HOW TO INTERPRET RADAR IMAGES
On the left, the varying degree of brightness or storm
intensity is hard to distinguish. On the right, in contour
mode, the area of greatest intensity is blacked out,
making it easy to see.
Early weather radars used a TV-style indicator
called a display storage tube. As the radar
scanned, the display would light up for a
few seconds to show the location of echoes,
but the picture faded away until the next
antenna sweep. The brighter the image on the
display, the more intense the echo. But it was
difficult to tell what areas of the display were
really bright, therefore showing the strongest
storms, which were merely pretty bright.
The solution was to black out the brightest
THE VFR PILOT NEEDS TO APPROACH ALL NEXRAD
areas of the display so pilots could see
a contour of the less bright around
the strongest echo. When a storm was
“contouring” on radar, it was very strong and
it was time to head the other way.
TARGETS OF ANY LEVEL WITH CAUTION AND BE
PREPARED TO MAKE BIG COURSE CHANGES, OR
EVEN REVERSE COURSE.
Nexrad stations, and some are the new,
more advanced high-definition type. Over
most areas of the contiguous United
States, the radar beams from the network
overlap, providing a very detailed radar
view and also limiting the impact of failure of individual radars.
Private companies—XM Weather and
WSI being the two mostly widely available
for pilots—use software algorithms to
piece together returns from the individual
radar sites into a mosaic that represents
the radar returns over the entire country.
That takes some time before it is ready to
be sent to the satellites for transmission
down to us. And then the transmission
itself takes time to reach our cockpit.
The various steps in the process of creating the image we see add up to around
10 minutes or a little more. That delay is
not a big deal for generalized rain systems, but when the air is unstable and
thunderstorms are popping, a lot can
change in 10 minutes.
The significance of the latency for us as
pilots is that we can’t use Nexrad to plan
close-in storm avoidance because the
weather probably moved or changed since it
was scanned by the Nexrad ground station.
Airborne weather radars complete a scan of
the weather ahead every few seconds so you
are looking at real-time conditions, but with
satellite-delivered Nexrad mosaic pictures
we are seeing recent history.
For all of these reasons satellite Nexrad
is a planning tool. A great planning tool, the
best we have ever had, but unless we are
flying in visual conditions and can see the
building storm clouds and avoid them, we
need to use Nexrad to miss the strong
echoes by several miles. It has been my
experience that flying into clouds that show
no Nexrad return but that are associated
with convection nearby is invariably turbulent. The bumps may not be strong enough
to break the wings off, but you will sure
wish you were flying somewhere else.
In spite of Nexrad’s limitations I would
not consider making IFR trips in thunderstorm season without it. After so many
hours of blundering along with such limited
information, Nexrad takes the scary out of
IFR flying and gives you the tools to avoid
the bad stuff by a lot of miles.
J. Mac McClellan, EAA 747337, has been a pilot for
more than 40 years, holds an ATP certificate, and owns
a Beechcraft Baron.