Thunderstorms — Page 263, Lesson 228

I want to walk you through a critical hazard that thunderstorms produce: windshear, and then we'll move into tornadoes. Let's start with windshear. First, I need to distinguish it from gusts. Gusts are erratic, short-lived fluctuations in wind speed. Windshear, by contrast, gives rise to airspeed fluctuations of a more sustained nature. That sustained quality is what makes windshear likely to be more dangerous than simple gusts. And importantly, gusts are likely to accompany windshear conditions, so you won't always get a clean separation between the two. Now, thunderstorms frequently produce windshear. While it's hazardous at all flight levels, it's in the lower levels that windshear may have more drastic consequences. Here's the mechanism: cold air rushes out from the base of a thunderstorm cell. That outflow of cold air has been known to change wind conditions in shallow layers of only a few hundred feet. The changes can be dramatic — as much as 80 knots in speed and 90 degrees or more in direction, all within that shallow layer. Why is this so dangerous for an aircraft? Because of inertia. An aircraft in flight will tend to maintain its ground speed. So when you encounter a sudden, sustained change in wind direction or speed — that's windshear — it produces airspeed variations. Those variations can be large enough to be extremely dangerous, especially when you're low and slow during takeoff or approach. Let's move on to tornadoes, which present a very serious threat to aircraft. I'll give you a real-world example: a Fokker F-28 was flying in cloud at 3,000 feet shortly after take-off from Rotterdam. On 6 October 1981, it was destroyed by a tornado. That shows you this isn't just a theoretical risk. Tornadoes are generally associated with organised severe local storms. They occur frequently in the United States, but they can also arise in the UK and Europe, although there they are less common and seldom as violent. Here's a key point for you as a pilot: there is evidence that tornado circulation may extend throughout the depth of the storm. That means a tornado can constitute a hazard to aircraft at all heights, not just near the ground. How do tornadoes form in the context of thunderstorms? The most violent thunderstorms draw air into their cloud bases with great vigour. If that incoming air has any initial rotating motion, it often forms an extremely concentrated vortex that extends from the surface well into the cloud. Meteorologists have estimated that wind velocities in such a vortex can exceed 200 knots. Because the pressure inside the vortex is quite low, the strong winds gather dust and debris — and that debris, along with the violent winds themselves, is what makes a tornado encounter catastrophic for an aircraft.