Thunderstorms & lightning

Which cloud changes suggest the cap is being overcome?

Follow vertical growth and the transition from liquid-looking towers to an icy upper cloud.

Ingredients, precursors and cloud identification

A thunderstorm is a convective storm with lightning and thunder, associated with cumulonimbus. Sufficient moisture, instability through a suitable depth and a lifting mechanism are needed. Surface heating, convergence, a front, terrain or an existing gust front can supply the lift. A cap can delay or prevent release of instability even with warm moist air below; once overcome, rapid development may follow.

Rapidly growing cumulus congestus or towering cumulus indicates strong vertical development. Altocumulus castellanus can indicate instability in an elevated moist layer and potential for later elevated convection, but it is not by itself proof that a thunderstorm will occur at the surface location. A change from sharp cauliflower towers to a smoother fibrous upper portion, often spreading as an anvil, indicates ice and cumulonimbus development. An overshooting top can mark a strong updraught above the main anvil level.

Deep cells can be hidden within layered cloud or behind haze, terrain or darkness. Thunder, lightning or hail is evidence of cumulonimbus activity even if the tower cannot be seen. A dark flat base alone does not identify storm severity, and an anvil can remain after the original low-level cell weakens. Watch the evolution and combine it with the profile and remote observations.

Can a frontal storm also be part of a squall line?

Use separate descriptions for why the air rises and how cells are organised.

Air-mass, frontal, orographic and organised storms

Air-mass thunderstorms commonly develop from local heating or convergence within a moist unstable air mass, often over land in the afternoon or evening. With weak shear, an ordinary single cell tends to rain into its own inflow and decay relatively quickly. Night-time or elevated air-mass convection can also occur; the label does not require daylight. A single pulse can still produce hail or a damaging downburst.

Frontal thunderstorms occur where frontal lifting releases instability, including along cold fronts and embedded within warm-frontal or occluded cloud. They can occur at any time of day if forcing and instability persist. Orographic thunderstorms are triggered or anchored by forced ascent over terrain; cells may remain near a ridge, regenerate there or move away with the flow. Moisture, stability and shear determine whether the lifting produces shallow cloud or deep storms.

A multicell cluster contains cells at different stages, with new development sustaining the system. A squall line is an elongated organised band, often along or ahead of a cold front but also possible away from fronts. A supercell has a persistent rotating updraught and a particularly organised internal structure. These are organisation categories, so a frontal storm can also be a supercell or part of a squall line. The same regional system can evolve between forms as its environment changes.

When do updraughts and downdraughts coexist?

Separate a typical ordinary-cell clock from the lifetime of a regenerating system.

Developing, mature and dissipating stages

The developing or cumulus stage is dominated by updraughts that lift moist air, form cloud and build precipitation particles aloft. As the cell deepens, latent heating supports buoyancy while entrainment and the surrounding stability influence the growth. The absence of surface rain at this stage does not mean the tower is harmless or that it will remain shallow.

In the mature stage, precipitation and a substantial downdraught coexist with the updraught. In the classical wet-cell model, the first rain reaching the ground marks the transition, but dry sub-cloud evaporation can keep much precipitation as virga while a strong downdraught develops. Surface outflow spreads as a cold pool and gust front, and the upper cloud commonly spreads into an anvil. The combination of up/down currents, precipitation, ice and electrical activity produces the broadest hazard set.

During dissipation, the downdraught/outflow dominates and cuts off or overwhelms the warm moist inflow. Precipitation weakens and the lower cloud decays, leaving a remnant anvil, though lightning, gusts and ice can remain. A typical ordinary cell lasts of order 30–60 minutes, often around half an hour; each stage lasts minutes to tens of minutes rather than one fixed interval. Multicell systems and supercells can persist for hours because organisation or replacement growth sustains them. Do not start a clearance timer from one observed cell.

Does every supercell pass through a tornado stage?

Follow the rotating storm while keeping tornado development conditional.

