Aviation Weather

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Dry convection

• Superadiabatic layer
• LRgt3C/1,000
• Very unstable air
• Creates bubble of air
• Thermals
• Few hundred to few thousand feet wide
• Lifetime 5 to 10 minutes
• Vertical speeds of up to 2,000 feet per minute

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Dust Devil

• Air around obstructions creates eddies
• If thermal forms in the same area you get Dust
  Devils
• Speed up to 20 kts
• Light wind hot surface
• 5 100 wide
• 100 300 ft. high
• 4 minutes or less

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Vortex Ring

• Thermals have distinct internal circulation

• Narrow core of upward motion
• Surrounded by weaker sinking motions
• Draws outside air into the thermal as it rises
• Halts when it reaches stable air

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Cloudy convection

• Convective condensation level
• Flat base of the clouds where air first reaches
  saturation
• Above air cools at the SALR
• Equilibrium level
• Updraft temperature is equal to the surrounding
  air so updraft slows
• Cloud top is where the updraft stops

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Downdraft development

• Precipitation induced
• When the cloud top exceeds the freezing level
• Updraft can no longer support the large
  particles, they begin to fall dragging air with
  them.

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Thunderstorm structures / types

• Airmass thunderstorm
• Severe thunderstorm
• Single cell
• Supercell
• Multicell

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Thunderstorm Stages

• Cumulus stage
• Air rises throughout cloud
• Grows to 20,000
• Reaches next stage of development in 15 minutes

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Thunderstorm Stages

• Mature Stage
• Begins with rain
• Lightning and thunder start
• Downdrafts produce gusty surface winds
• Develops anvil shaped top (reached the
  Stratosphere gt36,000)

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Thunderstorm Stages

• Dissipating Stage
• Begins in about 30 minutes
• Downdrafts cut off updrafts
• Takes on a stratiform appearance
• Life of a single-cell thunderstorm is usually
  lt hour

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Products of thunderstorms

• Tornados
• Hail
• Lightning

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Multicell Thunderstorms

• Compact cluster of thunderstorms
• Airmass thunderstorms at different stages

• Cell Regeneration
• Storms vary in intensity

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Supercell Thunderstorm

• Occurs in middle latitudes (most common in
  southern Great Plains in summer)

• Much larger
• Updrafts and downdrafts dont mix
• Steady-state

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Supercell Thunderstorm

• Right side (NW) is downdrafts
• Left side (SW) is rain-free flanking line

• Middle (W) is main updrafts
• Tornadoes occur here

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Tornadoes

• Winds exceeding 180 kts
• Usually associated with severe thunderstorms
• Diameters from 300 to 2,000
• Move SW to NE at 30 kts
• Funnel clouds Water-spouts

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Hail

• When ice crystals grow we normally expect snow
• Snow collides with water droplets and freeze
  (accretion)
• Strong updrafts suspend hail
• Can be found in clear air

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Lightning

• Visible electric discharge
• Several forms
• In-cloud, cloud-to-cloud, cloud-to-ground, sheet
  lightning, heat lightning.
• 300,000 volts per foot, 50,000F
• Typically 3-4 flashes per minute

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Requirements for development

• Sufficient water vapor
• Unstable lapse rate
• Initial upward boost (lifting)

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Instability patterns

• Lifted index
• Difference between observed temperature at 500 mb
  and temperature a parcel of air lifted from
  surface to 500 mb
• If observed is colder than lifted, then the
  lifted index is negative (unstable)
• Lifted index evaluates the severity of the
  thunderstorm not the probability

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Instability patterns

• K index
• Useful in determining the probability of a
  thunderstorm
• T Temp, D Dewpoint

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Stability Chart
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Weather Radar

• Uses electromagnetic radiation to detect objects
• Consists of a transmitter and a receiver
• Doppler Radar
• Has the ability to determine the velocity
• NEXRAD
• More powerful Doppler radar
• Can determine winds at a distance of 60 miles out

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Color Radar

• Black 0 20 dbz
• Green 20 30 dbz
• Yellow 30 40 dbz
• Red gt 40 dbz (1/2 inch of rain per hour)
• Flashing Magenta gt50 dbz

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Is It A Hazardous Storm?

• Is the atmosphere significantly unstable?
• Dewpoint gt 10C?
• Temperature/dewpoint spread gt 17C?
• Movement gt 10kts?
• Visible evidence?
• Southernmost cell in a line?

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How Hazardous Is It?

• Height greater than 15,000?
• Gradient or shape asymmetrical?
• Reflectivity gt 50 dbz?
• Casting a shadow?

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Round or Oval Shape
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Gradients
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Gradients
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Pendant Shape
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Notches, Fingers, Hooks
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Radar Shadows
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Thunderstorm Avoidance

• Don't land or takeoff in a thunderstorm
• Don't attempt to fly under a thunderstorm
• Don't fly without airborne radar into embedded
  thunderstorm
• Do avoid by at least 20 miles any thunderstorm

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Thunderstorm Avoidance

• Do clear the top of a known severe thunderstorm
  by at least 1,000 feet altitude for each 10 knots
  of wind speed
• Do circumnavigate the entire area if the area has
  6/10 thunderstorm coverage
• Lightning indicates the probability of a strong
  thunderstorm.

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Thunderstorm Avoidance

• Do regard as extremely hazardous any thunderstorm
  with tops 35,000 feet or higher

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Thunderstorm Penetration

• Tighten your safety belt
• Hold your course, dont turn back
• Penetrate below the freezing level or above the
  level of minus 15 degrees Celsius to avoid
  critical icing conditions
• Establish power settings for turbulence
  penetration airspeed

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Thunderstorm Penetration

• Turn up cockpit lights
• Do maintain constant attitude let the aircraft
  "ride the waves."