Steep gradient from green to yellow is
cause for concern
Narrow bands of green and yellow
around orange, red, and magenta
signifies a strong thunderstorm
In the cockpit, Nexrad uses a four-color system, like the one on this
Avidyne EX500. Radar images available through online subscription
services show as many as 19 different intensity levels indicated by
various colors, similar to the image below from Baron Wx Worx.
In cockpit view
Again, there is no absolute answer to what
flight conditions are like in areas of green or
yellow Nexrad returns because of the power
and sensitivity of the radar ground stations.
Each Nexrad station can take more than five
minutes to complete a 360-degree sweep as
the antenna looks from the horizon and then
far up into the atmosphere analyzing each slice
of airspace for precipitation targets. Many
times Nexrad will find layers of precip beginning thousands of feet up but with no precip
reaching the lower altitudes where weather
conditions can remain perfectly good VFR.
I have flown many, many miles through
areas of level one green Nexrad returns
(image 1) without the airplane hitting a drop.
The precip is really up there, but the atmosphere at the lower levels is dry so the
moisture evaporates as it descends. But other
times the exact same green Nexrad return is
from an area of steady rain and often low
clouds that would be a disaster for a VFR-only
pilot. There is no way to know for sure what
kind of precip and weather a green target
actually represents. The IFR pilot can plunge
in, but the VFR pilot needs to approach all
Nexrad targets of any level with caution and
be prepared to make big course changes, or
even reverse course, if the target turns out to
be from below VFR conditions weather.
WHAT TO DO ABOUT YELLOW RETURNS?
When it comes to the middle level two yellow targets, the most important indicator of
significant turbulence is the shape of the
yellow target area, and the so-called gradient. Often Nexrad will show flecks of yellow
in broad areas of green (image 2), and that is
not usually an indication of significant turbulence. Or, if there is a fairly large yellow
area displayed in a much bigger target of
green (image 3), you are almost certainly not
seeing a thunderstorm unless you have some
other reason—such as the presence of lightning—to believe convective activity exists.
But a steep gradient from green to yellow (image 4) is cause for concern. Gradient
refers to the distance between the change
from one level of intensity to the next on the
display. With a “steep” gradient the targets
jump from level one to two or three in a very
short distance, and that is an indicator of
convective activity. A signature of a strong
thunderstorm is to show very narrow bands
of level one and two around an area of level
three or four on the display (image 5). The
steepest gradient on the display is typically
on the leading edge of the storm with a shallower gradient trailing behind.
While a steep gradient is always something
to avoid when the core of the target is level
three or four, a steep gradient from level one
green to yellow can also indicate the presence
of more turbulence than you want to endure.
The actual Nexrad target intensity that
each level on the airborne display shows can
vary from one display system to another.
Nexrad radars rank the intensity of an echo
using a measure called dBZ, which is a measure of the power of the radar energy
reflected back to the antenna. The actual
Nexrad system uses as many as 19 different
thresholds to change display color to show
the dBZ of a target. You can see all of these
levels when you dial into individual Nexrad
stations on the Internet (image 6).
But for airborne display the avionics makers decided to show only three, or four, levels
of intensity because that is what airborne
weather radars show. Not every display sets
the dBZ level exactly the same to transition
from green to yellow, for example, so on one
system level one green may be very light precip, but on another brand green may
represent more significant rain. The same
differences can exist for changeover to the
other levels. So if your buddy says, “I fly
through green and yellow targets all the time,
and it’s not bad,” make sure you both have the
same display system before you swap advice.
NEXRAD TAKES TIME
Another limitation of satellite-delivered
Nexrad is what the weather types call latency.
In other words, the radar image can be fairly
old by the time it appears in your cockpit.
Part of the reason for this display delay is
that the Nexrad radars scan slowly, needing
five minutes or more for one sweep. So the
area scanned first in a sweep is five minutes
old when the sweep is complete.
The other delay is caused by the time it
takes to assemble the Nexrad images into a
mosaic picture for transmission to your display. There are more than 150 individual