Janet Kenkel, age 12, of Le 'bars, Iowa, for her question:
How many drops are in an inch of rainfall?
This poses a problem somewhat like trying to measure the contents of a swimming pool with a spoon a very small spoon. No doubt one member of the family can state the contents in gallons. This person would likely be the one who watches the meter and nays the water bills. Other family members know the depth of water at both ends, plus the width and length of the pool.
Rainfall, naturally, descends from the clouds in drops. But the drops may be whopping splashers, fine misty drizzles or some size between these soggy giants and midgets. What's more, a shower may shed just a few big splashers or just a few misty sprinkles. Another downpour may deluge down zillions of whoppers or a fine misty rain that does on for days and days.
To a weatherman, the day's rainfall means the depth of water that falls on the around as if none sinks down or drains away. Obviously he cannot depend on drops that measure anywhere from five to five thousand to the inch. He sets up scientific equipment to trap samples of falling rain in selected spots. He measures the depth of this captured rainfall in inches and fine fractions of inches.
An amateur can take a rough and ready measurement with a ruler and an open topped can. Place the can in a sheltered spot, protected from gusty breezes. When the sun comes out, use the ruler to measure the depth of the water in the can, naturally a larger can catches more water, but the depth of rainfall is the same as in .a smaller one.
Suppose your trapped rainfall is one inch deep. It would not help to measure the contents of your make shift rain gauge in drops either large or small, ones. This would not include the rain that fell all over your back yard and perhaps over the whole county. A weatherman solves this problem by estimating the depth of water in many sample rain gauges, placed throughout a large area. His estimate is the depth of rain that fell.
It is possible to estimate the weight of each inch of rainfall over large areas and these figures are downright stunning. A long day's downpour can dump an inch of rainfall over several states. In this case about 113 tons of water falls on every acre of city and iogmiond. One inch of rainfall on every square mile weighs 72,300 tons. And one inch of rainfall on your entire home state of Iowa weighs more than four billion tons.
In everyday language, a drop is the smallest pourable amount of liquid. This is too vague for fine figuring. Druggists use a more precise drop measurement called the ''minim". There are 480 minim drops in an ounce. You could multiply minims, ounces and tons to find the number of drops in an inch of rainfall over, say an acre. But the weatherman's tray of measuring the death of rainfall in inches is neater and less complicated.