human blood and claimed to have observed a greenish pigment that was associated with shifts in the visible absorption spectrum of hemoglobin. Although he was aware that he had probably produced a mixture containing unstable and denatured products, which resulted in turbidity and precipitation and made the absorption spectra suspect, he still thought that the mixture contained a new form of hemoglobin. He called it sulfhemoglobin and thereby launched one of the most confused areas in hematology. It led to the hypothesis some still subscribe to—namely, that hydrogen sulfide is a blood poison like sodium nitrite and carbon monoxide. No matter that animal experiments clearly demonstrated that it was a respiratory toxin, or that no such pigment has ever been identified in the blood of animals or humans fatally poisoned with hydrogen sulfide. Sulfhemoglobin generated by hydrogen sulfide appears to be a strictly in vitro phenomenon, and it has yet to be prepared in pure form.

Sulfmethemoglobin

No less a scientist than Linus Pauling and his associates described the magnetic properties of another blood pigment, which they called sulfmethemoglobin. This pigment is easily prepared in pure form by mixing hydrogen sulfide with methemoglobin, and it is chemically analogous to cyanmethemoglobin, in which the cyanide ion is bound to ferric irons of heme. The hydrosulfide anion (HS^–) also binds to ferric heme, albeit not quite so tenaciously as cyanide. Indeed, this reaction has been exploited medically as an antidote to cyanide poisoning. One can deliberately inject sodium nitrite intravenously to generate a tolerable level of methemoglobin. The methemoglobin will temporarily bind free cyanide as the inactive complex cyanmethemoglobin. Over time the cyanide is slowly released, at a rate at which the body’s natural detoxification mechanisms can deal with it.

At least three laboratories have demonstrated that the same principles can be applied to hydrogen sulfide poisoning and that induced methemoglobinemia can indeed be lifesaving. At least a half dozen successful human resuscitations have been reported in the literature. The odds against its successful application, however, are high. Few poisons are more rapidly acting than inhaled hydrogen sulfide, and inhalation is invariably the route of exposure. Sulfide poisoning tends to occur in remote locations, and there is seldom a medically qualified individual on the scene who is prepared with a parenteral form of nitrite and trained to make intravenous injections. Most successful resuscitations from cyanide poisoning have occurred in individuals who ingested soluble salts of cyanide, where absorption is delayed.

Chemical Similarities Between Cyanide and Sulfide

In addition to the similarities in their physiological effects, cyanide and sulfide have chemical similarities. The undissociated forms of both hydrogen cyanide and hydrogen sulfide are flammable, volatile gases. Hydrogen sulfide (whose vapor density, or d, is 1.19) is heavier than air (d = 1.0) whereas hydrogen cyanide (d = 0.941) is lighter. Both are weak acids with acid dissociation constants (pK_a) that are of some physiological significance: hydrogen cyanide 9.2–9.3 and hydrogen sulfide 7.04. Both form salts with sodium and potassium as well as with some alkaline earths. Both anions bind to methemoglobin as noted above, and each of those complexes has its distinct visible absorption spectrum. And both are inhibitors of cytochrome coxidase, the terminal enzyme in the electron transport chain that reacts with molecular oxygen in aerobic organisms. Blockage of that key enzyme is believed to be the mechanism of action in the rapidly lethal form of cyanide and sulfide poisonings.

New Biological Roles

Inhibition of cytochrome c oxidase results in a decrease in oxidative phosphorylation (the metabolic pathway that produces ATP). This in turn lowers the metabolic rate and body temperature in mice. These phenomena accompany states of suspended animation. When mice were exposed to hydrogen sulfide at concentrations of 80 ppm, dramatic effects were observed in the first 5 minutes. By 6 hours, their metabolic rate had dropped by 90 percent and body temperature to 2 degrees Celsius above ambient temperature. When the mice were then returned to room air, the metabolic rate and temperature returned to normal with no detectable behavioral or functional deficits. As already noted, lethal levels for hydrogen sulfide in mice are much higher than for dogs, but this state in mice must also occur over a fairly narrow range of concentrations.

