Ground II: Weather Hazards

Thunderstorms

• A Thunderstorm is a local storm produced by a cumulonimbus cloud

• These clouds have the greatest turbulence of all cloud types

  • The presence of cumuliform clouds is indicative of turbulence

• It is always accompanied by lightning and thunder

• May be noted in a SPECI

Encountering Turbulence

• In turbulence, it is almost impossible to maintain altitude

• Maneuvering in attempts to maintain altitude increase stress on the aircraft

• The pilot should instead
  ◦ Slow down to the turbulence penetration speed recommended in the aircraft manual
  ◦ Attempt to maintain a level flight attitude

Thunderstorm Stages: Cumulus or Developing

• Characterized by continuous updrafts

• Also known as a towering cumulus cloud

Thunderstorm Stages: Mature

• Characterized by precipitation at the surface

• The storm reaches its greatest intensity

  • The beginning of precipitation marks the start of the mature stage

  • The cloud is now a cumulonimbus cloud

Thunderstorm Stages: Dissipating

Characterized primarily by downdrafts
Thunderstorms

Thunderstorm Cells:◦ Single Cell

Single thunderstorm seldom lasting more than 1 hour

Thunderstorm Cells: Multicell

Cluster of thunderstorms interacting with one another

Thunderstorm Cells: Supercell

Severe thunderstorm lasting two hours or longer

Thunderstorm Structures: Gust Front

• All storms have them, but multicell storms are stronger and more organized

• Boundary of cold air downdrafts from the storm that spreads out below

Thunderstorm Structures: Supercell Thunderstorm

• Air is twisted to a different direction as it rises to a higher altitude

• Two downdrafts develop, but remain separated from the updrafts

Tornadoes

• Violently rotating column of air beneath cumulonimbus clouds

• Called a funnel cloud if it does not reach the surface

• Called a waterspout if touches water

• Often created by supercell thunderstorms

Hail

◦ Accretion of ice inside thunderstorm as it circulates

◦ Precipitates out when it grows too large

Lightning

• Electrical discharge produced by thunderstorm

• Always present in and near a thunderstorm

• May be cloud to cloud, cloud to ground, in cloud, and cloud to clear air

T-Storm: Required for Development

• Moisture

  • Sufficient water vapor to form cumulus, towering cumulus, then cumulonimbus

• Unstable air

  • If stable, stratiform clouds form

• Lifting Action

  • Unstable air will not rise without a lifting action

  • Warming from below

  • Orographic lift

  • Frontal systems

• Upward currents, combined with a lifting action, also enhance the growth rate of precipitation

Thunderstorm Lines: Squall Line

• A non-frontal, narrow band of active thunderstorms, usually producing the most severe conditions

• Usually preceding a fast-moving cold front

• Produce the most intense hazard to aircraft

• Sudden increase in speed of at least 16 knots knots or more, lasting for at least one minute

Thunderstorms: Embedded Thunderstorms

• Thunderstorms embedded in clouds and cannot be seen

• If encountered, reduce to turbulence penetration speed and attempt to maintain level attitude

  • Altitude may be impossible to maintain

  • Turning will increase load on aircraft

Mechanical Turbulence

Turbulence due to features on the surface

Convective Turbulence

• Warm pockets of air rise until they
  meet the capping stable layer

• Typically stronger during warmer
  months and around mid-day

Clear Air Turbulence

• Usually at high altitude, especially
  near the jet stream

  • Can occur at any altitude

Mountain Wave Turbulence

• Stable air blowing 40 knots or more
  blowing across mountain ridges

• Lenticular clouds or roll clouds may
  develop

  • Lenticular

    • Lens-shaped clouds which form on top the mountain or on wave crests

      • Also known as Standing Lenticular

      • Altocumulus or cirrocumulus Standing
        Lenticular

Wind Shear

• Any change in wind speed and/or direction

• Can occur at any altitude, in all directions

  • Is present at specific phenomena

• Present at

  • Frontal Boundaries

  • Just before a warm front passage

  • Just as or after cold front passage

  • Inversion Layers

  • Strong surface winds

  • Mountain wave turbulence

  • In and around thunderstorms
    Wind Shear

Microburst

Strong, short-lived downdraft of air from thunderstorms

Wind Shear Alerting

• Low Level Wind Shear Alert System (LLWAS)

