Section 5.6:
Precipitation Measurement

Learning Objective

Describe the instruments used to measure precipitation, including the standard rain gauge, snow pillow, and weather radar.

Section Content

The most common form of precipitation, rain, is probably the easiest to measure. Any open container that has a consistent cross-section throughout can be a rain gauge (Figure 5.26A). In general practice, however, meteorologists use more sophisticated devices to measure small amounts of rainfall more accurately and to reduce loss from evaporation.

Figure 5.26
Precipitation measurement

A. The simplest gauge is any container left in the rain.
B. The standard rain gauge increases the height of water collected by a factor of 10, allowing for accurate rainfall measurement to the nearest 0.025 centimeter (0.01 inch). Because the cross-sectional area of the measuring tube is only one-tenth as large as the collector, rainfall is magnified 10 times.
C. The tipping-bucket rain gauge contains two “buckets,” each holding the equivalent of 0.025 centimeter (0.01 inch) of liquid precipitation. When one bucket fills, it tips, and the other bucket takes its place. Each event is recorded as 0.01 inch of rainfall.

Video - Global Precipitation (Click to watch the video)

Measuring Rainfall

The standard rain gauge (Figure 5.26B) has a diameter of about 20 centimeters (8 inches) at the top. Once the water is caught, a funnel conducts the rain through a narrow opening into a cylindrical measuring tube that has a cross-sectional area only one-tenth as large as the receiver. Consequently, rainfall depth is magnified 10 times, which allows for accurate measurements to the nearest 0.025 centimeter (0.01 inch). When the amount of rain is less than 0.025 centimeter (0.01 inch), it is generally reported as being a trace of precipitation.

In addition to the standard rain gauge, several types of recording gauges are routinely used. These instruments not only record the amount of rain but also its time of occurrence and intensity (amount per unit of time). Two of the most common gauges are the tipping-bucket gauge and the weighing gauge.

As Figure 5.26C illustrates, the tipping-bucket gauge consists of two compartments, each capable of holding 0.025 centimeter (0.01 inch) of rain, situated at the base of a funnel. When one “bucket” fills, it tips and empties its water. Meanwhile, the other “bucket” takes its place at the mouth of the funnel. Each time a compartment tips, an electrical circuit is closed, and 0.025 centimeter (0.01 inch) of precipitation is automatically recorded on a graph.

A weighing gauge collects precipitation in a cylinder that rests on a spring balance. As the cylinder fills, the movement is transmitted to a pen that records the data.

All rain gauges are susceptible to inaccuracies. The tipping-bucket rain gauge is known to underestimate heavy rainfall by perhaps 25 percent because of the rainwater that is not collected during the tipping movement of the bucket. Also, wind can lead to measurement errors, by causing either too much or too little precipitation to enter the collecting container. In addition, because a rain gauge provides data for a specific location, the significant variations in the amount of precipitation that reaches the ground over a large area cannot be accurately estimated.

Measuring Snowfall

When snow records are kept (see Severe & Hazardous Weather Box 5.1), two measurements are normally used: depth and water equivalent. One way to measure the depth of snow is by using a calibrated stick. The actual measurement is not difficult, but choosing a representative spot can be. Even when winds are light or moderate, snow drifts freely. As a rule, it is best to take several measurements in an open place, away from trees and obstructions, and then average them. To obtain the water equivalent, samples may be melted and then weighed or measured as rain.

Severe & Hazardous Weather 5.1

Worst Winter Weather

Extremes, whether the tallest building or the record low temperature for a location, fascinate many people. When it comes to weather, some places take pride in claiming to have the worst winters on record. In fact, both Fraser, Colorado, and International Falls, Minnesota, have proclaimed themselves the “ice box of the nation.” Although Fraser recorded the lowest temperature for the 48 contiguous states 23 times in 1989, neighboring Gunnison, Colorado, recorded the lowest temperature 62 times, far more than any other location.

Such facts do not impress the residents of Hibbing, Minnesota, where the temperature dropped to −38°C (−37°F) during the first week of March 1989. But this is mild stuff, say the old-timers in Parshall, North Dakota, where the temperature fell to −51°C (−60°F) on February 15, 1936. Not to be left out, Browning, Montana, holds the record for the most dramatic 24-hour temperature drop. Here the temperature plummeted 56°C (100°F), from a cool 7°C (44°F) to a frosty −49°C (−56°F) during a January evening in 1916.

Although impressive, extreme temperatures represent only one aspect of winter weather. What about snowfall (Figure 5.A)? Cooke City holds the seasonal snowfall record for Montana, with 1062 centimeters (418.1 inches) during the winter of 1977–1978. And cities like Sault Ste. Marie, Michigan, and Buffalo, New York, can accumulate several feet of snow in a single storm due to the legendary snowfalls associated with the Great Lakes. Even larger snowfalls occur in many sparsely inhabited mountainous areas.

Figure 5.A

A winter snowstorm of historic proportions struck Chicago, Illinois, on February 2, 2011.

Try telling residents of the eastern United States that heavy snowfall alone makes for the worst weather. A blizzard in March 1993 produced heavy snowfall along with hurricane-force winds and record low temperatures that immobilized much of the region from Alabama to the Maritime Provinces of eastern Canada. This event quickly earned the well-deserved title Storm of the Century.