Finally, and most astonishingly of all, experimental evidence contributed in 2008 indicates that like carbon monoxide and nitric oxide, hydrogen sulfide is an important signaling molecule in biology, and it may find a role in medicine. It is physiologically generated in mice by cystathionine γ-lyase, and genetic deletion of that enzyme markedly reduces hydrogen sulfide levels in the serum, heart, aorta and other tissues. Mutant mice lacking the enzyme have marked hypertension and diminished endothelium-dependent vasorelaxation, consistent with an important vasodilator role for hydrogen sulfide. The enzyme is physiologically activated by the calcium-binding protein calmodulin, which is a mechanism for hydrogen sulfide formation in response to vascular activation. Thus, hydrogen sulfide appears to be a physiologic vasodilator and regulator of blood pressure. Its relative contribution vis-à-vis the similar nitric oxide is not yet clear.

What a strange and wondrous journey this odiferous and violently toxic chemical, associated with the excrement of humanity, has led us on for five centuries. It’s a history that could fill a book, one that covers a vast range of territory, from the search to determine the cause of workplace injuries to fascinating discoveries about how hydrogen sulfide interacts with chemoreceptors in the body. And for all the false leads, in the end it may yet turn out to have some useful applications in medicine—even if only a new Viagra.

Bibliography

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ScienceDaily (Jan. 22, 2010) — Dispelling the notion that urban "green" spaces help counteract greenhouse gas emissions, new research has found -- in Southern California at least -- that total emissions would be lower if lawns did not exist.

Turfgrass lawns help remove carbon dioxide from the atmosphere through photosynthesis and store it as organic carbon in soil, making them important "carbon sinks." However, greenhouse gas emissions from fertilizer production, mowing, leaf blowing and other lawn management practices are four times greater than the amount of carbon stored by ornamental grass in parks, a UC Irvine study shows. These emissions include nitrous oxide released from soil after fertilization. Nitrous oxide is a greenhouse gas that's 300 times more powerful than carbon dioxide, the Earth's most problematic climate warmer.

"Lawns look great -- they're nice and green and healthy, and they're photosynthesizing a lot of organic carbon. But the carbon-storing benefits of lawns are counteracted by fuel consumption," said Amy Townsend-Small, Earth system science postdoctoral researcher and lead author of the study, forthcoming in the journal Geophysical Research Letters. The research results are important to greenhouse gas legislation being negotiated. "We need this kind of carbon accounting to help reduce global warming," Townsend-Small said. "The current trend is to count the carbon sinks and forget about the greenhouse gas emissions, but it clearly isn't enough."

Turfgrass is increasingly widespread in urban areas and covers 1.9 percent of land in the continental U.S., making it the most common irrigated crop.

In the study, Townsend-Small and colleague Claudia Czimczik analyzed grass in four parks near Irvine, Calif. Each park contained two types of turf: ornamental lawns (picnic areas) that are largely undisturbed, and athletic fields (soccer and baseball) that are trampled and replanted and aerated frequently.

The researchers evaluated soil samples over time to ascertain carbon storage, or sequestration, and they determined nitrous oxide emissions by sampling air above the turf. Then they calculated carbon dioxide emissions resulting from fuel consumption, irrigation and fertilizer production using information about lawn upkeep from park officials and contractors.

The study showed that nitrous oxide emissions from lawns were comparable to those found in agricultural farms, which are among the largest emitters of nitrous oxide globally.

In ornamental lawns, nitrous oxide emissions from fertilization offset just 10 percent to 30 percent of carbon sequestration. But fossil fuel consumption for management, the researchers calculated, released about four times more carbon dioxide than the plots could take up. Athletic fields fared even worse, because -- due to soil disruption by tilling and resodding -- they didn't trap nearly as much carbon as ornamental grass but required the same emissions-producing care.

"It's impossible for these lawns to be net greenhouse gas sinks because too much fuel is used to maintain them," Townsend-Small concluded.

Previous studies have documented lawns storing carbon, but this research was the first to compare carbon sequestration to nitrous oxide and carbon dioxide emissions from lawn grooming practices.

