Garmin GTN 725 Pilot's Manual page 339

12.3.2 Radar Signal Reflectivity
12.3.2.1 Precipitation
Precipitation or objects more dense than water, such as earth or solid
structures, will be detected by the weather radar. The weather radar will not
detect clouds, thunderstorms or turbulence directly. It detects precipitation
associated with clouds, thunderstorms, and turbulence. The best radar signal
reflectors are raindrops, wet snow or wet hail. The larger the raindrop the
better it reflects. The size of the precipitation droplet is the most important
factor in radar reflectivity. Because large drops in a small concentrated area are
characteristic of a severe thunderstorm, the radar displays the storm as a strong
return. Ice, dry snow, and dry hail have low reflective levels and often will not be
displayed by the radar. A cloud that contains only small raindrops, such as fog
or drizzle, will not reflect enough radar energy to produce a measurable target
return.
190-01007-03 Rev. D
Figure 12-46 Precipitation Type and Reflectivity
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