Thunderstorms — Page 260, Lesson 224

I want to start by making sure we have a clear picture of what we're dealing with. All thunderstorms are potentially dangerous. That's the opening fact, and I want you to hold onto it because it sets the tone for everything we're about to cover. But within that general warning, there are two specific facts I need you to remember. First, a severe storm can occur in practically any geographical area where thunderstorms are known to happen. That means you cannot assume a particular region is safe just because severe storms are rare there. Second, and this is critical for a pilot: there is no useful correlation between the external visual appearance of a thunderstorm cloud — or what you see on your weather radar — and the actual turbulence and hail inside it. A storm that looks mild on the outside or on the radar screen can still contain violent turbulence and large hail. You cannot judge a thunderstorm's internal severity by looking at it. Now let's move into the flight hazards, starting with turbulence associated with thunderstorms. The air movement inside a thunderstorm is generally called turbulence, but I want to break that down into two specific components. The first is draughts — these are sustained vertical or sloping currents of air. The second is gusts — these are irregular, local variations in the airflow. Together, these can become violent, dangerous, and even destructive. The maximum intensity of this turbulence occurs in developing and mature cells — that's the growing stage and the fully developed stage of a thunderstorm cell. What does that mean in practical terms for an aircraft? Pilots have experienced high rates of roll and large pitching motions inside these storms. They've also experienced large vertical displacements — meaning the aircraft is pushed up or down by thousands of feet. The excerpt mentions displacements of as much as 5000 feet. Now, I want to be clear: these extreme variations only occur in the most severe conditions. But you need to know they are possible. There's another equally important point. Eddies — which you feel as gusts — can occur some distance outside a thunderstorm cell. So the regions around or between adjacent cells are likely to be turbulent, sometimes severely so. In fact, severe turbulence is often found 15 to 20 miles downwind of a severe storm core. That's a significant distance. You cannot assume you're safe just because you're not inside the cloud. Conditions at or near the surface in the vicinity of thunderstorms are also often rough. During the mature stage of the cells, the outflow from the base of the storm is turbulent. That outflow air is colder than its surrounding environment, which produces what is essentially a miniature cold front. This is often accompanied by heavy precipitation and squally conditions — meaning strong, sudden gusts of wind. When this outflow is associated with a line of thunderstorms, its effects can be felt as much as 40 miles away from the storm itself. So to summarise what I want you to take away: thunderstorms are always potentially dangerous, you cannot judge severity by appearance or radar, turbulence is worst in developing and mature cells, vertical displacements can reach 5000 feet in severe cases, turbulence extends 15 to 20 miles downwind of a severe core, and surface outflow from a mature storm can create cold-front-like conditions felt up to 40 miles away when associated with a line of storms.