Thunderstorms — Page 238, Lesson 198

I want to walk you through the thunderstorm hazards and the tools we use to detect them. Let's start with the detection equipment. First, we have Airborne Weather Radar, abbreviated as CCWR in some texts. This is a Plan Position Indicator, or PPI, radar — meaning it scans horizontally around the aircraft to show a map-like view of weather ahead. Ground-based radar is also mostly PPI, but it may also use RHI, which stands for Range-Height Indicator — that gives a vertical cross-section of the weather. I won't go deep into how CCWR works here, since it's covered elsewhere in the course, but I do want you to see Figure 14.9 — it's called a Radar Mosaic. That's where returns from many different radars are combined into a single area display, and it's multicoloured to identify different precipitation intensities. So on that display, different colours tell you how heavy the rain or hail is. Next is the Stormscope. This is a highly sophisticated system. It detects, locates, and maps areas of electrical discharge activity — in other words, lightning — that are contained within thunderstorms. The purpose is straightforward: it permits you, as a pilot, to avoid the hazards associated with those thunderstorms by showing you where the electrical activity is. Now let's move into the Summary of Thunderstorm Hazards. There are three main categories I want you to understand. First: Turbulence. Turbulence can be violent both inside the cloud and at the sides of the cloud. Below the cloud, turbulence can be dangerous during take-off and landing, and there can also be windshear — a sudden change in wind speed or direction. It is possible for a pilot to overstress the airframe in these conditions. Also, loose articles being thrown about inside the cabin can injure passengers. And there's an instrument effect: pressure instruments can be in error due to lag — meaning they don't respond instantly to rapid pressure changes caused by turbulence, so your readings may be temporarily wrong. Second: Hail. Hail can be encountered at any height inside the cloud, also below the cloud, and also below the anvil — that's the flat, spreading top of a mature thunderstorm. When the hail is large, severe skin damage to the airframe can occur. Damaging hail can occur up to a height of 45,000 feet — so that's well into the flight levels. Third: Icing. Icing can occur at all heights in the cloud where the temperature is between 0°C and -45°C. That's a very wide temperature range. Heavy concentrations of droplets and large droplet size result in severe clear icing — that's a particularly dangerous form of ice because it's transparent and hard to see, but it builds up quickly. There's also carburettor icing, which can occur at temperatures between -10°C and +30°C, and it can be particularly severe between -2°C and +15°C. That's important because carburettor icing doesn't require visible cloud — it can happen in clear air with high humidity. Let me show you the relevant figures. shows the building stage of a thunderstorm, and shows the mature stage. shows the dissipating stage, where the cloud extends to the tropopause and is spread out by the upper wind. And also includes the mature stage in that same figure. Figure 14.10 illustrates the thunderstorm hazards visually, Figure 14.11 relates to hail, and Figure 14.12 shows ice accretion on the tailplane and underwing. So to summarise: we have radar mosaics and Stormscopes to detect thunderstorms, and the three main hazards are turbulence (with windshear and instrument lag), hail (up to 45,000 feet), and icing (clear icing between 0°C and -45°C, plus carburettor icing between -10°C and +30°C, especially severe between -2°C and +15°C).