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Thunderstorms Pilots dealing with thunderstorms don't just face obvious problems such as lightning and turbulence - there are hidden ones too.Before examining these, let's look at three basic requirements for thunderstorm formation: 1 Water vapour absorbed in the air in massive amounts. 2 An unstable atmosphere, allowing rapid vertical ascent of air. This releases moisture as cumulonimbus (Cb) cloud when the ascending air cools through expansion in the lower pressure at higher levels, and cannot hold its water vapour. If instability allows, the rising air accelerates to the tropopause and 'towering cumulus' (TCu) clouds form in huge chimneys. This acceleration, along with heat release from water condensation provides thunderstorm energy. 3 A trigger action starting the humid air on its journey upwards. This trigger can be upflow over a hill, convection from ground heating, or even a new front forcing warm air upwards. But a thunderstorm has a finite life, and formation of an anvil top to the towering cumulonimbus cloud signals its decay. During its lifespan, pilot-unfriendly phenomena abound. The central core of rising air within the cloud causes severe turbulence, easily sufficient to overstress aircraft. Heavy precipitation (rain and hail), as the released moisture needs to grow to unusual size by coalescence, before overcoming strong upflows to drop to earth. Sudden ground wind direction changes as more air is drawn from ground level. These sudden changes seriously affect takeoff and landing distances within a few miles of the storm. Thunderstorms do not just contain ascending air. Colder, higher air is displaced down and away from the cloud, with the following effects: wind direction and strength changes for a few miles around. Direction reversal problems are common, and marked wind shear is present. This is the second hidden danger - airspeed on approaches to land can suddenly reduce significantly to the stall due to this shear. In severe cases - mainly in the tropics - downward localised airbursts have been known to slam nearby aircraft into the ground. The cold outflow can squeeze under nearby warm humid air, forcing it upwards, and actually trigger a new storm nearby. This forms a slow chain reaction of storms, each one the 'parent' to a new 'child', keeping the danger alive for much longer. The back-outflow can give a final 'kick', just as an impatient pilot thinks the danger is past. The friction between the upflow, downflow and actual water or ice droplets within a cloud provides the static electricity - discharged as lightning. Thunderstorms can arrive at any time of year, but are usually associated with the end of a long, warm hazy spell. These weather patterns originate from stable high pressure systems warming the atmosphere, thus allowing a high water vapour content. When the high pressure and its stability finally breaks down, it takes only a sufficient 'trigger' to meet all three requirements for a thunderstorm. These 'breakdown' storms often develop in the afternoon or early evening, when land heating and convective turbulence finally destroy a weak inversion, allowing the vertical ascents required. Such familiar thunderstorm conditions are predictable and visible when the storms form. There are other causes of thunderstorms, disguised from the pilot, and associated with frontal systems. Warm fronts are usually associated with flat stratus cloud, and thunder- storms are rare, but not completely unknown. However, the common place for a thunderstorm is on the cold front, or just ahead of it in a squall line. Cold air rapidly forces up warmer more moist air ahead, causing cumulonimbus clouds. If stability conditions encourage further upward movement, all the conditions are met for 'embedded' thunderstorms - very dangerous to the pilot as they're not individually visible, being hidden by the surrounding frontal cloud. To best appreciate a nearby thunderstorm, every pilot should experience one at an airfield from a safe vantage point on the ground. Feel the changes in air temperature, observe the sudden wind changes, damaging gusts and frightening precipitation.
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| This article first appeared in FLYER magazine's July 2000 edition |
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Weatherwatch
with Irv Lee