Thunderstorms

Three ingredients, three stages, and seven ways to hurt you. Everything else about thunderstorms follows from those.

Weather Information — FI.III.C · CA.I.C · PA.I.C

The three ingredients

A thunderstorm needs moisture, an unstable atmosphere, and something to lift the air. Take away any one and you get no storm — which is why a humid, unstable afternoon can stay quiet until a front, a sea breeze or a mountain slope supplies the lift, and why the storm then appears very quickly once it does.

This is also the most useful forecasting question you can ask. When a forecast hedges on convection, it is usually because two ingredients are certain and the third is not.

The life cycle

Towering cumulus is the building stage: a strong convective updraft, the cloud growing visibly, and no precipitation reaching the ground yet.

Mature begins the moment precipitation reaches the surface. That is the definition, and it matters — falling precipitation drags air down with it, so the storm now has a powerful downdraft running alongside its updraft. Adjacent columns of air moving hard in opposite directions is what makes this the most violent stage.

Dissipating is when the downdraft has spread through the cell and cut off the supply of warm moist air the updraft was feeding on. The storm starves itself. Note what this does not mean: a dissipating cell is still full of precipitation and downdrafts, and the gust front it has already pushed out can be miles ahead of it.

Types, and why single-cell is a red herring

Single cell storms — one cell, one life cycle — are genuinely rare. Nearly every storm you will meet is one of the other two.

Multicell is a cluster of cells at different stages. As the first matures it drifts downwind and a new cell forms upwind to replace it, and this continues as long as the ingredients hold. A multicell line can run laterally for hundreds of miles, which produces the situation with no good answer: too high to fly over, too long to fly around, and far too dangerous to fly under.

Supercell is a single quasi-steady rotating updraft, organised enough to sustain itself for hours, with updrafts reaching 9,000 feet per minute. That organisation is exactly what makes it produce the most dangerous weather.

The hazards

Lightning, damaging winds and microbursts, severe turbulence, icing, hail, rapid altimeter changes, static electricity, and tornadoes. Two are worth dwelling on because they catch people out:

Hail can be thrown outside the cloud. Clear air next to a storm is not proof of safety.

The altimeter lies during rapid pressure changes. A storm passing the field moves the setting faster than you will think to update it.

Microbursts

The most severe form of wind shear, with downdrafts reaching 6,000 feet per minute. What makes a microburst lethal is the sequence, not the strength alone. Flying into one you meet a performance-increasing headwind first — the aircraft climbs, the airspeed rises, and everything looks better than it is. Then the downdraft. Then the same wind on the far side as a rapidly increasing tailwind, taking away the performance it just gave you, close to the ground.

A pilot who corrects for the first part of that sequence — trimming off the unexpected climb — has made the second part worse.

Seeing one coming

Two visual cues are worth memorising: an intense rain shaft at the surface while the cloud base shows virga, and a ring of blowing dust spreading outward on the ground. Both say air is coming down hard.

Alerting systems — LLWAS-NE, TDWR, ASR-9 WSP — exist at major airports. Most smaller fields have none, which is where the visual cues and PIREPs stop being supplementary and become the only warning you get.

The avoidance rule is blunt because it has to be: do not take off, land or operate at low level near an active thunderstorm. If you are caught in shear, the recovery is maximum power and an aggressive pitch for climb — short of the stall.

What is in force right now

Convective SIGMETs are issued for thunderstorm areas, lines and severe cells. Below are the ones active over the United States at this moment — read one, and notice how much of what it tells you is movement and tops rather than position alone.

Keep going

• PIREPs, AIRMETs, SIGMETs and TFRs
• Structural icing and the freezing level
• Turbulence, shear and mountain waves
• How to read a METAR

Sources

Life cycle, storm types and microburst behaviour are from the FAA Aviation Weather Handbook (FAA‑H‑8083‑28); convective SIGMET issuance is in the AIM, chapter 7.