The most common form of precipitation, rain, is the easiest to measure. Any open container having a consistent cross section throughout can be used as a rain gauge (▼A). In practice, rain gauges are designed to measure small amounts of rainfall accurately and to reduce loss from evaporation.

A standard rain gauge has a diameter of about 20 centimeters (8 inches) at the top (▲B). When the water is caught, a funnel conducts the rain into an approximately 50-centimeter (20-inch) tall cylindrical measuring tube that has a cross-sectional area only one-tenth as large as the receiver. Consequently, rainfall depth in the standard rain gauge 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, it is reported as a trace of precipitation.

As ▲C illustrates, the tipping-bucket gauge consists of two compartments, each one 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. Tipping-bucket gauges are widely used due to their high accuracy in measurements and because they are self-emptying, allowing continuous measurement of rainfall.

Measuring Snowfall

Snowfall is typically measured by 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, a core taken from the snow may be weighed, or snowfall captured in a gauge may be melted and then measured as though it were 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  inch of water.

Precipitation Measurement by Weather Radar

Using weather radar, the National Weather Service (NWS) produces maps like the one in Figure 17.35, in which different colors illustrate precipitation intensity. Weather radar gives meteorologists an important tool to track storm systems and the precipitation patterns they produce, even when the storms are as far as a few hundred kilometers away.

Radar units have transmitters that send out short pulses of radio waves toward a storm system. The specific wavelengths selected depend on the objects being detected. Wavelengths between 3 and 10 centimeters are employed when monitoring precipitation. Radio waves at these wavelengths can penetrate clouds composed of small droplets, but they are reflected by larger raindrops, ice crystals, and hailstones. The reflected signal, called an echo, is received, and displayed as a reflectivity map. Because the echo is “brighter” when the precipitation is more intense, modern weather radar is able to depict both the regional extent and the rate of precipitation. Also, because the measurements are in real time, they are particularly useful in short-term forecasting.

• Two instruments commonly used to measure rain are the standard rain gauge and the automated tipping-bucket gauge. The two most common measurements of snow are depth and water equivalent.

• Modern weather radar has given meteorologists an important tool to track storm systems and precipitation patterns, even when the storms are as far as a few hundred kilometers away.

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