By Greg Illes
They are beautiful, dangerous, and full of mystery. As a femme fatale in an action movie, thunderstorms are lovely to look at, unpredictable and menacing all at the same time. As we enter the storm season in the interior of South Africa a refresher on these beautiful beasts seem appropriate.
So much has been written about these phenomena, it is difficult to discuss them without repeating age-old conventional wisdom's: keep 20 miles away from thunderstorm cells and stay on the upwind side. These guidelines are useful, but some additional information can help you creatively circumnavigate cells and still fly safely.
Thunderstorm “cells” are cumulonimbus clouds in various stages of development. The two major types of cells are distinguished by how the beasts are generated. Air-mass storms tend to have a lifetime of one to two hours, move slowly, and are often visible from many miles away. Frontal storms are more difficult to deal with; they are created by frontal energy and can last for many hours. They move quickly, bunch up, and cause poor visibility all around, and are frequently embedded in IFR weather conditions.
Cell location is of paramount importance, and therefore is the first consideration in dealing with thunderstorms. The closer a cell is to your route, the greater the problem it will pose. Astute weather planning—via weather radar and flight planning websites—is a cornerstone of cell avoidance. Weather radar from ATC is sporadic at best, and their radar is designed to see aircraft, not weather, so readouts may lack inaccuracy. Often, your best detection tools are your eyes.
Knowing where a cell is travelling is critical. Going around on the upwind side is the best choice, so the storm blows away from you. Racing past on the leeward side risks being chased downwind while you pray no hail is falling from the anvil above.
Air-mass cells are slow travellers and usually demand circumnavigation. Frontal cells will move quickly, and good planning can route you behind them on the safer, windward side. Cells that are not moving will have wider areas of influence, requiring greater clearance.
Most aircraft can’t top thunderstorm build-ups, which reach well into the flight levels. So why worry about storm cell height?
Because cell height is a crucial power indicator.
The convective activity builds storm height, and convection makes the storm a threat: turbulence, heavy rain, lightning, and hail are all proportional to convection.
The term “tops” is used to describe the upper limits of cells. Tops near 20,000 feet indicate cells that cause aggravation and make objects fly inside the cabin. Tops of 30,000 to 40,000 feet are in the “basher” category. Vertical air currents reach 4,000 fpm: a 45 mph gust, straight up or down. Vertical flows energise electrical and thermal dangers like lightning and golf-ball-sized hail. The rising air reaches atmospheric levels where horizontal winds accelerate, creating the “anvil head” of the storm. At its highest, coldest level, rain and hail whip off to the leeward side and fall downwind, outside the cell boundary. These effects result in the caution zone of 20 miles around a thunderstorm.
Tops of 50,000 feet and higher mean that a true monster is alive and ready to kill people. These behemoths produce baseball-sized hail and can spawn tornadoes which have become more common in the past few years here in “Sunny South Africa”. Vertical air currents exceed 60–80 mph and can simply destroy an aircraft. GA Aircraft and big jets alike avoid these systems like the Plague. Tune to ATC sometimes in an area of storms like these and listen to the non-stop diversion requests.
When judging storm height, be aware that reported low tops could be due to immature storms, particularly in the morning. Exercise caution if you’re on top since tops can climb at 1,000 fpm or more. Note the history of a weather area, and understand the life cycles of air-mass thunderstorms.
For air-mass thunderstorms, the duration is a predictable key to circumnavigation. After its energy peaks, a storm becomes more benign with age. For frontal systems, such is not the case. New frontal-mixing energy continues to re-create the thunderstorm cell, and duration can be indefinite.
Weather snapshots are of limited use; you must have history and frequent updates for good understanding. An air-mass storm two hours away may be gone before you get there, but one just birthing at the time of take-off may be a significant factor when you arrive. In many cases, air-mass storms can be waited out. Typical afternoon build-ups dissipate by late afternoon, and an early-evening arrival can be safe, calm, and beautiful to boot.
For frontal storms, the life cycle does not follow a birth-build up-decay pattern. These continuously generated cells last until the front weakens or moves away. Fortunately, frontal systems generally have movement, unless they are occluded or stationary fronts. Storm paths for fronts in motion can be generally predicted, and a route behind the storm track is the safest play.
If visibility is decent (20 miles or more), flying visually around isolated cells is a reasonably safe technique. But always be sure of a clear escape path. Remain 20 miles or more away from cells as you pick your route through them. Never fly directly under a cell, or nearby on the downwind side. Rain and turbulence are intense and visibility can quickly drop to nothing. Resist the temptation to squeeze between two cells. Although it’s “lighter” there, cells can merge together rapidly. Finally, don’t fly too low in thunderstorm territory. Vertical currents can get you really close to the terrain, and aircraft don’t fly well through dirt.
You did your homework, but you still got caught in the middle of a thunderstorm. Stuff is flying around the cabin, your passengers are terrified, and the turbulence is so bad you can’t read the instrument panel or dial the radio. The aircraft is out of control, rolling violently, sometimes 90 degrees. Visibility is dropping, and nearby lightning strikes are blinding, adding to the difficulty of control. Now what?
As always, fly the aircraft. Reduce speed quickly and smoothly to manoeuvring speed; don’t jerk the aircraft up, which can increase air-frame stress. Use gentle control inputs to counteract the buffeting. Don’t worry about altitude deviations unless you are close to terrain; instead, keep a level pitch attitude and control your airspeed. Even if you are not instrument-rated, concentrate on the artificial horizon and keep pitch and roll variations minimal. Using care and concentration, gently turn the aircraft back toward a known safe location. Stay at manoeuvring speed and avoid a steep, turbulent turn. Don’t look outside, just watch the AH and follow a heading.
If you encounter hail or lightning, don’t panic. Lots of aircraft have survived direct lightning hits, and hail dents are a small price compared to people’s lives. Control your airspeed, fly the aircraft back to safety, and join the ranks of lucky thunderstorm survivors.