Rain is made of drops. That statement may seem trivial, but the consequences of the discrete nature of rain are hardly insignificant. For example, radar measurements of storm intensity, often presented on maps as shown in figure 1, only work because rain falls as drops. Radar sensors—on ground-based instruments and from satellites—estimate the amount of precipitation from the intensity of echoes reflected off the raindrops. Because of their reliance on an indirect measurement, radar measurements can be inaccurate. Nonetheless, the technique allows scientists to study spatial variability over a range of about 500 km; for larger distances, the return echo is too noisy. In contrast, the collection area of rain gauges and the disdrometers that measure the size and speed of falling drops is roughly 50–1300 cm2.
The big-picture view of radar helps in many hydrological applications. For example, radar measurements of the spatial distribution of rainfall...
