Experiment

Measuring the amount of rain that falls is a very important step in managing fresh water supplies for farming and other uses. This SEED Engineering Challenge is to design and construct a rain gauge that will measure rainfall as accurately as possible. Read this short history of rain gauges, then make one for yourself.

Measuring Rainfall : A Brief History

People have measured rainfall for thousands of years. The earliest known records were taken in Greece around 500 BC. A century or so later, Indian rulers sent bowls to the villages in their kingdom as an official tool to measure the farmers’ potential harvest. These rainfall measurements were then used to determine what the farmers’ land taxes should be.

The Standard Rain Gauge

The most common rain gauge – currently used by official forecasters and at airports – was invented over 100 years ago. It consists of a large cylinder with a funnel and a smaller measuring tube inside it. The “official” rain gauge specified by the United States Weather Bureau is a 50 cm tall cylinder with a 20 cm diameter funnel. Water is collected in a measuring tube that has exactly one-tenth the cross-sectional area of the top of the funnel. As a result, the height of the water collected in the measuring tube is precisely ten times what it would be if it had been collected in the cylinder alone. For example, one-tenth of a centimeter of rainfall would fill one centimeter of the measuring tube. This exaggeration of the height of water in the tube enables meteorologists to make more precise rainfall measurements. A special measuring stick inserted into the measuring tube is scaled to take the exaggeration into account.

The standard rain gauge can measure up to 5 cm (1.97 in) of rain. If rainfall exceeds five centimeters, water overflows into the cylinder surrounding the measuring tube. To find the total rainfall, the observer empties the 5 cm in the full measuring tube, then takes the water in the cylinder and very carefully pours it into the now-empty tube. That measurement added to the five cm gives the final rainfall amount.

Knowing When It Rained

The rain gauge described above is very accurate, but measurements are made only when someone empties out the water. This might be just once a day or even less. One method of recording the time during which it rains is to use a mechanical drum recorder. Inside the water collection cylinder there is a float attached to a pen. If the water level in the cylinder rises, the float is lifted and the recording pen traces the change in height on a chart. The chart is attached to a clockwork-powered drum that rotates very slowly. When the collection cylinder is full, it automatically empties through a siphon tube into a larger collecting can below. When this happens, the recording pen sinks from the maximum back to the zero line of the chart and, if it continues raining, begins to record a rising line again. The chart must be replaced regularly – normally weekly or monthly – at which time the contents of the collecting can are also measured.

In 1622, the British architect Sir Christopher Wren designed the first tipping bucket rain gauge. This device punches a hole in paper tape after a particular volume of rainfall. The tipping bucket rain gauge is still widely used today, but it uses electronic measuring devices instead of paper tape to record the volume and time of rainfall.

Chart from a mechanical recording drum rain gauge
​The Tipping Bucket Rain Gauge

The tipping bucket rain gauge records the time when one of two specially designed buckets tip, which happens when a particular volume of water falls into it (typically 0.1 cm or 0.1 inches). When one bucket tips, the other bucket quickly moves into place to catch the next unit of rainfall. Each time a bucket tips, an electronic signal is sent to a recorder linked to a clock. In most tipping bucket gauges, water drains out of the bottom, so it does not need to be manually emptied. This device makes it possible to determine how much rain fell during certain time periods without anyone actually being present at the weather station. In addition to knowing the volume of rain that has fallen over a period of time, it is also useful to know its intensity. For example, five centimeters falling throughout a day would probably drain safely away, but the same amount falling within one hour would likely cause a flash flood.

The tipping bucket rain gauge is especially good at measuring light-to-medium rainfall. During a very light shower, rain collected in a bucket might not be sufficient to tip it and could evaporate before more is added. During very heavy rain events, such as thunderstorms, water may continue to pour into the bucket while it is emptying, before the next bucket moves into place. As a result, the tipping bucket rain gauge underestimates the rainfall. Failure can also result from the tipping mechanism getting jammed, often by a spider’s web. In addition, hail, snow, birds’ nests, insects, spiders’ webs, and leaves may block the funnel, leading to overflow. For this reason, it is common to place two independent gauges side by side, so that errors can be quickly detected and corrected.

The Weighing Rain Gauge

Another variety is the weighing rain gauge. It consists of a container sitting upon a scale. The scale is adjusted for the container and measures the weight of the collected rainwater.