|Home | Welcome | What's New | Site Map | Glossary | Weather Doctor Amazon Store | Book Store | Accolades | Email Us|
Weather Almanac for July 2000
RAINDROPS, SO MANY RAINDROPS
What better time than midsummer to look a little closer at raindrops? As I write this piece, a light rain is falling over southern Vancouver Island. The drops are barely perceptible in the puddle on the flat rooftop of the building next door. And if I were walking out there at this moment, I would feel only the gentlest touch from the falling drops. At the same time, from the central Plains states of the US, come reports of torrential rains and flash flood warnings as a line of severe thunderstorms bullies its way eastward. Similar forecasts are posted for the Great Lakes region. Were I walking along the street in any one of the hundreds of communities there, I am sure I would feel the sting of the drops from those storms.
When we think of raindrops, we tend to have a very narrow vision of what they are, how they look, what their size and composition are. While individually they do not have the delicate beauty of a snow crystal or leave the hard evidence of a hail stone, raindrops hold a spectrum of variability within their watery mass.
The main questions concerning the nature of raindrops surfaced in few meteorologists' minds until the middle of the twentieth century. Even today, you will find few discussions about raindrops in either the popular weather literature or introductory meteorology texts. As far as I have been able to determine, only a handful of researchers focus on the details of the individual raindrop today. Yet, raindrops are as interesting and diverse as that other well studied form of precipitation: snow.
Not until the late nineteenth century did scientists begin to inquire into the true size and shape of raindrops. As is often the case in science, two investigators independently devised methods by which to determine raindrop size and shape. Both began around 1898, and both published their results in 1904, never knowing of the other's work prior to publishing. The two men were: German Philipp Lenard and American Wilson A. Bentley.
Raindrop Shape: No More Tears
Perhaps our biggest misconception about raindrops concerns their shape. Ask anyone to imagine a raindrop, and the odds are high that they will mentally picture a tear-drop shape. Literature, poetry and song are filled with allusions to raindrops as heaven/sky tears. The analogy likely began when someone noticed that when raindrops hit a surface such as a window, they roll off like tears flowing down the cheek, having a round front with a pointed tail. The title of this essay derives from a phrase in a popular song from the 1950s: Raindrops sung by Dee Clark. In it the boy singing assures us that "these must be raindrops, falling from my eyes" and not tears of sorrow for the loss of his girl.
In truth, raindrops are spherical in shape as they begin to fall. Then, unless they are small, they become shaped more like hamburger buns -- flattened base and rounded top -- than teardrops. The distortion is caused by the air flow which pushes against the lower drop surface and thus flattens its base as it falls. This aerodynamic force can further deform the largest drops into a sagging dumbbell shape, eventually causing the biggest ones to split into smaller drops.
In actual fact, even the hamburger-bun shape, which is based on the observations of single drops in a steady flow, is idealized, particularly in heavy rains. As we shall see later, when rain falls, its drops have many different sizes. And each drop size "falls" at a slightly different speed. Indeed, the smallest drops may not fall at all, being suspended or perhaps forced upward by ascending currents of air until they grow large enough to fall. As a result, there are many collisions between raindrops. Some collisions cause distortions in the drops' shapes as they bounce off one another. Others cause drops to coalesce, forming a large drop in the process. And some collisions cause one or both drops to break apart into smaller drops.
If we could isolate single drops in a rainstorm and follow each from its formation until its final splashdown, we would see, not a tear drop or a sphere or even a bun-shaped mass of water throughout its lifetime, but an ever-changing, quasi-spherical shape. And as it falls to earth, our drop may grow in size by collecting other drops or perhaps split apart due to collisions or attaining an unstable size.
Philipp Lenard was the first to publish experimental results showing the actual shape a raindrop took while falling. By suspending drops of known size in a vertical wind tunnel, he was able to determine that small drops to about 2 mm (0.08 inches) "fell" as spheres. Larger drops deformed their shape, having flat bottoms and rounded tops. The largest drops became unstable at diameters greater than 5.5 mm (0.21 inches), lasting less than a few seconds before breaking apart into smaller drops. [For more on Lenard and his study, click here.]
Despite the knowledge that raindrops are not tear-drop shaped, we continue -- yours truly included -- to depict raindrops in that old familiar shape. Perhaps the streamlined shape of the tear drop implies falling motion more than the true shape does. Although I know better, it is likely that I will continue to use artistic license when depicting raindrops for just this reason.
A second common belief about raindrops is that they are all the same size in a given rainfall. As the accompanying table shows, a typical drizzle drop in about 1/5 the diameter of the largest raindrops forming in heavy showers.
However, although different precipitation types may have a characteristic drop size, the distribution of sizes in a rain usually contains a wide range of drop diameters. Small drops generally outnumber large drops, but as the intensity of the rainfall increases, the number of larger drops grows. The very largest drops are found only in downpours with rainfall rates greater than 5 cm (2 inches) per hour.
Although Lenard investigated the size of raindrops using blotter paper dusted with a water-soluble dye as a raindrop collector, he only collected ten field samples and never related drop size distribution to weather conditions. The first evidence for the wide distribution of raindrops sizes within rainfalls was gleaned from a series of experiments carried out in the Northeastern United States by a Jericho, Vermont farmer who also had a passion for photographing snowflakes: Wilson A. (Snowflake) Bentley.
