Thunderstorms — Page 266, Lesson 244

I want to walk you through some very important practical guidance on using airborne weather radar to avoid thunderstorms. This is the kind of knowledge that directly affects how you plan a diversion or decide whether to fly through a gap between storms. Let's start with a key limitation of weather radar that can trick you. The radar return intensities you see on the display may be misleading because of attenuation — that means the radar beam gets weakened or absorbed as it passes through heavy rain. The intervening heavy rain between the radar antenna and the storm cell beyond actually reduces the strength of the echo coming back. This can lead to a serious underestimation of how severe the rainfall really is in a large storm. You might think the heaviest rain is at a certain position on the screen, but in reality, the echo from the part of the storm furthest from the radar will be relatively weaker. So the actual position of the maximum rainfall at the far edge of the storm area will be further away than what the radar display shows you — sometimes by distances up to several miles. And even worse, a storm cell that lies beyond that first storm could be completely masked — you simply won't see it at all on the radar. Now, here's another critical point. Despite all the research and operational experience we have, it still seems impossible to use radar to detect with certainty areas where large hailstones exist. Why? Because clouds containing rain or hail can produce identical radar pictures. You cannot reliably tell them apart just by looking at the return. Some operators have claimed success in avoiding hail by keeping well clear of cloud echoes that have scalloped edges or pointed or hooked 'fingers' attached to them. But the best advice is simple: give radar echoes a wide berth when you are detouring storms visually. Don't try to thread the needle based on a hope that the radar is showing you hail-free areas. You also need to remember the high rate of growth of thunderstorms. A storm can build upward very quickly. The danger of flying over or near to the tops of both the main storm and the small convective cells close to it must always be in your mind when using weather radar for storm avoidance. That innocent-looking top you think you can clear by a few thousand feet might be growing right into your flight path. Let's now look at the specific avoidance guidance given in the table. This is the recommended minimum distances you should keep from thunderstorm echoes on your radar. The guidance is broken down by flight altitude and by echo characteristics. For flight altitudes from 0 to 20,000 feet, you should avoid by 10 miles any echoes that show certain features. First, echoes with 'hooks', 'fingers', scalloped edges, or other protrusions from the main storm return — avoid by 10 miles. Second, echoes with sharp edges or strong intensities — avoid by 10 miles. Third, echoes with strong gradients of intensity — avoid by 10 miles. And fourth, echoes showing rapid change of shape, height, or intensity — avoid by 10 miles. Let me explain what 'gradient of intensity' means, because it's important for sets with Iso-Echo or a colour display. Iso-Echo is a feature that produces a hole in a strong echo when the returned signal is above a preset value. Where the return around that hole is narrow, that indicates a strong gradient of intensity — the rainfall intensity changes very sharply over a short distance, which is a sign of a severe storm. For flight altitudes from 20,000 to 25,000 feet, the rule changes: avoid all echoes by 20 miles. Same for 25,000 to 30,000 feet — avoid all echoes by 20 miles. And above 30,000 feet — avoid all echoes by 20 miles. Now, the table can be simply summarised. If you are above 20,000 feet, avoid by a minimum of 20 nautical miles. If you are below 20,000 feet, avoid by a minimum of 10 nautical miles. One more thing: where weather information is available from ATC radar, it should be used to supplement your aircraft's weather radar. But note the cross-reference to paragraph 3.2(c) of that Annex — there are limitations to ATC radar as well, so it's supplementary, not a replacement.