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

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.

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.

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.

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.

• snow pillow: An instrument consisting of a panel on the ground that measures the weight of snow that collects on it.

• standard rain gauge: A gauge that has a diameter of about 20 centimeters and funnels rain into a cylinder that magnifies precipitation amounts by a factor of 10, allowing for accurate measurement of small amounts.

• tipping-bucket gauge: A recording rain gauge consisting of two compartments (“buckets”), each capable of holding 0.025 centimeter of water. When one compartment fills, it tips, and the other compartment takes its place.

• trace of precipitation: An amount of precipitation less than 0.025 centimeter.

• weather radar: An instrument run by the National Weather Service that is used to create maps showing the location and movement of precipitation.

• weighing gauge: A recording precipitation gauge consisting of a cylinder that rests on a spring balance.

• The instruments most commonly used to measure rain are the standard rain gauge, which is read directly, and the tipping-bucket gauge and the weighing gauge, which record the amount and intensity of rain. The two most common measurements of snow are depth and water equivalent.

• Weather radar gives 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.