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From Raindrops to Volcanoes
Adventures with Sea Surface Meteorology
By Duncan C. Blanchard, Berton C. Heinrich Jr.
Dover Publications, Inc.Copyright © 1995 Duncan C. Blanchard
All rights reserved.
THE FLIGHT OF THE RAINDROPS
"Attend now, and I will explain how rain collects in the clouds above, and how the showers are precipitated and descend upon the earth." These words were written two thousand years ago by the Roman poet Lucretius. In his great poem, On the Nature of Things, he ranged over a vast number of subjects that included clouds, rain, thunder, and lightning. But, typical of the thinkers of his day, he never put his ideas to the test of experiment. Some were subsequently shown to have much merit. Others, however, were either so vague and general that they said nothing at all, or else they were little more than interesting and sometimes amusing speculations. For example, he said that one of the causes of rain was the wind pressing against "swollen clouds." And the cause of lightning and thunder? Lucretius reasoned that when clouds collide they produce sparks and noise, as sometimes happens when two stones are struck together.
But it is not my intention to criticize the writings of Lucretius. It is all too easy to look backward down the long corridor of time and find error in anything. The ancients were struggling against a background of fear and superstition to find rational explanations for a multitude of natural phenomena. We should not criticize those who make an honest effort to understand the world around them. Rather, we should criticize those (they appear to be in the majority in every day and age) who unquestioningly will accept speculation and hypothesis before they have been put to the test of experiment.
The Beginning of Raindrop Studies
The ideas of Lucretius and his contemporaries on the formation of rain were carried down through the centuries and little attempt was made to improve upon them. Well over a thousand years went by, and nothing was added to our knowledge of how a raindrop is formed. It has been only within the past two hundred years that detailed daily or weekly measurements of rainfall, temperature, and atmospheric pressure have been made. By the end of the last century this collecting of data was being carried out with such zeal that the meteorological magazines devoted page after page to tables of this information. The rainfall was measured with great accuracy and reported to the nearest one-hundredth part of an inch.
In spite of the tremendous labors that went into the thousands of measurements of rainfall, no one seemed to ask the next questions: What are raindrops like? How big are they? Are they all the same size? Do they vary in size from rain to rain? Could I perhaps learn something of their origin if I determine their size and how they are distributed in rainfall? No one, I say, so far as we know, thought to ask these questions until, in the 1890s, a few men, one in this country and several abroad, began to wonder what raindrops were really like. I want to tell you about one of these men, about the ingenious and simple method he developed to measure the size of a raindrop, and what he found out about rain.
Wilson Bentley was a farmer who lived in the small town of Jericho, Vermont. But no ordinary farmer was he. Although he had no formal education beyond the public schooling available in Jericho and had his farming to attend to, he was somehow able to carry out a program of research on the mysteries of rain and snow. He is best known today for the thousands of beautiful photographs he made of snow crystals. But that is another story; we must stay with the raindrops.
In the year 1898 Bentley began his studies on rain, for he had "the desire to add, if possible, a little to our knowledge regarding rainfall phenomena...." And add he did. For seven years, from 1898 through 1904, he made 344 measurements of the sizes of raindrops from seventy different storms.
How did he measure the raindrop size? Very simple; he let the rain fall for a few seconds into pans of fine uncompacted flour. If the flour was at least one inch deep, the raindrops did not splash, and each drop produced a dough pellet. He let the pellets dry and then measured the diameters. But what relationship was there between the size of the dough pellet and the original raindrop? Again Bentley showed his ingenuity, but we'll let him speak for himself:
Drops of water about 1/12 of an inch and 1/6 of an inch in diameter, suspended from the end of a broom splint and from a glass pipette, respectively, were carefully measured, and then allowed to drop into flour from heights of from 12 to 30 feet. The smaller pellets (1/12 of an inch) were of so nearly the same diameter that it was difficult to detect any difference, although in some cases the pellets were slightly flattened by impact, with a corresponding slight increase in the longer diameter. The larger artificial raindrops (1/6 of an inch) produced pellets that were considerably flattened and had a longer diameter, exceeding by about one-third the diameter of the drop.
In 1904, Bentley published his findings in a scientific paper that is, in my opinion, among the very best ever written on the subject. He found that the largest raindrops are about one-quarter of an inch in diameter (about 6 mm). He suggested that in some cases the size was determined by the size of snowflakes within the cloud—the flakes had melted before they got to the ground. Bentley went on to tell how he had found different sizes of raindrops in different types of storms. He believed that there was a connection between lightning and raindrop size. And from an examination of his hundreds of raindrop samples he deduced that rain could have its origin either from melting snow or from a process that involved no ice or snow at all. But sometimes, he concluded, the sizes of the raindrops indicate that both processes may have operated at the same time.
