Page 23 - Volume 16 Number 8

What Triggers The Thunderstorm?Lifting is the mechanism that initiates the birth of cumulonimbus clouds, which can be present at several sources. Commonly, unstable air masses provide the fuel for expansion of a moisture-laden air parcel, but there has to be a starting trigger, something as small as thermal updraft from a parking lot or open field. Once begun, the lifting of damp air continues, feeding on itself.A convenient upslope with a windflow oriented perpendicular to the ridgeline will surely springboard unstable air upward to create mid-day thunderstorms, and some coastlines with onshore winds will generate storms. However, the most-feared thunderstorm generators are opposing air masses crashing into each other like atmospheric tectonic plates. Cold fronts act as snowplows, using the density of cooler air to shove lighter, warmer air upward with its load of moisture. The influence of such frontal attacks is felt far ahead of the actual surface front, as squall lines form and dry lines of wind gusts make their presence known in the warm air sector.Individual thunderstorms form and die in a matter of a couple of hours. But, the complex of storms can continue generating a “train” of storms to take the place of fallen comrades. As pilots, we need to observe the dynamics of the situation and adjust our plans accordingly. Is the area moving out of our way, or expanding into it? Two movements are usually seen; the individual storms drift one way, while the area itself goes off in another. In addition, a rapidly-expanding complex can simply grow outward to engulf what had been an open route, in a third direction. Therefore, you need to continually revise your exit strategy and alternates available, not sit on plans made earlier.Thunderstorms influence airport operations to a great degree, so arrival delays must be factored into one’s planning. Thunderstorms in the arrival gates or on a final approach course will ruin an air traffic controller’s day; an airplane arriving with minimal fuel only adds to the muddle. Be ready to switch approach procedures as storms pass by, and expect to calculate landing distance based on tailwinds and wet runway surfaces. The preferred strip of concrete may be unavailable, due to the barbarians at the gate, so you’ll have to land in a best-option manner. Windshear is an ever-present possibility when thunderstorms are moving through the area, requiring extra care during landings and takeoffs.If you fly where thunderstorms are a seasonal occurrence, you’ll need to draw upon your experience from past years and deal with them as best you can. Modern aircraft and flight management tools have given us more capability, but the rules of the •game haven’t changed. Avoid, plan and stay flexible – that’s still the way to stay safe in thunderstorm country. T&TAUGUST 2012TWIN & TURBINE • 21