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From the Flight Deck
by Kevin R. Dingman
The Tempestuous Troposphere
Turbulence Facts, Fiction and Fairy Tales
Many nursery rhymes express fear, suffering and disaster. Ring Around the Rosie was about the bubonic plague; Peter Peter Pumpkin Eater couldn’t keep his wife, so he imprisoned her; a farmer’s wife amputated the tails of three visu- ally impaired mice and the parenting techniques of The Old Woman Who Lived in a Shoe would today warrant the in- tervention of Child Protective Services. Of course, we all know that when it’s raining, it’s pouring, and the old man is snoring, that we’ll bump our head (on the cockpit ceiling, no doubt) and won’t get up in the morning. King James and Mary of Modena’s baby-napping scandal notwithstanding, we’ve also been taught that if it’s windy and the bough breaks, the baby will fall – likely due to a royal microburst.
Perhaps for us valiant aviators, these were a childhood introduction to the potentially traumatic and un- forgiving effects of weather. The list of atmospheric monsters has lengthened since we were kids and pilots can’t
outgrow or ignore them. No longer a fairy tale, turbulence demons live in the heart of our f lying territory.
Our Non-Terrestrial Territory
Weather and turbulence are the re- sult of uneven heating of the Earth by the sun. Combine this with pressure, temperature, moisture differences, plan- etary rotation with surface friction and you have the ingredients for a changing and sometimes turbulent atmosphere. The troposphere is the lowest layer of the atmosphere, 3.7 to 6.2 miles (19,500 ft. to 32,500 ft.), and it’s where nearly all weather conditions take place. The top of the troposphere varies with latitude (it is lowest over the poles and highest at the equator) and by season (lower in winter and higher in summer). It can be as high as 12 miles or 65,000 feet near the equator, and as low as four miles or 23,000 feet over the poles in winter. It contains approximately 75 percent of the atmosphere’s mass and is by far the wettest layer of the atmosphere, con- taining 99 percent of the total mass of
all water vapor. And it’s normally this water vapor that causes a bumpy ride. But there are other monsters lurking in our non-terrestrial territory.
The Turbulence Tempests
• ClearAirTurbulence.CATnormally occurs outside of clouds at altitudes above 15,000 feet MSL, and it’s caused by strong wind shears in the jet stream.
• Thermal Turbulence. Localized columns of convective current that result from surface heating or cold air moving over warmer ground. For every rising current, there is usually a compensating downward current also causing turbulence.
• Temperature Inversion Turb- ulence. Even though a temperature inversion produces a stable atmo- sphere, inversions can cause turbu- lence at the boundary between the inversion layer and the surrounding atmosphere.
• Mechanical Turbulence. When the air near the surface flows over obstructions, such as trees, plateaus, mountains or structures, the normal wind flow is transformed into swirling eddies and currents.
• Frontal Turbulence. Frontal turbulence is caused by the lifting of warm air or the abrupt wind shift between warm and cold air masses. The most severe cases are associated with fast-moving cold fronts.
• Mountain Wave Turbulence.
As air flows over mountains and down the leeward side, a standing mountain wave is formed and air currents oscil- late between altitudes. It can extend for hundreds of miles downwind of the mountain range.
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