Aviation Weather: Weather Hazards

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Aviation Weather Weather Hazards

• AF 202 Chris Dimoulis

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Objectives

• Thunderstorms
• Wind Shear
• Turbulence
• Icing
• Fog
• Additional Hazards

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Thunderstorms
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Thunderstorms

• Requirements for thunderstorms
• Moist air
• Instability
• Initial lifting action

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Thunderstorms

• If you have the moisture and instability, you
  need a lifting action to set off the
  instability
• Heating (thermals)
• Orographic lifting
• Fronts
• Low-level Convergence
• Upper-level Divergence

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Thunderstorms

• Heating (Thermals)
• When the ground becomes much warmer than the air
  above it, the Lapse Rate is GREATER than standard
  (2-3 deg /1000 ft)
• The slightest displacement of that warm air (say
  a wind gust) will send that warm air into a
  vertical motion
• This creates bubbles of rising warm air that we
  call thermals

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Thunderstorms

• Heating (Thermals) cont.
• Thermals can have horizontal dimensions of a few
  hundred to a few thousand feet.
• Vertical speeds can be a few hundred F.P.M. to
  2,000 F.P.M
• Thermals can happen as long as the ground is
  warmer than the air above it (i.e. cold front
  passage)
• Most common in hottest months and time of day
  (i.e. Southern Illinois in the summer)

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Thunderstorms

• Orographic lifting
• A fancy name for when air is lifted due to a
  mountain or other terrain
• Fronts
• The wedge of the warm or cold front lifts the
  warm air mass and sets off any instability

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Thunderstorms

• Low-Level Convergence
• Convergence is when the air horizontally
  converges into an area
• This creates a higher pressure and the only place
  for that air to go is up
• Upper-Level Divergence
• Divergence is when the air horizontally diverges
  from an area
• This creates a upper area low pulling air up from
  the surface

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Thunderstorms

• Three stage of the thunderstorm
• Cumulus
• Mature
• Dissipating

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Thunderstorms
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Thunderstorms

• Cumulus Stage
• Updrafts predominate and are continuous
• Updraft area is much larger than just one
  individual thermal
• You can visually see one particular cloud growing
  faster than all others (and all others may even
  shrink as the feed the one)
• This stage could grow the cloud all the way to a
  towering cumulus (about 20,000 feet)
• Usually does not produce precipitation, but can
  (though it will not become mature)

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Thunderstorms

• Mature Stage
• Begins when precipitation-induced downdrafts
  reach the ground
• Lightning and thunder
• Well organized circulation in the cloud
• Top of cloud reaches into the lower stratosphere
• The well known anvil shape is formed
• A cumulus stage that produces precipitation does
  not reach mature stage and does not produce
  lightning and thunder

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Thunderstorms

• Dissipating Stage
• Characterized by precipitation and downdrafts
• Updrafts are cut off so there is no more
  development
• Ends with stratiform clouds since the anvil layer
  is often an ice cloud, it still lingers after the
  storm dissipates

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Thunderstorms

• Single Cell
• Cumulus stage lasts 15 minutes
• Mature stage lasts about 30 minutes
• Total lifespan of a single cell storms is about 1
  hour

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Thunderstorms

• Multicell Thunderstorms

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Thunderstorms

• Multicell Thunderstorms
• The key to prolonged life is the gust front
• Cumulus stage clouds develop
• As they move into mature or dissipating stage,
  more cumulus stage clouds develop

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Thunderstorms

• Multicell Thunderstorms

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Thunderstorms

• Supercell Thunderstorms

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Thunderstorms

• Supercell Thunderstorms
• Produces the extreme convective activity
• Strong updrafts producing cloud bulges above the
  anvil top
• Updrafts and downdrafts remain separated which
  prolongs its life

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Thunderstorms
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Thunderstorms
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Thunderstorms
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Thunderstorms
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Thunderstorms
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Hazards

• Tornados
• A violently rotating column of air
• Winds up to 200 miles per hour (or more!)
• Incredible localize damage

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Hazards
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Hazards
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Hazards
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Hazards

• Hail
• A result of very strong updrafts
• Can be thrown out the top of the thunderstorm
• Tenths of an inch to larger than a softball

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Lightning

• Electric discharge
• Heats surrounding air to 50,000 deg. F. causing
  air to rapidly expand
• Thats what causes the shockwave called thunder
• In Cloud, Cloud to cloud, cloud to ground

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Hazards
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Hazards

• Downdrafts
• Can cause low level wind shear

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Pilot advice

• Stay 20 NM away from a thunderstorm
• And dont fly through or under one!

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Wind Shear and Turbulence
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Wind Shear

• Definition
• A change in wind direction and/or speed within a
  very short distance
• Causes of Wind Shear
• Microburst
• Passing fronts
• Temperature Inversions

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Wind Shear

• Low Level wind shear is most hazardous to pilots
• A sudden increase in wind or a switch from tail
  wind to head wind could give the pilot false
  indications

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Turbulence

• Technically defined as bumpiness in flight
• Types
• Low Level Turbulence (below 15,000ft)
• In and near thunderstorms
• Clear air
• Mountain wave

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Low Level Turbulence

• Mechanical Turbulence
• The stronger the wind, the higher mechanical
  turbulence can reach

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Low Level Turbulence

• Thermal Turbulence
• An unstable cold airmass over warm ground is
  prime ingredients for thermals
• Hot air rising causes vertical motion

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Clear Air Turbulence

• Occurs in the free air away from any convective
  activity
• Above 15,000 ft where beyond the affects of low
  level turbulence
• Cannot usually be seen unless there are any
  clouds to show affect

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Clear Air Turbulence
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Clear Air Turbulence
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Mountain Wave Turbulence

• Stable air flowing over mountains
• Can affect air patter for 100s of miles

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Mountain Wave Turbulence
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Icing
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Icing

• Icing occurs
• When there is visible moisture or precipitation
• Temperatures of aircraft skin are below freezing

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Icing

• Types
• Clear
• Rime
• Mixed

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Icing

• If you pick up icing
• 180 to get out of cloud
• Climb to find temperature inversion or sun
• Descend to warmer temps

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Fog
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Fog

• A cloud below 50 feet
• Needs condensation nuclei and moisture
• Types
• Radiation
• Advection
• Steam
• Upslope
• Ice
• Precipitation

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Fog

• Radiation (Ground Fog)
• Little or no wind
• Common in valleys and low lying areas
• Ground is cooledrapidly
• Air above is then cooled to dew point

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Fog

• Advection Fog
• Wind above 15 knotswill make stratus clouds
• Water changes tempslower than terrain
• At night water remainswarmer
• In the morning waterremains colder

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Fog

• Steam Fog
• Cool air over warm water
• Resemblessmoke

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Fog

• Upslope Fog
• Moist stable air is forced up sloping land
• As air climbs, temperature is dropped to dew
  point.

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Fog

• Ice Fog
• Temps below freezing
• Water vapor sublimateto ice crystals
• Blinding with light

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Fog

• Precipitation Fog
• Warm rain falls through cool air
• As rain falls some evaporates and saturates the
  air
• Does not enduntil rain does

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Additional hazards
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Volcanic Eruptions
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Sand/Dust Storms
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Cows