Ch 7 Quiz: Thunderstorms — Formation, Stages & Hazards

Thunderstorm ingredients: (1) Moisture (high humidity/dew point), (2) Instability (positive CAPE — Convective Available Potential Energy), (3) Lifting mechanism (fronts, orography, surface heating, convergence lines). All three must be present.

Cold air, moisture, and latitude

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Sufficient moisture, atmospheric instability, and a lifting mechanism

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Wind shear, cold front, and mountains

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CAPE, LLWS, and jet stream

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Air-mass (ordinary) thunderstorm life cycle: (1) CUMULUS stage — updrafts only; (2) MATURE stage — both updrafts and downdrafts, heaviest rain, lightning; (3) DISSIPATING stage — mainly downdrafts, rainfall decreasing, anvil remains.

Build-up, mature, dissipating

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Cumulus, mature, dissipating

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Initiation, anvil, clearance

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Cumulonimbus, Cb incus, clear

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Thunderstorm updrafts in the mature stage can reach 50–100 kt (25–50 m/s), exceeding 150 kt in extreme supercells. These extreme vertical currents cause catastrophic structural loads on aircraft entering the storm.

10–20 kt

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50–100 kt (can exceed 150 kt in severe storms)

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200–300 kt

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5–10 kt

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Lightning results from charge separation in the storm: lighter positive ice crystals migrate upward, heavier negatively charged graupel settles lower. When the electric field exceeds ~1 MV/m, leader strokes discharge the potential.

Friction between raindrops

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Charge separation between ice crystals and graupel in the mixed phase zone

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Static buildup on the airframe

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Solar radiation ionising the atmosphere

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Supercell: highly organised, long-lived thunderstorm with a MESOCYCLONE (rotating updraft, 2–10 km diameter). Produced by strong vertical wind shear. Responsible for most large hail (>5 cm), violent tornadoes, and extreme turbulence.

Less moisture

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A rotating updraft (mesocyclone) and long-lived organised structure

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Two cells side by side

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Higher cloud base

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Squall line (SLGT): a linear or curved band of active thunderstorms, typically ahead of or along a cold front. They can extend 500–1,000+ km, produce severe weather continuously, and are difficult to navigate through at any altitude.

Isolated thunderstorm cells

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Linear or curved bands of thunderstorms, often forming ahead of cold fronts — can extend hundreds of kilometres

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Mountain wave clouds

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Frontal fog bands

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Gust front: leading edge of the thunderstorm’s cold outflow at the surface. Marked by sudden wind shift, gust, temperature drop, and pressure rise (presquall line). The gust front can precede the visible storm by 10–20 nm.

The updraft region

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The boundary at low level between the cold outflow and the warm environmental air — marked by gusty winds and possible windshear

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The anvil edge

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A secondary cloud band

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Hail forms in the mixed-phase zone above the 0°C isotherm. Large hail can be lofted to very high altitudes within the updraft and ejected downwind up to 20 nm from the visible Cb. Severe hail causes catastrophic airframe damage.

Below the cloud base

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Above the 0°C isotherm, often to high altitudes — it can be thrown out of the storm anvil up to 20 nm downwind

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Only in tropical thunderstorms

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Only in mature squall lines

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CAPE (J/kg): area on a thermodynamic diagram between the parcel temperature curve and the environment curve above the Level of Free Convection (LFC). CAPE > 1000 J/kg indicates moderate convective potential; > 2500 J/kg severe.

Cloud cover

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The amount of energy available for convective updrafts — larger CAPE = stronger potential storms

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Wind speed at the jet stream

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Pressure gradient

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Thunderstorm hazards that may occur simultaneously: extreme turbulence (gust loads exceeding structural limits), severe icing, large hail, lightning strikes, static discharge, engine flameout (water ingestion, ice crystal icing), and severe windshear.

The rain damages the paint

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Severe turbulence, hail, icing, windshear, and lightning simultaneously — structural failure risk

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Lightning alone is fatal

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Only the engine is at risk

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Embedded thunderstorms (CB EMBD): Cbs concealed within layers of altostratus, nimbostratus, or cirrostratus. Visually indistinguishable from surrounding cloud. Only detected by onboard weather radar. The unexpected turbulence/hail is extremely dangerous.

They produce no lightning

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They are hidden within extensive cloud layers (As, Ns, Ci) and not visible on weather radar unless radar is used

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They are always small

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They only occur over oceans

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ICAO and IATA recommend maintaining at least 20 nm lateral clearance from active Cbs, and more if possible (30–50 nm from large supercell clusters). Hail and severe turbulence can extend 20+ nm from the visible Cb boundary.

