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Weather Hazards Chapter 9, Section B Thunderstorms • Conditions – Unstable air – Lifting action – High moisture content • Stages – Cumulus stage – Dissipating stage Mature stage Hazards • Embedded thunderstorms may be obscured by cloud layers • Wind shear can be found on all sides as well as directly under it • Greatest intensity during mature stage, which is signaled by precipitation at the surface Types of Thunderstorms • Airmass thunderstorms are usually isolated or scattered over a large area • Frontal thunderstorms associated with frontal activity • Squall line is a narrow band of active thunderstorms normally containing severe weather Hazards • Lightning is always associated with thunderstorms • Hail is often associated with cumulonimbus clouds but can be found in clear area several miles from the cloud • Funnel clouds - tornado or waterspout Thunderstorm Avoidance • Use weather radar to avoid thunderstorms • Avoid echoes by at least 20 miles Do not fly between if less than 40 miles apart • Weather radar does not pick up fog or clouds Turbulence • Low-level Turbulence below 15,000 feet consists of – Mechanical Turbulence – Convective Turbulence – Frontal Turbulence – Wake Turbulence Mechanical Turbulence • Wind forms eddies as it blows around hanger, stands of trees or other obstructions Convective Turbulence • Thermal Turbulence is a daytime phenomena which occurs over land in fair weather • Capping stable layer begins at the top of the convective layer. It can be identified by a layer of cumulus clouds, haze or dust Frontal Turbulence • Occurs in the narrow zone just ahead of a fast-moving cold front Wake Turbulence • Wingtip vortices occurs when an airplane generates lift • They can exceed the roll rate of an aircraft • Greatest when an aircraft is heavy, slow and clean Wingtip Vortices • Tend to sink below the flight path of the generating aircraft • Most hazardous during light, quartering tailwinds • Land beyond where a large aircraft has touched down Wingtip Vortices • Lift off before the point a large aircraft departing in from of you lifted off climb out above his flight path or turn upwind • Helicopters in forward flight produce wingtip vortices like circulation of air Clear Air Turbulence • Turbulence above 15,000 feet AGL not associated with cumuliform cloudiness is reported as CAT • CAT is common in a upper trough on the polar side of the jet stream Jet Stream • A curving jet stream associated with a deep low pressure trough can be expected to cause great turbulence • Jet stream can sometime be identified by long streaks of cirrus clouds Mountain Wave Turbulence • Greatest turbulence occurs approaching the lee side of a mountain range in strong headwinds • Standing lenticular and rotor clouds indicate the possibility of strong turbulence Reporting Turbulence • Light - slight erratic changes in altitude or attitude • Moderate - aircraft remains in positive control • Severe - large abrupt changes in altitude and attitude and may be momentarily out of control Wind Shear • Sudden, drastic shift in wind speed and/or direction over a short distance • May be associated with a strong lowlevel temperature inversion, a jet stream, a thunderstorm or a frontal zone Microbursts • Intense, localized downdrafts seldom lasting longer than 15 minutes • Downdrafts can be as strong as 6,000 feet per minute • Performance changes drastically as an aircraft flies through a microburst Low-Level Wind Shear Systems • LLWAS - system of anemometers compares wind speed at several locations around the airport • Terminal Doppler Weather Radar provide a clearer, more detailed picture of a thunderstorm • Visual - Virga Restrictions to Visibility • Fog, haze, smoke, smog and dust • Fog requires moisture and condensation nuclei • Industrial areas produce much fog since they have more condensation nuclei Fog • Radiation Fog - ground fog - forms over fairly flat land on clear, calm nights • Advection fog- forms near coastal areas when moist air moves over colder ground or water Fog • Upslope fog forms when moist stable air is forced up a sloping land mass • Steam fog occurs as cool air moves over warmer water • Precipitation-induced fog forms when warm rain falls through a layer of cooler air near the surface Fog • Ice fog occurs in cold weather when the temperature is much below freezing and water vapor sublimates directly as ice crystals Icing • Freezing rain is most likely to have the highest rate of accumulation • Ice, snow or frost having the thickness and roughness of sandpaper and reduce lift by 30% and increase drag by 40% Cold Weather Operations • Preheat the cabin as well as the engine • Warm crankcase breather lines since they may be clogged by ice from vapors that have condensed and subsequently frozen