"Tom S." wrote in message
...

How is it (Nexrad) different?

Nexrad and radar are similar but Nexrad is better than radar at showing
intensities inside a given storm cell.

An isolated radar or Nexrad site base reflectivity return shows ground
clutter and other artifacts. For example, radar and Nexrad get less
sensitive at the fringes of a given radar site's coverage area.

When we talk of "Nexrad" we are usually talking about the composite national
Nexrad image. That image is created by digitally processing the images from
multiple sites into a national mosaic -- this process is usually partly
automated and partly done manually, but in any event there is some
subjectivity involved.

The end result is that a composite Nexrad image usually is easier to
interpret than comparing isolated radar sites. And Nexrad weather datalink
allows a pilot to view the big picture of the whole country rather than just
the coverage area of an airplane's onboard radar.

The primary disadvantage of Nexrad composite images compared with onboard
radar is that Nexrad provides no information regarding how high the
reflected precip goes. So what appears as precip on Nexrad may really be
no more than low-level mist; I have seen this often in flight.
Airplane-mounted radar does not have nearly the range of composite Nexrad
but does have the advantage that the pilot can tilt the radar's energy up or
down and thus -- with training -- calculate how high precipitation goes.

There is a solution to the problem of calculating Nexrad precip heights --
NOAA maintains a national network of doppler precip tops radar.
Unfortunately this government-generated precip tops radar data is not
available to FSS (!), but several subscription websites and some of the
weather datalink vendors do indeed have make available the precip tops radar
data.



--
Richard Kaplan, CFII

www.flyimc.com