So a Cb is necessary, but it's not sufficient on its own — Page 228, Lesson 192

I want to walk you through the beginning of Chapter 14, which is all about thunderstorms. This is a major topic for us as pilots because thunderstorms produce some of the most severe weather hazards we can encounter. Let's start with the conditions needed for a thunderstorm to form. First, the formal definition: a thunderstorm, abbreviated TS, occurs in a well-developed cumulonimbus cloud, which we call a Cb. But here's an important distinction right away — not every cumulonimbus cloud produces a thunderstorm. So a Cb is necessary, but it's not sufficient on its own. For a thunderstorm to actually develop, we need three specific conditions to be present simultaneously. The first condition is a lapse rate greater than the saturated adiabatic lapse rate, or SALR, through a layer that is at least 10,000 feet thick. And that layer must extend above the freezing level. So we need a deep, unstable layer that reaches above the altitude where the temperature drops to 0°C. The second condition is sufficient water vapour in the air to form and maintain the cloud. Without enough moisture, the cloud simply can't sustain itself. The third condition is what the book calls "trigger action" — something that forces the air to rise and produce early saturation, which enhances the instability. The book lists four specific triggers or lifting forces. The first is convection — that's the daytime heating of the ground that causes warm air to rise. The second is orographic uplift, which happens when wind forces air up over high ground like mountains. The third is convergence, where air flows together horizontally and has nowhere to go but upward. And the fourth is frontal uplift, where a colder, denser air mass forces warmer air to rise ahead of it. Now, thunderstorms are classified into two main types. The first is air mass type thunderstorms, which are more common in summer. The second is frontal type thunderstorms, which are more common in winter. Let's look at air mass type thunderstorms in detail. They are isolated in nature, and they use all the triggers I just mentioned except frontal uplift. They are most frequent over land in summer, and they usually form by day and clear by night. They tend to form in cols — that's the area of relatively low pressure between two high-pressure systems — or in weak lows, which are areas of low pressure that aren't particularly intense. There's an important note here: thunderstorms formed by advection — that means the horizontal movement of air bringing in warm, moist conditions — can occur day or night, over land or sea, at any time of the year. So that's an exception to the typical daytime-summer pattern. Now for frontal type thunderstorms. These are most frequent in winter. They can form over land or sea, day or night — so they're not tied to daytime heating. They usually form in a line at a cold front or an occlusion. They're found in active depressions or troughs — so in well-developed low-pressure systems. They are often accompanied by a line squall, which is a sudden, sharp increase in wind speed along the front. And importantly, frontal thunderstorms are the fastest-moving type of thunderstorm. So to summarise what we've covered: thunderstorms need a deep unstable layer above the freezing level, plenty of moisture, and a trigger to start the uplift. We have two classifications — air mass type, which is isolated and mostly summer daytime over land, and frontal type, which forms in lines at cold fronts and occlusions, is most common in winter, and moves the fastest.