• Terminal Doppler Weather Radar (TDWR)

• Allows for wind shear prediction due to presence of thunderstorms

• Pilots can expect wind shear due to an inversion layer when winds 2,000’- 4,000’ AGL are 25 knots or more

Fog: Advection

• Warm, moist air over low, flatland areas on clear, calm nights.

• Commonly from an air mass moving inland from the coast in winter.

Fog: Radiation

At night in still air, ground cools air above

Fog: Upslope

Moist air moved upslope

Fog:Precipitation-induced

Precipitation falls through a layer of cold air, saturating it

Fog: Steam

• A type of advection fog caused by the movement of cold air over warm water

• Low-level turbulence and icing can be hazardous

Fog: Ice

Extremely cold temperatures cause water to sublimate as ice crystals

Fog: Smoke

Combustion particles suspended in the air

Fog: Smog

Mixture of smoke and fog

Volcanic Ash

• Ash plume may not be visible, especially at night or in IMC

• If visible, may be indistinguishable from clouds

• When ingested into engine, melts and adheres to fuel injectors, valves, and clogs air filters

• Piston aircraft are less likely to lose power, but severe damage almost certain

• Abrasive damage to exterior of aircraft will occur

How does structural icing form?

• For structural icing to form:

  • Below freezing temperatures at on the surface

  • Visible moisture

    • Rain, clouds, fog, etc.

Ice Formation: Causes

◦ Temperature below freezing at the surface
◦ Visible moisture

Ice Formation: types

• Induction

  • E.g. Carb Ice

  • Impact

• Structural

  • Clear

  • Rime

  • Mixed

  • Ice Formation

Frost

• When the temperature and Dewpoint meet and the Dewpoint is below freezing

• Frost spoils the smooth flow of air over the wings, thereby decreasing lifting capability.

  • Changes the effective shape of the airfoil

• If frost or icing is suspected, pilots should use a faster than normal approach speed

• The changed airfoil shape will likely have a lower critical angle of attack

Clear Ice

• typically forms when temperatures are around 2 ° C. to -10° C

• Often a result of freezing rain

• most dangerous type of structural ice

• changes the shape of the airfoil

Rime Ice

• Forms when droplets freeze immediately on contact

• Typically occurs with temperatures between -15° C. and -20 ° C

• Milky, opaque appearance resulting from air trapped when it strikes the leading edge of an airfoil and freezes

Mixed Ice

• Combination of clear ice and rime ice that has the worst characteristics of both

• Can form rapidly when ice particles become embedded in clear ice and build a very rough accumulation

• Most likely to form at temperatures between -10° C to -15° C

Supercooled Large Droplets

• Much larger water droplets that may freeze beyond protected surfaces

• Freezing Rain is most likely to have the highest accumulation rate

• Look for:

• Unusually extensive coverage of ice, visible ice fingers, or ice feathers on parts of the airframe not normally covered by ice.

  • Probes and antennas are likely to first signify icing accumulation

Impact Icing

• Formed by moisture laden air at temperatures below freezing, striking and freezing on elements of the induction system which are at temperatures of 32 degrees F° or below. (AFFECTS CARBURETOR PLANES)

Tailplane Icing

• Caution should be used for a tailplane stall

• Use partial or no flaps to prevent disruption of airflow over the horizontal stabilizer

  • If flaps were just increased, the pilot should immediately retract flaps to the previous setting and apply appropriate nose-up elevator pressure

• Increase airspeed appropriately for the reduced flap extension setting
  

• Apply sufficient power for aircraft configuration and conditions

• Make nose-down pitch changes slowly

• If a pneumatic deicing system is used, operate the system several times in an attempt to clear the tailplane of ice.