The UCI study was supported by the Kearney Foundation of Soil Science and the U.S. Department of Agriculture.

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Last Decade Was Warmest on Record, 2009 One of Warmest Years, NASA Research Finds

ScienceDaily (Jan. 22, 2010) — A new analysis of global surface temperatures by NASA scientists finds the past year was tied for the second warmest since 1880. In the Southern Hemisphere, 2009 was the warmest year on record.

Although 2008 was the coolest year of the decade because of a strong La Nina that cooled the tropical Pacific Ocean, 2009 saw a return to a near-record global temperatures as the La Nina diminished, according to the new analysis by NASA's Goddard Institute for Space Studies (GISS) in New York. The past year was a small fraction of a degree cooler than 2005, the warmest on record, putting 2009 in a virtual tie with a cluster of other years --1998, 2002, 2003, 2006, and 2007 -- for the second warmest on record.

"There's always interest in the annual temperature numbers and a given year's ranking, but the ranking often misses the point," said James Hansen, GISS director. "There's substantial year-to-year variability of global temperature caused by the tropical El Nino-La Nina cycle. When we average temperature over five or ten years to minimize that variability, we find global warming is continuing unabated."

January 2000 to December 2009 was the warmest decade on record. Looking back to 1880, when modern scientific instrumentation became available to monitor temperatures precisely, a clear warming trend is present, although there was a leveling off between the 1940s and 1970s.

In the past three decades, the GISS surface temperature record shows an upward trend of about 0.36 degrees F (0.2 degrees C) per decade. In total, average global temperatures have increased by about 1.5 degrees F (0.8 degrees C) since 1880.

"That's the important number to keep in mind," said GISS climatologist Gavin Schmidt. "The difference between the second and sixth warmest years is trivial because the known uncertainty in the temperature measurement is larger than some of the differences between the warmest years."

The near-record global temperatures of 2009 occurred despite an unseasonably cool December in much of North America. High air pressures from the Arctic decreased the east-west flow of the jet stream, while increasing its tendency to blow from north to south. The result was an unusual effect that caused frigid air from the Arctic to rush into North America and warmer mid-latitude air to shift toward the north. This left North America cooler than normal, while the Arctic was warmer than normal.

"The contiguous 48 states cover only 1.5 percent of the world area, so the United States' temperature does not affect the global temperature much," Hansen said.

GISS uses publicly available data from three sources to conduct its temperature analysis. The sources are weather data from more than a thousand meteorological stations around the world, satellite observations of sea surface temperatures, and Antarctic research station measurements.

Other research groups also track global temperature trends but use different analysis techniques. The Met Office Hadley Centre in the United Kingdom uses similar input measurements as GISS, for example, but it omits large areas of the Arctic and Antarctic where monitoring stations are sparse.

Although the two methods produce slightly differing results in the annual rankings, the decadal trends in the two records are essentially identical.

"There's a contradiction between the results shown here and popular perceptions about climate trends," Hansen said. "In the last decade, global warming has not stopped."

For more information about GISS's surface temperature record, visit: http://data.giss.nasa.gov/gistemp/

For related video and still images, visit: http://svs.gsfc.nasa.gov/goto?010557

NASA (2010, January 22). Last decade was warmest on record, 2009 one of warmest years, NASA research finds. ScienceDaily. Retrieved January 22, 2010, from http://www.sciencedaily.com­ /releases/2010/01/100121170717.htm#

Non Linear Term in the Saint Venant Equation of SWMM 5

The flow equation has six components that have to be in balance at each time step:

1. The unsteady flow term or dQ/dt

2. The friction loss term (normally based on Manning's equation except for full force mains),

3. The bed slope term or dz/dx

4. The water surface slope term or dy/dx,

5. The non linear term or d(Q^2/A)/dx and

6. The entrance, exit and other loss terms.

All of these terms have to add up to zero at each time step. If the water surface slope becomes zero or negative then the only way the equation can be balanced is for the flow to decrease. If the spike is due to a change in the downstream head versus the upstream head then typically you will a dip in the flow graph as the water surface slope term becomes flat or negative, followed by a rise in the flow as the upstream head increases versus the downstream head.

