The life of an ordinary cell

The three stages

An ordinary single cell lives about 30 to 45 minutes. The Field Guide gives the wider range as 30 to 60 minutes for a cell, and over eight hours for a system that keeps generating new updrafts, which is a distinction lesson 5 turns into the most useful idea in this module. For one cell on its own, the clock runs like this.

1. 0 to 15 minCumulus stage. UpdraftThe column of rising air inside a storm. Everything a thunderstorm does, from hail to tornadoes, is downstream of how strong and how organised this is. only. The tower builds, the top is hard-edged and cauliflower-textured, and everything the storm has made so far is still being held up. Nothing has reached the ground.
2. 15 to 30 minMature stage. The downdraftAir descending through and out of a storm, driven by the weight of the precipitation and by rain evaporating into drier air below. It is what produces the gust front and the damaging winds. forms roughly 15 to 20 minutes after initiation, when the load of water and ice aloft becomes more than the updraft can carry. Updraft and downdraft now coexist, side by side. This is when the storm produces whatever it is going to produce.
3. 25 to 30 minThe updraft starts to weaken. The downdraft has spread out at the surface and is beginning to get underneath the inflow.
4. 30 to 45 minDissipating stage. Downdraft only. The top goes fibrous and glaciatedTurned from water droplets to ice crystals. A glaciated cloud top loses its crisp cauliflower edges and goes soft and fibrous, which is the visual sign a storm has matured., the anvilThe flat, spreading top of a thunderstorm, where the updraft has run out of buoyancy and is pushed sideways by the winds aloft. An anvil left behind by a storm that has died is an orphan anvil. detaches and drifts, and the rain tails off. The cold air the storm made is still there, and still moving.

What actually builds the downdraft

Three things push air down, and they arrive in that order rather than all at once.

1. Precipitation loading. The updraft is carrying the weight of everything it has condensed. Past a point it cannot, and the water falls, dragging air with it.
2. Evaporative cooling. Falling rain evaporates into unsaturated air below the cloud. Evaporation takes heat out of that air, which makes it denser than its surroundings, which makes it accelerate downward. This is the big one, and it is why the dryness of the sub-cloud layer matters so much in lesson 4.
3. Melting. Hail and graupelSoft, opaque ice pellets formed when supercooled droplets freeze onto a falling ice crystal. Collisions between graupel and ice crystals are what separate charge and make a storm electrified. falling through the freezing levelThe height at which the air reaches 0 °C. A hailstone has to survive the fall from there to the ground, so a high freezing level means more melting and smaller hail at the surface. absorb heat to melt, cooling the air around them the same way.

The result reaches the ground as a dome of cold, dense air that cannot go down any further, so it spreads sideways.

The storm cuts its own throat

cold pool

The dome of rain-cooled air sitting on the surface underneath and around the storm. Denser than the air it is displacing, so it spreads out under its own weight.

gust front

The leading edge of that spreading cold air, arriving as a sharp wind shift and a temperature drop, usually several minutes before the rain. Air ahead of it is forced up over it.

outflow boundary

The same edge, hours later, after the storm that made it is gone. It keeps moving and keeps lifting, and it is a favoured place for the next storms to form.

Here is the whole life cycle in one sentence. An ordinary cell has its updraft and its downdraft in the same place, so the cold poolThe dome of rain-cooled air a storm leaves sitting on the ground under and around itself. Denser than its surroundings, so it spreads out, and what it spreads under decides whether the storm lives or dies. it makes spreads out underneath its own inflow, undercuts it, and starves it. The storm is fed by warm, moist surface air, and the storm has just replaced that air with cold air of its own making.

Every organised storm mode in this module is a variation on solving that problem. A multicellA cluster of storms in which new cells keep forming on one flank while old ones die on the other. The cluster lasts far longer than any cell in it, which is the whole trick. solves it by building the next updraft somewhere the cold air has not reached yet. A supercellA thunderstorm with a single rotating updraft that can persist for hours. The storm type that produces most strong tornadoes, very large hail and the most damaging winds. solves it by tilting, so the precipitation falls out beside the updraft rather than into it. A squall lineA long, narrow line of thunderstorms moving as one system. It uses the cold air it produces as its own source of lift, which is how it keeps going for hours. solves it by using the cold pool as its lift. Nothing else in lessons 5 through 8 will be a surprise if you hold on to that.

What the top of the storm is telling you

• Hard, crisp, cauliflower tops mean the updraft is vigorous enough to keep the cloud droplets liquid and the edges sharp. The storm is still winning.
• Fibrous, soft, glaciated tops mean the updraft has slowed and the top has frozen out into ice crystals. Weakening.
• The anvil is the updraft spreading out horizontally at the equilibrium level, sheared downwind. Its shape is a rough read on the winds aloft.
• An overshooting topA dome punching up through the anvil, thrown there by an updraft strong enough to overshoot the level where it should have stopped. How long one lasts tracks how strong the updraft is. is the updraft punching above the equilibrium level on momentum alone. Nothing but a strong updraft can do it, and how long one persists tracks how strong.
• An orphan anvil is an anvil with no storm under it any more. The updraft has died and left its own exhaust behind, drifting.

That list is deliberately short. Identifying cloud features by eye is module 3, which spends twelve lessons on it and covers the lookalikes. What matters here is the physical link: the top of the storm is a readout on the updraft, and the updraft is what the rest of this module is about.

The Thunderstorm Project

The three-stage model is not folklore. It came out of the Thunderstorm Project, the first large-scale field study of convection, which flew aircraft through storms and ran dense surface networks in two summers: 1946 in Florida, around Orlando, Cocoa and Kissimmee, and 1947 in Ohio, at Clinton County Air Force Base near Wilmington. The report Byers and Braham published in 1949 runs 287 pages and is a US Government work, so it is public domain and readable in full today.

The first field phase happened in our own state, and the model it produced is still what you were taught this afternoon.

What makes it a thunderstorm

The National Weather Service definition is narrow and worth being precise about: a thunderstorm is a storm that produces lightning and thunder. Not heavy rain, not gusty wind, not a dark sky. Lightning. A storm with no lightning is a rain shower however unpleasant it is to stand in.

The charge that makes it happen comes from non-inductive charging in the mixed-phase region of the cloud, where supercooled liquid water, ice crystals and graupel all coexist. Graupel and small ice crystals collide and rebound, and each collision transfers charge. The sign of that transfer depends on temperature and on how much liquid water is present, reversing somewhere around −10 °C to −20 °C. The updraft then does the sorting: light ice crystals are carried up, heavier graupel falls, and the two charge regions separate vertically until the field between them breaks down.

That is as far as this module goes on lightning. It is here because it explains why a strong updraft and an electrified storm are the same observation. Lightning safety is module 8, entirely, and it is not optional reading.