Although Bentley's paper was clear and well written, his strikingly original work and ideas went unnoticed. No one went out to check his measurements. No one gave much thought to the questions he had raised about the nature of rain. Nearly forty years passed before anyone in this country continued on with the work he had started. Finally, in 1943, J. O. Laws and D. A. Parsons, of the Soil Conservation Service, utilized Bentley's flour technique and obtained more measurements of raindrop size. Since that time numerous scientists in many parts of the world have used the flour and other methods to discover much more about the sizes of raindrops.
Other Ways to Measure Raindrop Size
Before we turn to what has been revealed in raindrop studies, I want to tell you about two other methods that have been used to measure raindrop size. These methods, like Bentley's, are extremely simple and can be mastered by anyone who has the curiosity and desire to learn something about rain.
In 1895, Professor J. Wiesner of Germany exposed sheets of absorbent paper to the rain. When the raindrops fell upon the paper they were absorbed, and the size of the spots could be related to the original size of the raindrops. This method is in common use today.
A popular paper is the ordinary filter paper that can be found in any chemistry laboratory; circular sheets of Whatman's #1 paper of at least 15 cm diameter have been a favorite of many scientists. If these papers are dusted lightly with a water-soluble dye, such as Methylene Blue powder (also common in the laboratory or the corner drugstore), the spots from the raindrops will be recorded permanently on the paper. And, I must warn you, they will be recorded on you unless you are extremely careful with the dye. Put the dye on the paper only out-of-doors or under a ventilating hood. You can treat the papers by putting them one at a time into a large Mason jar in which one or two teaspoons of the dye have been placed. With the cover on the jar rotate it to tumble the dye completely over one side of the paper. Remove the paper and eliminate any excess dye by snapping the back of the paper with the fingers. Afterward the treated side of the filter paper may not appear to have any dye attached to it, but believe me, it does. You will find out when you expose it to the rain.
A third method of raindrop size measurement utilizes screens. Some years ago I was looking for a way to sample the large raindrops that often fall from thunderstorms. The flour method was just a bit too messy in heavy rain. The use of filter papers was out, as the large drops would splash. And so I tried exposing metal screens to the rain. The screens, similar to window screening but with a much finer mesh, were coated with soot from an acetylene torch. When the raindrops passed through a screen, they left an indication of their size, for they removed a circular spot of soot and carried it along with them. Although this method worked fairly well, it had its drawbacks. First, the use of acetylene was somewhat dangerous. Second, the long, stringy, soot particles that were produced ended up not only on the screens but on everything else in the laboratory. Needless to say, I was not too popular in the lab while doing this work.
Dr. Wallace Howell and his co-workers, then of the Mount Washington Observatory, came to the rescue just in time. They improved the screen method by substituting women's nylon stockings (60-gauge) for the wire screen, and confectioners' sugar (powdered sugar) for the black soot! Nylon stockings and confectioners' sugar hardly seem proper apparatus for measuring raindrop size, but they do the job extremely well. If you decide to try the nylon screen method, go to the dime store and get a set of embroidery hoops of about 15 cm diameter. Mount a piece of the nylon screening tightly on the hoops and coat it with confectioners' sugar. That's all there is to it; you're now ready to go out and measure raindrops. But one word of warning. The sugar may not stick at all well to stockings that are new. It appears that new stockings do not carry the natural body oils as do old, worn stockings. Therefore, unless you or someone else can wear the stockings in, I recommend you "wear them in" artificially by dipping them (after they're mounted on the hoop) into a trace solution of vaseline in benzene (not more than a quarter teaspoon of vaseline in one pint of benzene. Caution: benzene is inflammable). This method is guaranteed to leave the fibers coated with a thin but sticky layer of vaseline. You will have no trouble in making the sugar stick to it. But shake off any excess sugar before exposure to the rain. You can easily retreat a used screen. Dip it in a pan of water to remove the old sugar; let it dry, then sugar it again.
The sugar-screen method will give you very nice raindrop patterns. In Plate I you can see the clear, sugar-free spots that were produced when water drops of 2.65 mm and 5.4 mm diameter fell through the screen.