You get more than the normal flow based on the head difference because in addition to the head difference you also get a push from the non linear terms or dq3 and dq4 in this graph.

From Edge Magazine - http://edge.org/q2010/q10_16.html#dalrymple

DAVID DALRYMPLE

Researcher, MIT Mind Machine Project

KNOWLEDGE IS OUT, FOCUS IS IN, AND PEOPLE ARE EVERYWHERE

Filtering, not remembering, is the most important skill for those who use the Internet. The Internet immerses us in a milieu of information — not for almost 20 years has a Web user read every available page — and there's more each minute: Twitter alone processes hundreds of tweets every second, from all around the world, all visible for anyone, anywhere, who cares to see. Of course, the majority of this information is worthless to the majority of people. Yet anything we care to know — what's the function for opening files in Perl? how far is it from Hong Kong to London? what's a power law? — is out there somewhere.

I see today's Internet as having three primary, broad consequences: 1) information is no longer stored and retrieved by people, but is managed externally, by the Internet, 2) it is increasingly challenging and important for people to maintain their focus in a world where distractions are available anywhere, and 3) the Internet enables us to talk to and hear from people around the world effortlessly.

Before the Internet, most professional occupations required a large body of knowledge, accumulated over years or even decades of experience. But now, anyone with good critical thinking skills and the ability to focus on the important information can retrieve it on demand from the Internet, rather than her own memory. On the other hand, those with wandering minds, who might once have been able to focus by isolating themselves with their work, now often cannot work without the Internet, which simultaneously furnishes a panoply of unrelated information — whether about their friends' doings, celebrity news, limericks, or millions of other sources of distraction. The bottom line is that how well an employee can focus might now be more important than how knowledgeable he is. Knowledge was once an internal property of a person, and focus on the task at hand could be imposed externally, but with the Internet, knowledge can be supplied externally, but focus must be forced internally.

Separable from the intertwined issues of knowledge and focus is the irrelevance of geography in the Internet age. On the transmitting end, the Internet allows many types of professionals to work in any location — from their home in Long Island, from their condo in Miami, in an airport in Chicago, or even in flight on some airlines — wherever there's an Internet connection. On the receiving end, it allows for an Internet user to access content produced anywhere in the world with equal ease. The Internet also enables groups of people to assemble based on interest, rather than on geography — collaboration can take place between people in Edinburgh, Los Angeles, and Perth nearly as easily as if they lived in neighboring cities.

In the future, these trends will continue, with the development of increasingly subconscious interfaces. Already, making an Internet search is something many people do without thinking about it, like making coffee or driving a car. Within the next 50 years, I expect the development of direct neural links, making the data that's available at our fingertips today available at our synapses in the future, and making virtual reality actually feel more real than traditional sensory perception. Information and experience could be exchanged between our brains and the network without any conscious action. And at some point, knowledge may be so external, all knowledge and experience will be shared universally, and the only notion of an "individual" will be a particular focus — a point in the vast network that concerns itself only with a specific subset of the information available.

In this future, knowledge will be fully outside the individual, focus will be fully inside, and everybody's selves will truly be spread everywhere.

The actual percentages are approximate but I roughly have seven equally important tasks in the support of our Sewer and Stormwater products.

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Medieval Storms Portend Worse Hurricanes

By Phil Berardelli
ScienceNOW Daily News
12 August 2009

For more than a decade, scientists have been trying to determine whether climate change is linked to intense storms, such as 2005's Hurricane Katrina. Meteorologist Michael Mann of Pennsylvania State University, University Park, and colleagues attacked the question by turning to the past. They looked through drill cores from coastal waters for signs that sediments had been disturbed by major storms. Eight sites along the U.S. East Coast and Puerto Rico provided a reliable record of the number of significant hurricanes going back about 1500 years. Other climate data and models added clues to water temperatures and hurricane intensity.

