What makes a thunderstorm
A thunderstorm is a deep convective cloud system. It needs instability, moisture and a lifting trigger. Once a cumulonimbus is established, the most serious operational threats are not just the cloud itself, but the violent air movements around it.
Three ingredients, one result
For a thunderstorm to form, a substantial depth of air must have a steep lapse rate, there must be enough moisture for condensation and latent heat release, and some mechanism must start the air rising. A useful practical picture is a parcel that is warm enough to keep accelerating upward after it reaches its condensation level.
What can trigger the first rise?
| Trigger | What forces the air upward? | Typical result |
|---|---|---|
| Surface heating | Warm ground creates buoyant thermals. | Afternoon air mass storms over land. |
| Convergence | Surface winds meet and have nowhere to go except up. | Storms near lows, troughs and sea breeze boundaries. |
| Orography | Wind is lifted over rising terrain. | Cloud on the windward slope, often recurring in one area. |
| Fronts | Warm air is lifted by denser cold air. | Lines of active cells at a cold front or occlusion. |
Subsidence is the opposite of a trigger. It warms and stabilises descending air, which is why it is the least favourable setting for thunderstorm development.
Air mass and frontal thunderstorms
Air mass thunderstorms are usually isolated cells, favoured over land in summer and often reaching their peak in the late afternoon or evening. Frontal thunderstorms occur in organised lines near cold fronts and occlusions. They may occur over land or sea, at any time of day, and can move more quickly with the front.
The life of a cell
A conventional cell passes through three recognisable stages. The names matter because the air motion, precipitation and safest avoidance judgement change from stage to stage.
1. Cumulus or building stage
Strong updraughts only feed a rapidly growing tower. Droplets and ice particles are held aloft, so little reaches the ground. The building stage is often about 15 to 20 minutes, but it is already unsafe to penetrate because the vertical currents can be severe.
2. Mature stage
The first precipitation begins to fall and drags air down with it. Updraughts and downdraughts now coexist, often with intense turbulence, hail, icing, lightning and wind shear. The outflow spreading at the surface is the gust front; its first arrival can bring an abrupt wind shift and a squall.
3. Dissipating stage
Downdraughts dominate. They cut off the warm, moist inflow that powered the cell, leaving rain aloft and a spreading anvil. A complete ordinary Cb cell commonly has a life of roughly two to three hours, although its most violent mature period is much shorter.
When a storm becomes severe
Strong vertical wind shear tilts the updraught away from the precipitation shaft. This prevents the falling rain and downdraught from choking the inflow too early. The result can be a longer lived, more organised severe cell or supercell, often supported by a strong upper wind or jet stream.
The hazards a flight must avoid
Never treat a Cb as only a rain cloud. It is a concentrated weather system with hazards outside the visible cloud boundary, particularly beneath the rain shaft and near the gust front.
| Hazard | Where or when it is worst | Operational meaning |
|---|---|---|
| Turbulence | Inside the cloud, close to the updraught and downdraught boundaries, and below the anvil. | Can be severe or extreme. Do not attempt penetration. |
| Hail | Core and downwind of a strong cell, sometimes beyond visible rain. | May damage an aircraft even outside the cloud edge. |
| Icing | Supercooled water region, commonly 0 to −20°C for the strongest accretion. | Rapid, potentially severe icing. |
| Lightning | Most likely in the mixed phase region, about +10 to −10°C. | Avoid active cells and their anvil region. |
| Wind shear | Gust front, precipitation shaft and microburst outflow. | Critical during take off and approach. |
Hail, rain and radar
Hail grows by collision with supercooled water droplets while it is repeatedly carried up and down in a strong Cb. Heavy rain can mask a more dangerous hail core on weather radar through attenuation, so a dark gap is not automatically a safe route. Use radar to identify the whole structure, not to thread between close echoes.
Practical avoidance
Give a thunderstorm a wide margin and stay clear of its anvil and downwind side. A standard Oxford back question gives 10 NM visual avoidance and 15 NM with airborne radar for a Cb at flight level 300. Radar needs the larger margin because the cell may be stronger and wider than the displayed echo suggests.
Microbursts and regional severe weather
A microburst is a compact, violent descending current that strikes the surface and spreads outward. It is normally less than 4 km across, lasts only a few minutes and produces severe low level wind shear. Virga below a high based cell is a warning sign because evaporation cools the air and accelerates the downdraught.
The approach sequence
On approach toward a microburst, the first edge of the outflow may produce a headwind increase and apparent extra performance. Crossing the core brings a downdraught. On the far side, the flow becomes a tailwind and the aircraft can lose airspeed and lift when it is closest to the ground. The safe response is avoidance, not trying to fly through the sequence.
Severe convective weather in India
Norwesters are severe hot weather thunderstorms over north east India, especially Bengal and Assam, often moving from the north west. Dust storms, locally called Andhi, can accompany thunderstorms in north west India during the pre monsoon hot season. Tornadoes are rotating columns beneath a severe convective storm; they are rare but intensely destructive and require complete avoidance.