Thunderstorms — Page 260, Lesson 221

I want to walk you through the start of the thunderstorm chapter. This is a critical topic for any professional pilot, because thunderstorms are one of the most severe weather hazards you'll encounter. Let's begin with the structure of a thunderstorm cloud. A thunderstorm cloud — whether it's an air mass type or a frontal type — usually consists of several self-contained cells. Each cell is in a different state of development. That's a key point: a thunderstorm isn't one single, uniform cloud; it's a collection of individual cells, each at its own stage of life. And I must stress this: the storm clouds you see are only the visible part of a much larger turbulent system that extends over a much greater area. So what you see visually is just the tip of the iceberg. Now, how do you recognise the different stages of these cells? New and growing cells can be recognised by their cumuliform shape. They have a clear-cut outline and what's described as a 'cauliflower' top — that's the classic, sharply defined, bubbly appearance of a growing cumulus cloud. In contrast, the tops of more mature cells will appear less clear-cut. They'll frequently be surrounded by fibrous cloud — that's the ice-crystal anvil or the wispy, hazy appearance that comes as the cell reaches its upper levels and starts to spread out. But here's an important warning: the development of cells can be very rapid, and you won't always see it, even in daylight. Other clouds may obscure the view. For example, in frontal or orographic conditions — where forced ascent of air gives the impetus for vigorous convection currents — extensive layer cloud structures may hide the development of Cumulonimbus thunderstorm cells. They can also hide Altocumulus Castellanus. Let me define that term: Altocumulus Castellanus is a cumuliform cloud with a base above 8,000 feet. It's an indication of middle-level instability, which often precedes or is associated with the development of thunderstorms. So if you see Altocumulus Castellanus, you should be alert for possible thunderstorm development. Another cloud feature to watch for is Mammatus clouds. These are udder-shaped features seen beneath cumulonimbus clouds. They can also appear in association with medium-level altocumulus layer clouds — again, above 8,000 feet — or with high-level cirrus anvil cloud, which is above 20,000 feet. Mammatus clouds are an indication of strong vertical winds with associated turbulence. So if you see those pouch-like shapes hanging beneath a cloud, you know there's significant vertical motion and turbulence present. Let me move to the conditions that produce the most severe thunderstorms. The most severe thunderstorms require two things: an increase in the general wind speed with height, and a change in wind direction with height. That combination — increasing speed and changing direction as you go up — is vertical windshear. And it's needed to maintain a release of energy. The excerpt explains: with no vertical windshear, as the cloud grows and the updraught strengthens, precipitation forms in the... and the text cuts off there, but the principle is clear — without windshear, the precipitation falls back into the updraught and chokes the storm. The vertical windshear is what tilts the updraught and allows the storm to sustain itself. So to summarise what we've covered: a thunderstorm is made of multiple cells at different stages; new cells have clear cauliflower tops, mature cells have fibrous tops; development can be hidden by other clouds; Altocumulus Castellanus above 8,000 ft indicates middle-level instability; Mammatus clouds indicate strong vertical winds and turbulence; and severe thunderstorms need vertical windshear — increasing wind speed and changing direction with height — to sustain their energy release.