EAA Sport Aviation - March 2011

HOW TO INTERPRET RADAR IMAGES
Radar Before
Nexrad

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
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.

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.

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.