Garmin GTN 625 Pilot's Manual page 407

Rainfall data is color coded as follows:
Figure 11-73 FIS-B Weather Precipitation Legend
Gray shaded areas indicate that data is not available or that no data has been
received for a requested area. Gray shades may also indicate that the rainfall rate
for a given area is undetermined.
The FIS-B Function is based on a ground-to-air data link and requires that
the appropriate ground systems are broadcasting weather data and the aircraft is
within reception range of the Ground Broadcast Transceiver (GBT).
190-01004-03 Rev. P GTN 625/635/650 Pilot's Guide
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11-47