Short notes 13: Thunderstorm

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Thunderstorm Formation & Stages

4. Stages of Development

A. Cumulus Stage (Developing Stage)

• Dominated by strong updrafts (up to 3000 ft/min or more).
• Air parcels rise, cool, and condense to form towering cumulus.
• No precipitation yet, only growth by convection.
• Latent heat release maintains updraft.

🟢 Cloud Type: Cumulus Congestus → Developing Cumulonimbus.

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B. Mature Stage (Active Stage)

• Most violent and hazardous stage.
• Updrafts and downdrafts coexist.
• Precipitation begins, leading to downdrafts.
• Anvil top forms due to tropopause stability.
• Lightning, thunder, hail, turbulence, wind shear common.

🟡 Cloud Top: May reach 40,000–60,000 ft (sometimes higher in tropics).

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C. Dissipating Stage

• Downdrafts dominate, cutting off warm air supply.
• Precipitation weakens and stops.
• Cloud dissipates into stratiform remnants.
• Turbulence and lightning reduce.

🔵 Characteristic: Weak convection, rain ending, residual anvil remains.

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5. Energy Source

• Latent heat of condensation — each gram of water vapor releases ~600 cal when condensed, fueling vertical motion.

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6. Classification (by Trigger)

Type	Trigger Mechanism	Example
Air-mass Thunderstorm	Local convection due to surface heating	Summer afternoons
Orographic Thunderstorm	Uplift over mountains	Windward slopes
Frontal Thunderstorm	Lifting along cold/warm fronts	Mid-latitude systems
Convergence Thunderstorm	Surface wind convergence	ITCZ, sea-breeze fronts

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7. Key Data (ICAO Context)

• Cloud Type: Cumulonimbus (CB).
• Vertical Extent: 5,000 ft to >60,000 ft.
• Top Shape: Anvil (due to tropopause).
• Updraft Speed: Up to 6000 ft/min (severe).
• Precipitation: Heavy rain, hail, lightning.
• Temperature Range for Activity: Greatest at 0°C to –20°C levels.

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Thunderstorm Hazards

Thunderstorms present multiple hazards to aviation due to strong vertical motion, turbulence, icing, lightning, and wind shear within and around cumulonimbus (CB) clouds.

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1. Turbulence

• Caused by violent updrafts and downdrafts within the CB.
• Most severe in and near the mature stage, especially near the gust front and anvil edges.
• Can cause loss of control, structural stress, or altitude deviations.
• Avoid by at least 20 NM horizontally from any CB cloud.

⚠️ Never attempt to climb over a CB — vertical extent can exceed 60,000 ft.

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2. Lightning

• Result of charge separation within the cloud (positive top, negative base).
• Hazards:
  • Temporary blindness or distraction to crew.
  • Compass errors, radio interference, or electrical damage.
  • Pitting or burn marks on aircraft skin or propeller tips.
• Not necessarily an indication of the worst turbulence area, but always signals active convection.

Note: Aircraft are designed with static wicks to safely dissipate charges.

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3. Icing

• Occurs in 0°C to –20°C region where supercooled water droplets (SWDs) exist.
• Severe icing near updraft zones and edges of precipitation shafts.
• CB clouds often contain both SWDs and ice crystals, producing mixed icing (clear + rime).

⚠️ Avoid flight through the 0° to –20°C layer inside or near CBs.

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4. Hail

• Produced by repeated vertical circulation of ice pellets within strong updrafts.
• Can reach diameters >5 cm and fall outside the visible cloud.
• Severe airframe and windshield damage possible even in clear air near a CB.

Avoid anvil downwind area — hail may be ejected 10–20 NM from the storm.

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5. Precipitation & Downdrafts

• Heavy rain and downdrafts reduce visibility and cause rapid loss of lift.
• Microbursts: Extremely strong localized downdrafts, 2000–6000 ft/min, cause severe wind shear.
• Gust fronts: Horizontal spreading of cold air from downdrafts, leading to sudden wind shifts at the surface.

Low-level wind shear (LLWS) during takeoff or landing is one of the deadliest thunderstorm hazards.

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6. Pressure and Temperature Effects

• Strong convection can cause rapid altimeter fluctuations.
• Cold downdrafts may create local low-pressure zones, causing aircraft altitude errors if not corrected.

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7. Electrical and Static Effects

• Static charge builds up near CBs, especially around anvils and precipitation zones.
• Can lead to St. Elmo’s Fire — bluish glow at wingtips or windscreen edges.
• Though not dangerous, it indicates intense electrical activity nearby.

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8. Operational Impact

Hazard	Effect on Aircraft
Turbulence	Structural stress, loss of control
Lightning	Electrical interference, minor surface damage
Icing	Lift reduction, weight increase
Hail	Structural damage
Downdrafts/Microbursts	Sudden altitude loss
LLWS	Hazardous during takeoff/landing
Heavy Rain	Visibility reduction, engine ingestion

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Key Data (ICAO Context)

• Most dangerous region: Mature stage, near freezing level to –20°C layer.
• Lightning formation zone: Between –10°C to +10°C.
• Downdraft speed: Up to 6000 ft/min (microburst).
• Wind shear zone: Within 20 NM of CB and below 1500 ft AGL.
• Avoidance: Maintain ≥20 NM clearance, and never fly beneath the anvil.

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