Weather Safety

The National Weather Service (NWS) issues many different weather alerts to warn people of potential hazards. An Advisory is issued if the forecast indicates an inconvenience that could become hazardous. A Watch is issued when there is an increased risk for a hazardous weather, but the likelihood of occurrence, location, or timing is still uncertain. When a Warning is issued, hazardous weather is occurring or will occur shortly. A Warning indicates that the hazardous weather poses a threat to life or property. Here are the meanings of some common NWS alerts for winter weather events.

Winter Weather Advisory

An accumulation of snow, sleet, and/or ice that will cause an inconvenience.

Winter Storm Watch/Warning

Significant snow or sleet in a 12- to 24-hour period, and/or ice accumulation capable of causing damage to trees or powerlines, and/or a combination of wind with snow or ice that could threaten life or property.

Blizzard Warning

Snow and/or blowing snow that reduces visibility to ¼ mile or less for at least 3 hours and winds of at least 56 kilometers (35 miles) per hour. There are no temperature criteria for a blizzard.

Freezing Rain Advisory

Ice accumulation of ¼ inch or less that could be dangerous.

Ice Storm Warning

Ice accumulation of at least ¼ inch that will cause dangerous conditions for both drivers and pedestrians. Power lines and limbs will likely be damaged.

Wind Chill Advisory/Warning

A combination of low temperatures and high winds will cause dangerous conditions for those not dressed properly. A Wind Chill Warning is issued when conditions are life threatening and any exposed skin will freeze quickly.

Freeze Watch/Warning

Temperatures below freezing are expected over a large region. This warning is issued only during the growing season to warn farmers of impending crop loss.

Frost Advisory

This is issued when the low temperature will be near freezing on a clear, calm night. This alerts homeowners to bring in or cover outdoor plants.

The quantity of water in a given volume of snow is not constant. You may have heard media weathercasters say, “Every 10 inches of snow equals 1 inch of rain.” But the actual water content of snow may deviate widely from this figure. It may take as much as 30 inches of light and fluffy dry snow (30:1) or as little as 4 inches of wet snow (4:1) to produce 1 inch of water.

To measure the mountain snowpack, which produces 75 percent of the water supply for the western United States, automated weather stations employ snow pillows that have been installed at more than 600 sites. A snow pillow typically consists of two, three, or four large panels that measure the pressure exerted by the weight of snow that collects on them. Because snow pillows have large surface areas and measure the water equivalent of snow, they provide a good estimate of available water when the spring melt arrives.

Precipitation Tracking by Weather Radar

Using weather radar, the National Weather Service (NWS) produces maps like the one in Figure 5.27, in which colors illustrate precipitation intensity. The development of weather radar has given meteorologists an important tool to track storm systems and the precipitation patterns they produce, even when the storms are as far away as a few hundred kilometers.

Figure 5.27
Doppler radar display produced by the National Weather Service

Colors indicate different intensities of precipitation. Note the band of heavy precipitation along the U.S. Eastern Seaboard.

Radar units have transmitters that send out short pulses of radio waves. The specific wavelengths selected depend on the objects being detected. When radar is used to monitor precipitation, wavelengths between 3 and 10 centimeters are employed. At these wavelengths, radio waves can penetrate clouds composed of small droplets, but they are reflected by larger raindrops, ice crystals, and hailstones. The echo intensity, called radar reflectivity, is converted to decibels (dBz) and displayed on a map. Because the echo is more intense when more precipitation is present, modern weather radar can depict both the rate of precipitation and its regional extent. Also, because the measurements are in real time, they are particularly useful in short-term forecasting.

Despite its usefulness, weather radar does not always show what is occurring at Earth’s surface. In order to avoid “ground clutter” such as trees and buildings, the radar signal is directed slightly upward at an angle. As a result, radar detects only precipitation very high up at locations far from the radar unit. For example, radar may detect precipitation high in the atmosphere that doesn’t reach the surface (virga), or it may miss precipitation actually hitting the ground from low clouds. Occasionally, a radar unit will receive echoes produced by dense swarms of insects or birds. In addition, conventional radar systems cannot distinguish rain (liquid water) from solid forms of precipitation. Fortunately, the National Weather Service has upgraded most of its local forecast centers to dual polarization radar, which transmits both horizontal and vertical pulses and gathers significantly more data. This information helps forecasters distinguish rain, hail, snow, or sleet from other flying objects, including tornado debris.

Video - Record-Breaking Hailstorm as Seen by Radar (Click to watch the video)

Section Glossary

Section Summary

Section Study Questions

Try to answer the following questions on your own, then click the question to see the correct answer.

Although any open container can serve as a rain gauge, what advantages does a standard rain gauge provide?

The standard rain gauge magnifies the actual rainfall depth 10 times. This allows for accurate measurement to the nearest 0.025 cm or 0.01 in.

Why is it important to collect both snow depth and water content?

The quantity of water in a given volume of snow is not constant. It may take as much as 30 in. of light and fluffy dry snow (30:1) or as little as 4 in. of wet snow (4:1) to produce 1 in. of water.

Why is weather radar useful in detecting precipitation?

Weather radars send out short pulses of radio waves that are reflected by large raindrops, ice crystals, and hailstones. These “echoes” are brighter when precipitation is more intense, so modern radar can depict both the regional extent and rate of precipitation. Weather radar can also measure the rate and direction of storm movement.