Weather Hazards Chapter 9, Section B

Thunderstorms Conditions Stages Unstable air Lifting action High moisture content Stages Cumulus stage Mature stage Dissipating 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 low-level 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