2 nm

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5 nm

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20 nm (ICAO recommendation)

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50 nm

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At the tropopause, the temperature lapse rate reverses (isothermal or warming in stratosphere). This creates a very stable layer that caps convective updrafts. The anvil forms as the storm top spreads horizontally along the tropopause.

It is very cold

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Temperature increases with height in the stratosphere (stable), preventing further convective penetration

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It has no moisture

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Wind shear stops convection

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ICAO SIGMETs for thunderstorms are typically issued when CB activity with tops above FL250 is forecast or observed, or when embedded CBs, squall lines, or severe turbulence associated with Cbs is anticipated.

FL100

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FL250 — generally above FL250 thunderstorms are SIGMET-worthy

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FL350

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FL500

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The Cb anvil (Cb incus) = the flat, spreading top at the tropopause composed of ice crystals, shaped by upper-level wind. Anvil cirrus can spread hundreds of miles downwind. The anvil side downwind indicates wind direction aloft.

The storm is weakening

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The storm top has reached the tropopause and is spreading downwind — cirrus-like ice crystal cloud

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The storm is at FL100

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Lightning is ending

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Static discharge (St Elmo’s fire): luminous glow visible on windscreens/engine cowlings/wingtips in electrically active areas. It causes HF/ADF radio interference. Aircraft should avoid thunderstorm areas to prevent lightning attachment.

Compass errors only

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St Elmo's fire (corona discharge), radio interference, and potential lightning attachment

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No hazard

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Fuel to ignite

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Frontal thunderstorms: triggered by mechanical lifting along cold/warm fronts or orographic uplift, independent of solar heating. Can occur at any time including night and winter, and often form squall lines or embedded thunderstorm bands.

They require tropical conditions

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They are forced by frontal/orographic lifting rather than surface heating — can occur at night and in winter

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They are less severe

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They only occur over oceans

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LLWAS: network of anemometers at airport boundaries detecting wind shear. TDWR: Doppler radar detecting divergence signatures of microbursts. PIREPs from departing/arriving aircraft also provide real-time microburst reports.

NDB and VOR

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LLWAS (Low Level Wind Shear Alert System) and TDWR (Terminal Doppler Weather Radar)

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ILS only

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Lightning strike counters

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Standard 6-colour radar: Green (55 dBZ, extreme hail). Red/magenta = avoid.

Green

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Yellow

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Red/magenta (in 6-colour scale: green, yellow, orange, red, magenta, white for most intense)

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Blue

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Tornadoes most frequently spawn from supercell thunderstorms with a well-developed mesocyclone. The rotating updraft concentrates vorticity, tightening into a tornado vortex (Fujita scale EF0–EF5, wind speeds up to >300 km/h).

Ordinary air-mass cell

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Supercell with mesocyclone

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Squall line only

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Embedded Cb

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Multicell cluster: several cells at various stages (developing, mature, dissipating) coexist. New cells continuously develop on the gust-front flank. Can persist for 6–12+ hours. Produce large hail, heavy rain, and gusty winds.

No difference

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Multiple cells at different life-cycle stages exist simultaneously — new cells develop on the flank, maintaining the storm for hours

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They only occur over oceans

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They are always less severe

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Cb anvil: when the updraft reaches the tropopause, it spreads horizontally forming an ice crystal shield. The anvil points downwind, indicating upper-level wind direction. Can spread 100+ km downwind, producing widespread cirrus and possible light turbulence.

Water droplets only

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Ice crystals spread horizontally at the tropopause level by upper-level winds

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Smoke particles

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Virga only

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CAPE thresholds: 0–1,000 J/kg = weak instability; 1,000–2,500 J/kg = moderate (significant storms possible); >2,500 J/kg = strong (severe thunderstorm potential); >3,500 J/kg = extreme (supercell/tornado threat). Values vary by region.

>100 J/kg

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>2,500 J/kg

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>10,000 J/kg

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Lifted Index (LI): LI = T_environment(500hPa) − T_parcel(500hPa). Negative LI = parcel warmer = unstable. LI −2 to −6 = moderate to strong convective risk. LI < −6 = extreme risk. Used as a quick stability assessment from rawinsonde data.

A measure of cloud top height

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The temperature difference between a lifted parcel at 500 hPa and the environment at 500 hPa — negative = unstable, more negative = more unstable

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A wind measurement

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An icing index

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Pre-monsoon convection in India: a dry line (moisture discontinuity) forms over central India where dry north-westerly winds meet moist south-easterly flow from the Bay. Strong CAPE in the moist sector + lifting at the dry line → severe Cb, often with large hail.

