Thunderstorms — Page 269, Lesson 247

I want to walk you through a critical operational procedure for dealing with thunderstorms — specifically, how to safely overfly them and how to estimate cloud-top height using airborne weather radar. First, the fundamental rule: if you have to overfly storm clouds, you must always maintain at least 5,000 feet of vertical separation from the cloud tops. That's a hard minimum — not a suggestion. Now, you can estimate this separation using the technique I'm about to explain, and you may also have ATC or meteorological information on the actual altitude of the tops to cross-check. Let's break down the radar procedure step by step. Step a — setting up the radar for measurement. You start with the aircraft in straight and level flight. Set the aerial tilt to zero — that means the centre of the weather radar beam, which is called the bore-sight, is aligned exactly with the horizontal. Now, you adjust the gain control. One manufacturer recommends this: reduce the gain until the radar paint — that's the coloured return from the clouds on your display — just disappears. Then increase the gain until you get a solid paint — a full, continuous return. Leave the gain at that setting. Finally, record the range of the nearest part of that solid paint. So you now have a range value. Step b — measuring the cloud top angle. Next, raise the beam by adjusting the aerial tilt upwards until the return disappears again. Record the tilt angle at which that disappearance occurs. Then return the tilt to zero so you can continue monitoring the storm and your separation from it. Now, to get the approximate height of the cloud top, you can use either a look-up table provided by the radar manufacturer, or you can do a mental calculation. Here's the approximation method described: Take the notional beam-width quoted by the radar manufacturer — this is usually in the region of 3° to 4°. Subtract half of that beam-width from the recorded tilt angle. So if your recorded tilt angle was, say, 5° and the beam-width is 4°, you subtract 2°, leaving 3°. Then apply the 1:60 rule to this remainder, using the recorded range of the edge of the return. The 1:60 rule says that at a range of 60 nautical miles, 1° of angle subtends 1 nautical mile of height. So you calculate the height in nautical miles — and since 1 nautical mile is approximately 6,076 feet, you can convert to feet to check your 5,000-foot separation requirement. Let me make sure that's clear: the height you calculate is the approximate height of the cloud top above your aircraft's altitude. You then add your own altitude to get the absolute height of the top, and you ensure you're at least 5,000 feet above that when you overfly. That's the core technique — a practical, in-flight method using your weather radar to estimate cloud-top height and maintain safe vertical separation from thunderstorms.