Gene Turetsky, age 11, of Los Angeles, Calif., for his questions
How is rainfall measured?
Measuring rainfall would. be no problem if all the ground were as smooth and solid as glass, The fallen rainwater would then form an even puddle and we could measure its depth with a ruler. If the puddle were an inch deep, then we could report with confidence that one inch of rain had fallen,
But the face of the earth is not smooth and solid as glass, It is scarred with hollows and pimpled with bumps. A good deal of it is surfaced with soil which soaks up the falling rain like a sponge. What's more, there is the wind to consider and the wind is always gusty, blowing the rain here or there.
Measuring the amount of rain which falls is a job full of snags‑‑and it can never be done to perfection. A special instrument is used which measures the amount of rainfall over a small, selected area. This instrument is called‑a rain gauge. An open‑topped barrel will serve as a rain gauge if placed in the open, away from sheltering walls.
The depth of the water in the barrel, measured in inches, gives you the rainfall from a certain shower. This simple rain gauge is not precise enough to suit the weatherman. He needs to know the rainfall to a small fraction of an inch. So his rain gauge is designed to give measurements in ratios.
The professional rain gauge collects the rain from a sizeable area and drains it into a smaller area. The ratio of the large and small areas is known. As a rule, the small area magnifies the depth of the rain ten times. A layer of water one inch deep can be measured far more exactly than a layer of water one‑tenth of an inch deep.
The most common instrument is the eight‑inch rain gauge. Its top is an open funnel exactly eight inches in diameter. The funnel drains the falling rain down into a tube which is exactly 2.53 inches in diameter. In math you learned that the volumes of cylinders vary with the squares of their diameter. In this case, the ratio is one to ten. Every inch of rain collected in the eight‑inch funnel cylinder fills the 2.53‑inch cylinder to a depth of ten inches.
The measuring cylinder of the eight‑inch rain gauge is made 20 inches long. This means that it can give a precise measurement of rainfall up to two inches. The funnel and measuring cylinder are fitted into an outer casing which protects the instrument and catches any overflow.
The ordinary rain gauge needs constant checking. During excessive rainfall the gauge should be checked every ten minutes. Rainfall is rated as excessive when one‑fourth inch falls in five minutes, one inch in an hour, or two and a half inches in 24 hours.