Cold front only

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Convergence of dry continental air (NW) with moist marine air (SE) over central India — moisture boundary drives severe afternoon convection

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Monsoon trough

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Bay of Bengal cyclone

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Water loading: as precipitation forms in the updraft, the weight of water droplets/ice reduces the net buoyancy. When water loading exceeds buoyancy, the updraft weakens → precipitation falls → drags air down → downdraft begins (mature to dissipating transition).

Extra fuel is needed

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Rain and hail add weight to updraft air, eventually overcoming buoyancy and initiating the downdraft — key mechanism in thunderstorm mature stage

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No effect on aircraft

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Wind becomes calmer

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Precipitation (p-static): ice crystals or frozen precipitation striking the airframe transfer charge. This builds up to a discharge via static wicks, generating broadband radio frequency noise that interferes with HF, ADF, and occasionally VHF. Does NOT require a thunderstorm.

Only during lightning

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When aircraft accumulates static charge flying through precipitation/ice crystals — corona discharge produces wideband radio noise on HF/ADF/VHF

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Only in thunderstorms

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During clear air only

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Hail formation: embryo (snow pellet/graupel) lofted repeatedly by the strong updraft. Each ascent through the supercooled water zone adds a clear ice layer; each descent adds a rime layer. Cross-section shows onion-like rings. Large hail requires sustained updraft >25 m/s.

Sea spray freezing

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Repeated cycles of growth in the updraft — embryo (graupel) carried upward, collects supercooled water forming clear ice layers, then falls and repeats

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Radiation cooling

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Snow compressing under gravity

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MCS: organised convective systems larger than a single thunderstorm cell. Types: squall line, bow echo (damaging straight-line winds), MCC (Mesoscale Convective Complex — circular shape, long-lived). Major source of heavy rainfall, flash flooding, and aviation hazards.

Single thunderstorms

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Large clusters or lines of thunderstorms covering 100s of km — producing extreme rainfall, large hail, and severe winds at the mesoscale

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Tropical cyclones only

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Mountain waves

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Funnel cloud: rotating condensation funnel extending from Cb base. If it reaches the ground = tornado; if over water = waterspout. Associated with supercell mesocyclones but also non-supercell storms. Never fly below or near a visible funnel cloud.

Sea fog development

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A rotating column of air descending from a Cb — if it touches ground it becomes a tornado; waterspout if over water

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Only light turbulence

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Dry conditions

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Hook echo: distinctive radar signature of a supercell mesocyclone. The precipitation wraps around the updraft area, creating a hook. A bounded weak echo region (BWER/vault) may also be present. Hook echo = high probability of tornado in progress or imminent.

Light drizzle

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Rotation in a supercell — the hook shape results from the storm's RFD wrapping precipitation around the mesocyclone, indicating tornado potential

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A cold front

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MCS boundary

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High dew point (≥18–20°C) is a key severe thunderstorm indicator. High dew point = high moisture content = more latent heat released when air rises → stronger updrafts → higher CAPE → more severe storm potential. Summer days with Td > 20°C are prime severe thunderstorm days.

Below 0°C

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High (e.g. >18–20°C) — high dew point indicates abundant moisture and high latent heat available for deep convection

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Very low

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The same as temperature

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Tilted updraft: vertical wind shear tilts the storm’s updraft away from the downdraft. Without shear, rain falls into the updraft, killing it (single cell, 20–40 min lifespan). With shear, updraft persists → longer-lived, organised, more severe storms.

No effect

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Low-level shear tilts the updraft, separating it from the downdraft — this allows long-lived organised storms (supercells) to persist

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It weakens all storms

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It only affects tornado development

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PIREP sky condition codes: TS = thunderstorm. Example PIREP: UA /OV DPN/TM 1530/FL085/TP B737/SK BKN025 TOP040/WX TS/TB MOD/RM TS OBS N OF ARPT. Provides real-time confirmation of storm location and intensity.

Temperature stable

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The pilot directly observed or flew through/near a thunderstorm — critical information for ATC and other aircraft

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Time of sunset

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Turbulence light

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Supercell propagation: right-moving (NH) due to storm-relative helicity (SRH). The interaction of the rotating updraft with the low-level wind shear vector causes deviation to the right of the mean wind. This is important for storm hazard path prediction.

With the mean wind exactly

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Right of the mean wind in the NH — due to storm-relative helicity and the interaction of the updraft with the wind shear vector

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Left of the mean wind

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Perpendicular to the wind

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