Organisation, possible tornadogenesis and decay

A supercell develops in an environment where instability, moisture, lift and a suitable wind-shear profile support an organised rotating updraught, the mesocyclone. Tilting and stretching of environmental vorticity help produce the rotation. Separation of important updraught and precipitation/downdraught regions can preserve the inflow, allowing a quasi-steady storm to persist for hours instead of following a short ordinary-cell cycle.

The initial stage builds the deep cell and its rotating structure. In the mature supercell stage, the updraught, anvil, precipitation and downdraughts become strongly organised; large hail, damaging outflow and severe turbulence can occur. A lowered wall cloud or a hook-shaped radar feature can be clues in suitable cases, but neither a visible wall cloud nor a particular reflectivity shape is required for all supercells.

A tornado may develop if low-level rotation intensifies and connects the cloud circulation to the ground through the relevant near-surface processes. Most supercells do not produce a tornado, so the EASA sequence initial–supercell–tornado–dissipating describes a possible tornadic evolution, not an obligatory four-stage path. A tornado can weaken while the parent storm remains active or produces another one. Finally, disrupted inflow, outflow dominance or a less favourable environment weakens the storm; residual anvil and outflow hazards can persist.

Which zones matter beyond the main cloud body?

Compare a single mature cell with a regenerating multicell system.

Inside, beneath and around the storm

Within a mature CB, strong updraughts/downdraughts and their gradients can produce severe turbulence, large vertical acceleration, wind shear and loss of flight-path control. Supercooled liquid, hail and ice can damage the airframe or propulsion and impair sensors; intense rain or mixed precipitation can affect engine operation. Lightning and electrical effects, low visibility and rapidly changing pressure add further problems. No visual feature reliably lists every hazard in one cell.

Beneath the cell and ahead of the rain shaft, the downdraught, gust front and spreading outflow can cause dangerous low-level shear. Hail can fall outside the cloud and be carried downwind beneath the anvil; lightning can reach outside the main precipitation region. Upper/anvil ice and convectively generated turbulence can also extend beyond the strongest radar echo and visible tower. A clear-looking space under an anvil is not a guaranteed safe corridor.

On a single-cell section, mark the updraught, rain/hail downdraught, anvil/downwind region and gust-front outflow. On a multicell plan, add new growing cells along convergence/outflow boundaries and older cells/stratiform regions elsewhere. Individual cells can advect one way while repeated development shifts the whole system another way. Gaps can close, and the hazard footprint changes in three dimensions and with time. A route needs clearance from the evolving area of threat, not just the current coloured core.

How does the fair-weather field become a strong local storm field?

Connect particle collisions, different fall speeds and the growing potential difference.

Atmospheric field and storm electrification

In fair weather the atmosphere has a relatively weak electric field, generally directed downward near the surface: the Earth is negative relative to the upper atmosphere. The air is a weak conductor and the global electrical circuit is maintained by electrified weather systems. This background field is very different from the strong and rapidly varying fields near an active thunderstorm.

Within deep mixed-phase convection, collisions between ice particles, graupel and supercooled droplets transfer charge. Different particle sizes and vertical motions separate those charges: small crystals can be carried aloft while heavier graupel falls or remains lower. A common simplified structure has positive charge in the upper cloud, a main negative region in the middle/lower cloud and sometimes a smaller positive region near the base. Actual charge structure can be more complex or inverted; not every thunderstorm is the same fixed battery.

The separated charges create potential differences within the cloud, between neighbouring clouds and between cloud and ground. Charge on nearby surfaces is induced or redistributed, and the field can become strong near protruding objects. When local breakdown and leader development establish a conducting path, a lightning discharge transfers charge. A lightning flash can occur without heavy rain directly at the observer, and an aircraft can interact with the electrical field outside the visually most active core.

Which way does the leader start, and where does charge transfer occur?

Distinguish a flash type from the direction of one bright process within it.

Cloud, ground and upward discharges

Intra-cloud lightning transfers charge between regions within one cloud; cloud-to-cloud lightning connects separate cloud regions. A cloud-to-ground flash connects a cloud charge region with the surface. Both negative and positive cloud-to-ground flashes occur. Upward lightning begins with an upward-developing leader, often from an exposed tall object under a strong field; an upward bright return stroke in an ordinary downward-initiated flash is not the same classification.