The relation between the diameter of the raindrop and the diameter of the spot that it makes on Whatman's #1 filter paper or on a sugared nylon screen (60-gauge) is shown in Fig. 1. You can see that the spots on the screens are not much larger than the drops that made them, but those on the filter papers are very much larger. This "magnification" of raindrop size makes the filter-paper method ideal for light rain where the drops are small. But in heavy rain, where the drops may be as large as 5 mm, it becomes intolerable. A 5 mm raindrop makes a spot 36 mm in diameter, more than seven times the size of the raindrop! A few such drops as these and the filter paper will look just as if it had been held under an open faucet. Better use the nylon screens for heavy rain.
Distribution of Raindrop Size in Rain
Most of the meteorological studies on the numbers and sizes of raindrops (usually called the size distribution) are presented in terms of ND. This is a symbol for the number N of drops of size D per cubic meter of air, the subscript D representing a certain range of diameter. How do you get ND from the number of drops (call this C) that are counted on the filter paper or screen? It's not hard. For example, suppose you have counted all the drops on the collecting surface and have used the calibration curve (Fig. 1) to divide them into intervals of 0.2 mm in true size. You may wish to find ND for the size range that goes from 0.4 to 0.6 mm. If C is the number of drops in the size range, and T is the time in seconds in which the drops were collected, if A is the area (in square meters) of the collecting surface and S is the terminal velocity in meters per second (see Fig. 2) of these drops, then ND = C/T × A × S. If you think about it for a while, you will see why it is so. You should make this calculation for each size range that you have collected. When you finish, you will have the size distribution of raindrops. Remember, this is the distribution of the drops in one cubic meter of space just above your sampling surface.
Now let's turn to what has been found out about the sizes and numbers of raindrops. In the past twenty years, measurements have been made in many parts of the world. It has been found, in general, that the more intense the rain the larger and more numerous are the raindrops. Now of course anyone who has walked in the rain (and who hasn't?) could make this statement. But without the benefit of measurement I doubt that one could guess that the number of raindrops per cubic meter of air varies from about 1000 in light rains (an intensity of less than 1 mm per hour) to 5000 or more in the heavier rains. The size of the raindrops has been found to range all the way from 0.2 mm diameter (by definition they have to be this large to be called a raindrop) to drops of about 6 mm diameter. These large drops usually are found only in downpours greater than 50 mm per hour.
It doesn't take very many of these giant 6 mm drops to bring a lot of water to the ground. Let's imagine two rainfalls where the number of drops per cubic meter of air are equal. In the first the drops are all 1 mm diameter, and in the second they are 6 mm diameter. There is much more water in the cubic meter of air containing the large drops. Just how much more is given by the cube of the ratio of the diameters,
(6/1)3 = 6 × 6 × 6 = 216!
However, the 6 mm drops fall about 2.3 times faster than the 1 mm drops (Fig. 2). Consequently, the amount of water carried to the ground in a given time by the 6 mm drops will be 216 × 2.3 or about 500 times as much as that brought to the ground by the 1 mm drops. When we use the expression, "it's a downpour" or "raining cats and dogs," we are speaking of a rain which probably has a few of these giant drops.
It is the exception rather than the rule to find raindrops of all the same size in a rainstorm. Nor are they distributed in a random sort of way. One generally finds more small drops than large drops. If the rainfall is light, the largest drops may be only 1 mm in diameter, but as the intensity increases, larger drops begin to appear.
Shortly after World War II, a number of meteorologists attempted to express these facts in mathematical form. The equations they developed related the intensity of rainfall (which is easily measured by rain gauges) to the size distribution of the raindrops. For example, with a rain intensity of 25 mm per hour, the equations said that the largest drops would be between 5 and 6 mm but would be so rare that only one drop would be found in 10 cubic meters of air. On the other hand, drops less than 1 mm should exist in numbers of about 1000 per cubic meter of air.
For a few years all was well. But then, as more work was done on the measurement of raindrops, it became increasingly clear that for a given rate of rainfall the raindrop distribution could vary. In particular, it appeared that "cold" rain—that is, rain which originated from the melting of snowflakes—had an entirely different drop distribution from that found in "warm" rain, which evolved without the snowflakes. Cold rain contained relatively few but very large drops, while warm rain contained very many but small drops. The suggestion was made that the large drops in the cold rain resulted from the melting of large snowflakes. With this the study of raindrops came full circle, for Wilson Bentley had made this suggestion half a century before.
Excerpted from From Raindrops to Volcanoes by Duncan C. Blanchard, Berton C. Heinrich Jr.. Copyright © 1995 Duncan C. Blanchard. Excerpted by permission of Dover Publications, Inc..
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