Bentley's apparatus for gathering raindrops was a marvel of simplicity; he collected raindrops in a pan of wheat flour. Bentley had previously found that after exposing a pan of flour to the rain, dough pellets would form under the flour from the combination of the flour with water from the drop. If he let these pellets harden, he could then extracted them from the flour and measure their size. Through careful experiments using known drop sizes, Bentley determined the pellets to be approximately the same size as the falling drops. Thus, he could determine a size distribution of drops from each exposed pan. To better understand the distribution of various sizes of raindrops within a rainfall event, he divided his raindrop fossils from each of the 344 collections into five size categories: very small, small, medium, large and very large.
The next step was to determine if raindrop sizes or their size distribution varied with rainfall conditions. This would require many samples and a record of conditions under which the raindrops fell. Over several years, he amassed a total of 344 separate size distributions sampled from 70 individual storms including 25 thunderstorms. Unlike Lenard, Bentley kept extremely detailed records of the weather conditions under which he collected these raindrops. [For more on Raindrop Bentley's experiment and conclusions, click here.]
Bentley reported the results of his raindrop research in Monthly Weather Review in October, 1904. He found only a few samples (around 7.5%) showed little variation in drop size, being composed mainly of either all small drops or all large drops. Thus, he concluded that drops from all size categories were present in most rainfalls, although smaller drops greatly outnumbered larger ones. Low clouds generally produced few large drops. The largest drops (>5 mm or 0.2 inch) collected fell from thunderstorms that reached high into the atmosphere. Bentley found the maximum raindrop size to be around 6 to 8 mm (0.24 to 0.31 inches) in diameter, in agreement with the findings of Lenard that drops larger than 6 mm usually broke apart.
Unfortunately, Bentley was both ahead of his time and not fully appreciated by contemporary scientists who did not take this innovative, but not formally trained, farmer/scientist seriously. Forty years would pass before his work was finally given serious recognition when in 1943, researchers from the US Soil Conservation Service used the flour pellet technique to measure raindrop size as a function of rainfall intensity. These results when combined with the measurements of others would lead to the development by Canadian scientists Marshall and Palmer of a mathematical relationship between raindrop size distribution and rain intensity. Such relationships have been instrumental in the development of radar as a tool to observe precipitation and determine its accumulation rates.
Purity of Raindrops
A third misconception holds that raindrops are pure water. The fact is, rain would not fall without some degree of impurity in the air to act as a seed. This seed may be a chemical salt, an acid droplet, a speck of dust or soil, or even a bacterium. Cloud droplets or ice crystals which precede raindrop formation require such seeds to readily form from the water vapour in the air. Some seed materials are better than others in forming droplets or crystals. And when the drops formed from these cloud droplets or ice crystals fall earthward, they carry their seeds with them.
To get an idea of the amount of seed material required to form a raindrop, let's look at a simple example. Near the ocean surface, the air is filled with billions of minute salt particles (diameters between 0.5 and 10 micrometres) floating above the water. A typical rain cloud forming above the ocean waters contains billions of cloud droplets, each one likely formed on at least one sea salt or other particle. It takes about a million cloud droplets to provide the water contained in one average-sized raindrop, so we can safely assume that each raindrop will contain the material from a million or more salt particles. The same can be said of a raindrop formed from any other seed type, or combination of seed types.
In addition, as raindrops fall, they can collect chemicals, particles, bacteria, pollen, etc. from the air they pass through. This process is known as scavenging. How well a given raindrop scavenges material from the air around it depends on a number of factors related to both the drops and the potentially scavenged material. Scavenged particles are usually deposited onto the surface on which the drop eventually lands.
Scavenging can be a beneficial process when it cleans the air of "impurities" but the eventual deposition of these impurities on a surface may not always be welcome. In the last 25 years, the term acid rain has rising in recognition of a major environmental problem. ("Pure" rainfall is usually slightly acidic -- pH about 5.8 -- due to the presence of carbon dioxide gas in the drop.) Acid rain, or more correctly acidic precipitation, forms when cloud droplets and raindrops scavenge or use as seeds acidic materials released as air pollutants. When deposited on surfaces such as buildings or falls onto snow and lake surfaces, the acidity accumulates, eventually becoming a hazard to the health of plants and animals and causing damage to materials, particularly marble, limestone and iron.
Raindrops may also collect dust/sand particles or pollen grains in sufficient quantity to give the resulting rain a distinct colour. Reports of storms "raining blood" usually come from rainstorms that have scavenged red dust/sand from air blowing off desert areas such as the Sahara and Australian interior deserts. In ancient literature, such rains were seen as bad omens, portending death or the impending wrath of the gods rather than rain cleansing the air of impurities.
While we rather readily accept the notion that no two snowflakes are the same, the knowledge of large variations among raindrops is not widespread. But if we could see the various sizes and look into the internal contents of raindrops, we would see a world of great variation. Why we might even see a micro-aquarium of tiny life forms swimming in a drop of rain. So next time you are caught out in the rain, remember that other great rain song:
Raindrops keep falling on my head,
Oh, those aren't the right words? Well, you get the idea.
Learn More From These Relevant Books
To Purchase Notecard,
Now Available! Order Today!
The BC Weather Book: