Inside a tornado vortex, wind speeds can exceed 200 knots — Page 263, Lesson 229

I want to walk you through a critical part of thunderstorm hazards — the tornado vortex and hail. Let's start with the vortex itself. Inside a tornado vortex, wind speeds can exceed 200 knots. That's well beyond any aircraft's design limits. Because the pressure inside the vortex is quite low, the strong winds gather up dust and debris from the surface. That low pressure also generates a funnel-shaped cloud that extends downward from the base of the cumulonimbus cloud. Now, there's an important distinction based on whether that funnel cloud actually reaches the ground. If the cloud does not reach the surface, we call it a 'funnel cloud'. If it touches the land surface, it becomes a tornado. So the difference is purely about ground contact — the same rotating column of air, just at different stages or intensities. Tornadoes can occur with two types of thunderstorm: isolated thunderstorms and squall line thunderstorms. For an aircraft, the danger is extreme. An aircraft entering a tornado vortex is almost certain to suffer structural damage. But here's the really dangerous part — the vortex extends well up into the cloud itself. So any pilot flying on instruments in a severe thunderstorm could encounter a hidden vortex that they never see visually. You don't have to see the funnel to be in serious trouble. Families of tornadoes have been observed as appendages of the main cloud, extending several miles outward from the area of lightning and precipitation. That means any cloud connected to a severe thunderstorm carries a threat — not just the part directly under the visible funnel. Now let's move to hail, which is section 2.4. Despite all the work done in thunderstorm forecasting, no confirmed or fully reliable method has yet been developed for recognising a storm that will produce hail. So the safest assumption is that hail exists in one part or another of every thunderstorm at some stage in its life. You treat every thunderstorm as a potential hail producer. What conditions make hail more likely? The higher the lapse rate — that's the rate at which temperature decreases with altitude — and the greater the moisture content of the air mass, the stronger the convective activity will be. Stronger convection increases the likelihood of damaging hail forming. Stability in the upper atmosphere results in the characteristic anvil shape of the cumulonimbus cloud — that flat, spreading top you see in textbook pictures. Strong upper winds will often cause hail to fall from the overhang of that anvil. Flight beneath the overhang should be avoided. That's a direct operational instruction. Let me give you the numbers on hailstone size. The maximum size of hailstones found on the ground is around five and a half inches in diameter. But we also know that hailstones of four inches in diameter can be encountered at 10,000 feet, and damaging hail can exist up to 45,000 feet. So hail is not just a low-level phenomenon — it extends through a huge vertical range of the thunderstorm. That figure shows you the cumulonimbus cloud with the risk of icing and turbulence labelled as moderate to severe, and you can see the structure we've been discussing.