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Wilson A. Bentley:
The Raindrop Man, Too
The life-long work of Wilson Alwyn Bentley was farming his family's acreage in Jericho, Vermont. But his passion, from 1885 until his death in the waning days of 1931 was observing and photographing snow crystals. Over those 46 years he produced more than 5,000 photographs of snow crystals and several papers in the scientific journals on his work. Even during his lifetime, Bentley was known as The Snowflake Man or Snowflake Bentley to thousands of Americans.
But during a six-year period from 1898 to 1904, Bentley's summer interest turned to rain, not the usual concern for rain of a Vermont farmer, but a particular interest in the size of raindrops. Unfortunately, the results of that research were far ahead of the contemporary thinking in atmospheric sciences and were ignored or forgotten for almost four decades. Let us turn our attention away from The Snowflake Man for a moment and focus on W.A. Bentley, The Raindrop Man, by taking a close look at his study of raindrops.
The nature and formation of raindrops aroused the interest of many great thinkers over the ages including Aristotle, Descartes and Benjamin Franklin. Most of their theories, however, were philosophical in nature, based mainly on long-distant observation and deduction rather than hard data derived from experimentation. By the end of the nineteenth century, atmospheric scientists focused the study of rainfall only on measuring rainfall quantity and rates. Few bothered to consider looking into the nature of the raindrops themselves: their shape, composition and size variations within and between rainfall events.
A few European scientists had briefly looked into the nature of raindrops in the 1800s. The most prominent was Philipp Lenard, a German physicist who in 1905 would receive the Nobel Prize in physics for his work with cathode rays. Lenard began looking at raindrop properties such as size, shape and stability in 1898, publishing his results in 1904.
At about the same time that Lenard began his research, Wilson Bentley independently set out to study raindrops. (There is no evidence that either man knew of the other's work. And after both published their results in 1904, neither ever revisited the topic.) Bentley proclaimed in the paper that summarized his raindrop work that his motivation to undertake the study came from a "desire to add, if possible, a little to our knowledge regarding rainfall phenomena." And that he did. With as focused and thorough an effort as he exhibited with his snow crystal work, Bentley became the most diligent chaser of raindrops of his day.
By 1898 Bentley had been photographing snow crystals for 13 years. Bentley's inquiring nature also wanted to know how the crystals formed and where in the clouds they originated. He wrote in his first published article on snow crystals:
"...by means of these wonderfully delicate and exquisite figures, much may be learned of the history of each crystal and the changes through which it has passed in its journey through cloudland." (Popular Science Monthly, May 1898)
Such thinking likely lead him to wonder about the formation and history of all forms of precipitation. His recent contact with the academic world through Professor George Henry Perkins (who co-authored Bentley's first article) at the University of Vermont must have boosted his confidence that he was doing meaningful work -- not wasting his time as his father and brother frequently complained. Spending the winter fussing over snowflakes was crazy enough, but wasting valuable summer hours playing with raindrops; that was too much in their opinion.
With his background in photographic investigations, Bentley's first inclination was to use photography to study raindrops. According to his technical notebook, titled Snow which covering the period 1989 to 1902, he first succeeded in "photographing" raindrop impressions on October 30, 1898:
"I secured first photos of raindrops, magnified 4 diameters. They were impressions made in flour. After trying various mediums I found flour to be best to secure their impressions in."
The next day brought fog and drizzle and Bentley captured further imprints in the flour. But this method was not be the one he would use in his detailed rain studies over the next few years. While again photographing raindrop impressions on the evening of November 10th, Bentley made one of those serendipitous discoveries that fill the annals of scientific investigation.
"In the bottom of each raindrop impression in the flour there could always be found a roundish granule of dough nearly exact size of raindrop. After experimenting with artificial raindrops I could measure diameter before falling into the flour, and thus tell if the dough granule corresponded in size with the measured raindrop."
Bentley saw the utility of such an experimental technique (and perhaps as a good Yankee farmer the thrift of it as well). He then set out to assure himself that the dough granules related to the true raindrop size. He calibrated his device thus:
"Drops of water of about one-twelfth of an inch [2.1 mm] and one-sixth of an inch [4.2 mm] in diameter, suspended from the end of a broom splint and a glass pipette respectively, were carefully measured and then allowed to drop into flour from height of from twelve to thirty feet [3.7 to 9.1 m]."
The resulting dough pellets "were found by careful experiment to correspond very closely in size with the raindrops that made them." Thus assured of the validity of the results, Bentley had his collection apparatus -- a shallow tin pan and a cup of well-sifted baking flour.
"The method employed was to allow the raindrops to fall into a layer one inch deep of fine, uncompacted flour, with a smooth surface, contained in a shallow tin receptacle about four inches in diameter, which was generally exposed to the rain for about four seconds, although a longer time was given when the drops fell scatteringly. The raindrops were allowed to remain in the flour until the dough pellet that each drop always produces at the bottom of the cavity was dry and hard."
Bentley's niece Alice recalled watching him sift flour into small tin containers and then run quickly outside with them into the hard rain. "Then later on," she remembers, "I saw these little pellets, dried, so I asked what they were, and then he'd explain the whole thing to me. He said he was trying to measure the size of raindrops. In different storms they'd be different sizes. He had these little pellets all around the table."
