Thunderstorms — Page 238, Lesson 196

I want to walk you through the thunderstorm chapter, starting with how these storms can align themselves and how we forecast them, then moving into the special case of supercell thunderstorms, and finally the critical avoidance rules you'll use in the cockpit. Let's begin with alignment. Thunderstorm squall lines may occur at and some miles ahead of an active cold front. A squall line is a line of thunderstorms that can form along or ahead of the front itself. The figure on this page, Figure 14.6, shows that alignment. Now forecasting. Forecasting the occurrence of thunderstorms is largely a matter of assembling the conditions necessary for thunderstorm formation and the triggers that set them off. A combination of these two groups — the necessary conditions and the triggers — will indicate the probability of thunderstorms. In practice, satellite photography and computer modelling are used to predict this occurrence. So you're not just looking at a single factor; you're putting together a picture from multiple sources. Now we move to a special and dangerous type: supercell thunderstorms, also called severe local storms. These go through stages just like ordinary thunderstorms, but with much more intensity. Let's look at the initial stage first. The conditions necessary to initiate supercell thunderstorms are threefold. First, a great depth of instability in the atmosphere — that means the air is very unstable over a deep layer. Second, strong vertical windshear — that's a rapid change of wind speed or direction with height. Third, a stable layer between warm air below and cool air above, which is eventually broken down by insolation. Insolation is the heating from the sun. So you have a lid of stable air that the sun's heating eventually breaks through, releasing the instability. Figure 14.7 illustrates these conditions. Now the mature stage. The characteristics here are dramatic. Very strong updraughts and downdraughts are produced within one large cell — a single supercell. These give rise to violent weather and even tornadoes. The book notes that an average of 33 tornadoes per year have occurred in Britain over recent years, reminding us that tornadoes are not a phenomenon restricted to the USA. The mature stage of a supercell may last several hours, which is much longer than an ordinary thunderstorm cell. Movement of supercell thunderstorms in the Northern Hemisphere is usually about 20 degrees to the right of the 18,000-foot, or 500 hectopascal, wind velocity. So if you know the wind direction and speed at the 500 hPa level, the storm will track roughly 20 degrees to the right of that. Location matters too. Supercell thunderstorms are more common over continental land masses than over maritime areas. The classic example given is thunderstorms over the mid-west states of the USA producing tornadoes. Now the most operationally important part for you as a pilot: avoidance. The CAA — the UK Civil Aviation Authority — has produced recommended avoidance distances when using weather radar. These are shown in Figure 14.8. A key point to note is that the significance of a radar return of a given intensity usually increases with altitude. The principle underlying the use of airborne weather radar is that strong upcurrents, which will support strong turbulence, will also support large water droplets, and those large droplets will show a stronger radar return. Let me walk you through the table of recommended avoidance ranges. For flight altitudes from 0 to 20,000 feet, you must avoid by 10 miles any echoes with certain characteristics. Specifically, avoid by 10 miles echoes with shapes like hooks, fingers, scalloped edges, or other protrusions from the main storm return. Avoid by 10 miles echoes with sharp edges or strong intensities. Avoid by 10 miles echoes with strong gradients of intensity. And avoid by 10 miles echoes showing rapid change of shape, height, or intensity. For flight altitudes from 20,000 to 25,000 feet, the rule changes: avoid all echoes by 20 miles. For 25,000 to 30,000 feet, again avoid all echoes by 20 miles. And above 30,000 feet, avoid all echoes by 20 miles as well. There's a note about the gradient of intensity. This applies to radar sets with Iso-Echo or a colour display. Iso-Echo produces a hole in a strong echo when the returned signal is above a pre-set value. Where the return around that hole is narrow, there is a strong gradient of intensity — meaning the radar return changes from very strong to much weaker over a short distance, which indicates a sharp boundary and likely severe turbulence. So in summary: know the conditions that breed supercells, understand their movement 20 degrees right of the 500 hPa wind, and most importantly, apply those avoidance distances rigorously — 10 miles for specific features below 20,000 feet, and 20 miles for all echoes above that altitude.