Welcome to You Ask Andy

 

Barbara Hershorn, age 8, of Ottawa, On, for her question:

How do they measure rainfall?

The weatherman reports the rainfall in inches. He may say that ono inch of rain fell yesterday. This moans that enough rain f©11 to cover the ground with one inch of w7ter. But, of course, the rain does not sit on the ground waiting to be measured in neat inches. Some of it sinks into the soil. Some gushes away in streams. Some runs down slopes and. some gathers in hollows to make puddles.

What's morn, the rain may be gusty, blown slantwise in the wind. One side of a building may be very wet, the other almost dry. The weatherman must get an overall picture of the rninfall. And he must get it before the rain water runs away.

He uses a special gadget for the job called a rain gauge. The instrument catches and measures the rain as it falls. It is always put in an open place, away from trues and buildings which might keep off some of the rain.

You can make a simple rain gauge for yourself. Use a tin can with an open top. Draw straight line down the side from top to bottom and mark it off in inches, half and quarter inches. Set it in an open place and wait for the rain.

Go look at your rain gauge when the rain his stopped. There will be water in the bottom of the can. For the can collected the rain which fell over a small nrec and prevented it from running away. If the water reaches the first inch mark on the scale, then you have one inch of rainfall.

The export weathertnon needs a more refined rain gauge. The rainfall must be kept from evaporating and the scale for measuring must be more detailed. This refined rein gauge is an open funnel on top of a narrow cylinder. The cylinder is a hollow tube marked off in inches and fractions.

The most common is the eight inch gauge. The round mouth of the funnel is exactly eight inches across. It gathers all the raindrops falling in a circular area eight inches in diameter. This rain runs down into the hollow tuba below. There it is protected from the drying air.

But the width of the tuba is narrower than the funnel. The water in the cylinder will be: higher than it would be in the wide funnel. Surely this gives an exaggerated idea of the amount of rain which actu^lly fall. lend so it does.

Suppose the cylinder is exactly 2.53 inches wide. Ten inches of wFter in the 2.53 inch cylinder is than equal to only one inch in an eight inch cy linder. One inch of rain, then, falling through the eight inch funnel registers ten inches on the rain gauge cylinder .The weatherman must than divide the wcter level by ton. This helps him got a pore occurate picture of the true rainfall.

Most weatherman in the U.S. use the eight inch gauge with the 2.53 cylinder. In Canada the eight inch gauge is more often used with a 3.5 cylinder. In both cases, the smaller cylinders give nn exaggerated picture of the rainfall. This gives the weatherman a deeper column of water to measure, from which he can estimate the fractions of an inch mare accurately.

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