Over the tenure of this study, Bentley collected 344 sets of raindrop pellets from over 70 distinct storms, including 25 thunderstorms. For each collection period, he meticulously recorded the date, time of day, temperature, wind, cloud type and estimated cloud height. He also noted the type of storm, the presence or absence of lightning, and what sector of the storm from which the samples were collected.
For each set of raindrops, Bentley partitioned the pellets into one of five size categories based on their diameter:
The large number of samples collected by Bentley from a variety of storms allowed him to make generalizations on raindrop size distributions (or spectra) with storm and cloud types and collection position (east edge, west edge, center, etc.) within the storm.
Bentley published his results in a paper entitled "Studies of raindrops and raindrop phenomena" in the October 1904 issue of Monthly Weather Review. The following summarizes his findings.
From his study data, he concluded that drops of all sizes were usually present in most rains. However, small drops greatly outnumbered the large ones. The largest drops fell solely from cumulonimbus clouds that reached far up into the sky.
Bentley also observed that a small number of samples (7.5 %) were composed solely of one size category -- usually small drops but sometimes only large ones. Such distributions are called monodisperse, and their cause is still an issue of debate today. Bentley felt these distributions were due to the melting of snow crystals or granular snow which he had found to generally be of uniform size.
Bentley also used his data to conclude that the largest drops fell in rains associated with lightning overhead rather than flashing in the distance or being totally absent. From an analysis of nearly 80 data sets from each of the three lightning scenarios, the largest drops --those > 0.20 inches (5 mm) in diameter -- were thirty times more numerous when lightning flashed overhead compared to the no-lightning situation and ten times greater in number with lightning overhead than when it was present but distant from the sampling location. Bentley believed that the large drops caused the lightning because they had a higher concentration of electrical charge than smaller drops.
Duncan C. Blanchard, long-time researcher in the field of cloud and precipitation physics as well as Bentley biographer (click here for a review of The Snowflake Man), remarked that Bentley helped open a Pandora's Box of hypotheses on what really happens within a thunderstorm. Do the charged, large raindrops cause the lightning? Or does the lightning help form large raindrops? Although most atmospheric scientists would currently side with Bentley on this issue, "[t]his chicken-or-the-egg scenario is being played out in laboratories around the world today."
Another of Bentley's deductions drawn from his measurements proposed that, based on the raindrop size distribution in thunderstorms, most thunderstorm rain came from the melting of snowflakes. He drew his conclusions form the relative volumes of large and small drops collected in thunderstorm rainfall.
"Assuming, accordingly, that the larger raindrops issue from the upper portions of the clouds and are due to the melting of snow, or, conversely, that the smaller ones originate by liquid processes alone within the lower clouds, we have a means of approximately ascertaining which process produces the major part of the rainfall. We have only to group together the individual raindrops (flour pellets) within the samples secured during various rainfalls, and weigh each group separately."
Looking at the data from four thunderstorms, he found the total weight of the larger drops was almost twice that of the total weight of the smaller drops and thus concluded "the major portion of the rainfall of thundershowers is of snow origin."
Although Bentley's conclusions can be criticized today on a number of technical points, they were well ahead of their time in 1904. Interestingly, research by R. List and J.R. Gillespie at the University of Toronto published in 1976 independently reached similar conclusions on the dual origin of rain.
A paper, published in the leading American meteorological journal of its day Monthly Weather Review, so filled with innovative research and ideas should have attracted a lot of attention in the scientific community. But, according to Blanchard, it did not. "No one tried to repeat Bentley's experiments or extend his work. No one even commented on it. There was complete silence in the pages of Monthly Weather Review, not only in the year of publication but during his remaining lifetime. If anyone wrote to Bentley either to praise or criticize his efforts, Blanchard found no record of it.
Unfortunately for Bentley, the study of cloud physics and precipitation processes would not blossom until the 1940s. The first recognition of Bentley's raindrop experiments appears to have been by US Soil Conservation Service scientists J.O. Laws and D.A. Parsons, who published a paper in 1943 reporting measurements of raindrop size under various rainfall intensities using Bentley's collection method.
Laws' and Parson's data were later combined with additional measurements of drop size distributions and rainfall intensity by J.S. Marshall and W.McK. Palmer of McGill University. In 1948 they published a mathematical relationship (now known as the Marshall-Palmer equation) that related the raindrop size spectra with rainfall intensity. This equation has been instrumental in the development of radar as a tool to observe precipitation and its intensity, and thus has increased the ability to produce storm warnings.
Perhaps the lack of recognition of his peers led Bentley away from trying to further understand the precipitation process. He did continue photographing snow and ice crystals until his death in 1931, and would publish six articles on snow crystals in Monthly Weather Review after 1904. By 1920, he was know to thousands as The Snowflake Man. But had his 1904 paper received the recognition it deserved, perhaps we would also know him today as The Raindrop Man.
I am indebted to Duncan C. Blanchard for the extensive research on W.A. Bentley found in his biography of Bentley titled The Snowflake Man. The facets of Bentley's life presented here are based on materials from that book. I have reviewed the book in the Weather Reviews section of the Weather Doctor.
For more on Bentley, check out Wilson Bentley Digital Archives.
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