In a common negative cloud-to-ground sequence, a branching stepped leader progresses downward from the cloud. Upward streamers/leaders from objects or the ground may connect with it. A bright return-stroke wave then propagates upward along the established channel while charge transfer occurs. Further leaders and return strokes can reuse the path, creating the apparent flicker of one flash. The direction of the luminous wave should not be confused with the direction of electron motion or the net sign of charge transferred.

Positive ground flashes and upward-initiated flashes have different initiation details; the simple negative-flash sequence does not describe all lightning. For flight hazard assessment the key point is that electrified cloud and the surrounding field matter, not just visible ground strokes. Absence of a ground flash on a detector does not prove absence of intra-cloud activity or a developing electrical hazard.

What does a glow at a protruding aircraft surface indicate?

Separate local ionisation, precipitation static and a high-current discharge.

St. Elmo’s fire and flight consequences

St. Elmo’s fire is a luminous corona discharge produced when a strong local electric field ionises air around a protruding surface, such as an aircraft extremity or mast. It can appear as a blue/violet glow or brush-like light. It is not combustion and does not by itself prove that a lightning strike has occurred. Other windshield or surface discharges can look different; a cockpit description or video alone may not distinguish every electrical phenomenon.

The corona itself is generally much less energetic than a lightning channel, but it is evidence of an electrically active environment that deserves assessment. Precipitation static and associated discharge can also produce radio noise or interfere with reception. Neither the appearance nor disappearance of a glow gives a reliable timer or distance to the next lightning event. Continue assessment of surrounding convection and apply the aircraft/operator guidance.

A lightning strike can produce a bright flash, noise and startle, temporary visual impairment, electrical or sensor disturbances and local surface/radome damage. Large aircraft are designed with bonding, conductive paths and other protection to limit the effects; composite structures use suitable conductive protection as part of the design. That protection is not a reason to seek exposure or assume no damage. Monitor actual indications, apply the approved procedures, report the event and ensure the required post-flight inspection and maintenance assessment.

Which information identifies what is developing beyond the visible core?

Use current data for tactical decisions and older/forecast information for the broader plan.

Forecasts, radar, lightning and practical avoidance

Before flight, use convective forecasts and significant-weather charts, SIGMETs, satellite sequences, radar, soundings and recent reports to assess instability, forcing, organisation, motion and route alternatives. Air-mass heating suggests a different timing pattern from a forced frontal line; terrain can anchor repeated growth; shear can support persistent organised storms. In flight, watch cloud evolution and use current airborne radar, available lightning information and relevant ATC/air-reports. Each source needs its own issue/observation time and valid time.

Ground precipitation radar and airborne radar help locate precipitation structure, but weak reflectivity may occur in ice-rich upper cloud and attenuation can hide further cells. Beam height/tilt and range affect the sampled volume. Data-linked mosaics can be delayed and are suited to strategic routing rather than threading small gaps. A lightning detector or stormscope detects electrical discharges, not every new updraught, cloud boundary, hail region or non-electrified hazardous cell. Detection geometry and the types of flash sensed matter; no lightning symbol is not an all-clear.

Plan lateral avoidance early, with room for growth, uncertainty and the surrounding hazard area; an upwind route is often preferable to the hail/anvil side where practicable, but still needs a substantial margin. Manufacturer guidance commonly uses at least 20 NM lateral clearance from the identified convective threat, with more for rapid growth or other conditions; apply the aircraft/operator’s specific criteria. Do not convert this into a universal boundary outside which every storm hazard is absent. Going beneath a cell or squeezing between developing echoes can expose the aircraft to outflow, hail and closing gaps. Overflight is not automatically available or safe merely because a visible top is below cruise.

If the corridor cannot be kept clear, delay, turn away, divert or land at a suitable alternative while options remain. Obtain necessary ATC coordination and reassess before rejoining the original route. On approach or take-off, evaluate the actual storm/outflow position, alerts and escape options rather than the aerodrome rain report alone. In an inadvertent encounter, use the approved type-specific turbulence/wind-shear/icing procedures; avoid importing one generic aircraft-control sequence from a weather schematic.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.