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A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.") It's part of what makes aviation so exciting for all of us... just when you think you've seen it all, along comes a scenario you've never imagined.

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Aero-Tips 04.11.06

Radar is most closely identified with thunderstorms. But radar images do not tell us where clouds are stormy -- they only show precipitation. How can we accurately use radar reports, and how do we use radar returns to avoid thunderstorms?

Radar returns

All "official" aviation radar depictions and uplinked weather plots are derived from ground-based radars. Antennae send out strong electric pulses that reflect back when they encounter resistance; each radar transmitter can also receive, and therefore detects those signals that have been reflected back. Radar frequencies can be set for optimal return when hitting rain or snow.

Not every bit of the radar energy reflects back. Some will penetrate the precipitation until reflected back from a greater range. This allows radar plots to show precipitation at some depth into the area, instead of merely drawing a line at the outer edge of rain or snow. But some energy is absorbed in the process, either "on the way in" to the precipitation or after reflection "on the way out" en route to the receiver. Loss of radar signal is called attenuation.

Note: Airborne radar, for those pilots blessed with such systems, is subject to the same attenuation.

Consequently radar shows the outer edges of precipitation quite well, with varying success plotting "returns" inside the outer edge. In strong precipitation attenuation will leave gaps or "black holes" on the screen behind plotted returns-not because there's no activity there, but because all the radar energy was absorbed by the outer lines of precipitation. Some ground-based radar plots artificially "fill in" areas between images derived from different radar stations, using computer models to estimate likely rainfall rates in attenuated areas.

Radar and thunderstorms

Thunderstorms, nonetheless, usually deliver copious rainfall, so it's common to look to radar imagery when seeking the location of thunderstorms. Bear in mind that rain only begins to fall when the thunderstorm reaches the mature state-long after lightning begins to flash and dangerous turbulence exists in, around or under the cloud. Further, heavy rain (as indicated by strong radar returns) does not always mean a thunderstorm, and may in fact result in a smooth flight through the in-flight deluge. As a general rule, however:

• Avoid flying through areas of heavy radar returns.
• Don't assume a thin line of precipitation with no returns beyond means a safe passage-attenuation may not reveal heavier returns until you get closer.
• Don't think just because you have the picture in the cockpit means you know the nuances of flying using radar as a guide-it takes dedicated instruction (example) to learn how to use radar information safely.

Aero-tip of the day: Understand the use, and the limitations, of weather radar information.

FMI: Aero-Tips