Most likely the answer. Check the DBZ scale, if it starts with negative
numbers the radar is in clear air mode. In precip mode it starts at 5 dbz.

"Mick Ruthven" wrote in message
...
I've never used in-aircraft Nexrad. But the Nexrad images displayed on the
web by the National Weather Service has two modes, Precipitation and Clear
Air. When there isn't significant precip they use Clear Air mode that is
much more sensitive and will displays colors without any precip. Don't
know
if that applies to the in-aircraft version.