The course has met the cumulonimbus many times: the clouds module's king, the cold front's weapon, the ITCZ's engine, the worst case of every icing and shear lesson so far. This lesson finally takes the machine apart — the clouds that warn of it, the company it keeps, how a cell lives and dies, what a mature storm throws at an aircraft and where exactly it throws it, and the electricity, from the atmosphere's quiet background field to the strike itself. The next lesson adds the downburst, the tornado, and the art of staying away.
The warning clouds
Thunderstorms announce themselves in cloud before they announce themselves in thunder. The syllabus wants three names:
altocumulus castellanus — the clouds module's free warning, turrets betraying instability aloft: castellanus in the morning, storms in the afternoon;
cumulus congestus — the towering cumulus that is the storm's own adolescence;
cumulonimbus — the storm itself, whose transition from a hard cauliflower top to a fibrous top with the spreading anvil marks a mature storm.
The types of thunderstorm
All thunderstorms need the same familiar trio (the thermodynamics module's conditional instability through a deep layer, plenty of moisture, and a lifting trigger from the clouds module's list), and the types are named after their triggers:
Air-mass thunderstorms — summer afternoon heating over land: single or pulse cells, widely scattered, slow-moving, short-lived, the weakest of the family; the winter version fires when cold maritime air streams over a warm sea, the air-masses module's maritime polar cold, heated from below and unstable;
Frontal thunderstorms — organised along fronts, above all the cold front's line, moving with the front, by day or night, and often embedded in layered cloud, which makes them invisible until entered;
Squall lines — a pre-frontal line of storms ahead of a sharp cold front: the most violent organised form short of the supercell, with a long gust front and heavy hail;
Orographic thunderstorms — fired by forced ascent on windward slopes and anchored to the terrain for as long as the flow holds.
The life of a cell
An individual thunderstorm cell lives about an hour, in three acts:
The developing (cumulus) stage — roughly 20 minutes of updraughts only, the tower building;
The mature stage — 20 to 30 minutes, beginning when precipitation starts to fall: updraughts and downdraughts side by side, and every hazard at its peak;
The dissipating stage — downdraughts only, the anvil spreading as the rain weakens: the downdraughts have strangled the updraught that fed the cell.
Multicell storms cheat the one-hour clock by breeding new cells on the flank — the storm you watch for hours is a relay, not a single runner.