As the researchers report tomorrow in Nature, they found strong evidence that Atlantic hurricane activity peaked about 1000 years ago, producing up to 15 hurricanes a year on average--a level matched in recent times only over the past decade and a half. At the time, according to estimates constructed from other geologic data, Atlantic water temperatures were relatively warm, "though not as warm as today," Mann says. And Pacific temperatures were relatively cool, thanks to La Niña events. Warmer Atlantic waters whip up more storms, but warmer Pacific temperatures tend to create stronger jet streams that break up those storms. So the twin conditions a millennium ago produced kind of a "Perfect Storm" for hurricanes, he explains.

Of particular interest, the sediments reveal a close link between warmer water and the number of hurricanes during the past 150 years or so. Dropping temperatures produced seven or eight hurricanes a year, while a rising thermometer, such as in the earlier part of this decade, pushed the total to 15. "All other things being equal," Mann says, "this suggests that we are indeed likely to see not only stronger hurricanes in the Atlantic but perhaps more of them" in the near future.

Meteorologist James Elsner of Florida State University in Tallahassee agrees with the findings, but adds a caveat. The historical data do show that a link between warmer ocean temperatures and higher hurricane frequencies has existed for at least 1500 years, he says. However, there's a high degree of uncertainty in the data. That and the fact that the physics explaining the link haven't yet been established, Elsner explains, "indicates this is not the 'smoking gun' we've been looking for that would allow us to confidently project what will happen as the oceans continue to warm."

Atmospheric scientist Kerry Emanuel of the Massachusetts Institute of Technology in Cambridge agrees. The paper "shows that hurricane activity is indeed quite sensitive to climate," he says, and that global warming could have a dramatic impact on the severity of these storms in the future.

http://sciencenow.sciencemag.org/cgi/content/full/2009/812/1?rss=1

How a Raindrop Is Like an Exploding Parachute

By Karen C. Fox
ScienceNOW Daily News
20 July 2009

The idea has been that raindrops grow as they gently bump into each other and coalesce. Meanwhile, more forceful collisions break other drops apart into a scattering of smaller droplets. All this action would explain the wide distribution of shapes and sizes. But trying to unravel how the drops crash and break up led to a tough set of equations.

The new movies, however, show a much more straightforward process. Researchers snapped 1000 pictures a second of an isolated water drop as it fell through an ascending air stream. The drop first flattens into a pancake shape, which then balloons like a parachute. The bottommost rim of this chute has a thick, irregularly corrugated rim. Pressure from the air drag eventually breaks the chute apart into numerous smaller droplets--their wide range of sizes is due to the wide range of sizes of the bumps in the rim.

If a single raindrop breaks up in a statistically predictable way, then determining the wide range of sizes in an entire rain shower varies only with the intensity of the rainfall: Heavier rain leads to larger initial drops and a broader size distribution versus fine mists with homogenous, small drops. Villermaux and Aix-Marseille colleague Benjamin Bossa publish their findings online today in Nature Physics.

Physicist Jens Eggers who studies the dynamics of water drops at the University of Bristol in the United Kingdom is breathing a sigh of relief. "I was expecting things to get complicated, with lots of empirical relationships thrown together," he writes in an e-mail to Science. "Instead, based on a few physical ideas, the authors manage to explain a beautiful empirical relationship ... in a simple and universal way."

Atmospheric scientists, who have long believed that raindrop size is determined inside a cloud and from complex interactions as they fall, may take more convincing. "Mainstream cloud physicists will reject this thesis," e-mails Ramesh Srivastava, an atmospheric scientist who studies cloud dynamics at the University of Chicago in Illinois. Srivastava says that rain size distribution in practice does not seem to correlate to the paper's predictions; whereas Villermaux says evidence shows correlation at 100 meters below the cloud after the drops have had a chance to break up.

Regardless of who's right, the work isn't likely to see application any time soon. Villermaux says the findings are unlikely to aid weather forecasting or climate modeling, for example. "It's just for the pleasure of understanding."

http://sciencenow.sciencemag.org/cgi/content/full/2009/720/2?rss=1