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# How to Determine the Effect of Wind on Measuring Rainfall

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Author: Janice VanCleave

### What You Need to Know

A gauge is a scale of measurement. A rain gauge is a device used to collect and measure rainfall.

### How Does a Rain Gauge Work?

Rainfall measurement is the depth of accumulated rainfall in a period of time. Because rain generally sinks into the ground, runs off the surface into streams, collects in low areas, or evaporates, the amounts of rainfall cannot be measured with accuracy in natural places. Instead, a rain gauge is used. A rain gauge is usually a cylinder with a scale in inches or millimeters on its side. The diagram below shows two types of rain gauges, one with a funnel-shape top and the other with straight sides.

For a gauge like rain gauge A, the actual height of the rain collected in the cylinder is equal to the amount of rainfall. Rain gauge A shows 1 inch (2.5 cm) of water, thus the rainfall measurement is 1 inch (2.5 cm). Rain gauges with funnels, like rain gauge B, are used to collect and measure small amounts of rainfall. In gauges with a funnel top, the height of the water in the cylinder does not equal the amount of rainfall. Instead, the ratio of the diameter of the cylinder to the diameter of the funnel is used to make the measurement. To determine the amount of rainfall per 1 division on the scale of the rain gauge: (1) write down the ratio, (2) express the ratio as a fraction, and (3) divide the denominator of the fraction into the numerator. For example, if the ratio is 1:10, the scale would be determined as follows:

• 1:10 = 1 inch of rainfall/10 inch height of water in the cylinder
•         = 0.1 inch of rainfall / 1 inch height of water in the
•            cylinder

Rainfall is usually described as either light, moderate, or heavy. Light rainfall is less than 0.10 inches (0.25 cm) of rain per hour. Moderate rainfall measures 0.10 to 0.30 inches (0.25 to 0.85 cm) of rain per hour, and heavy rainfall is more than 0.30 inches (0.85 cm) of rain per hour.

### What Does This Have to Do with the Effect of Wind on Measuring Rainfall?

A rain gauge measures the amount of rainfall in a specified period of time. This means the amount of rain that would accumulate on a level surface if none of the rain soaked in, ran off, or evaporated. Catching rain that falls vertically is not a problem. But what about rain that is being blown by the wind and falls at an angle? Does this affect the amount collected?

### Fun Fact

One inch (2.5 cm) of rainfall produces 4.7 gallons (17.9 L) of water per square yard or 22,650 gallons (86,070 L) of water per acre.

### Real-Life Science Challenge

It's a challenge to measure rainfall at sea where it must be measured on ships. The motion of the ship presents a problem. Special rain gauges have been designed to improve the accuracy of rainfall measurement on moving ships, but better methods are still needed.

### Experiment

Now, start experimenting with determining the effect of wind on measuring rainfall.

Hints

• Design and build a rain gauge.
• A spray mister can be used to simulate rainfall.
• A fan can be used to simulate wind.