The life of an ordinary cell

An ordinary thunderstorm builds the thing that kills it. Watch the sequence once with that in mind and the timings stop being trivia.

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.

• 0 to 15 min: Cumulus stage. Updraft 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.
• 15 to 30 min: Mature stage. The downdraft 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.
• 25 to 30 min: The updraft starts to weaken. The downdraft has spread out at the surface and is beginning to get underneath the inflow.
• 30 to 45 min: Dissipating stage. Downdraft only. The top goes fibrous and glaciated, the 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 graupel falling through the freezing level 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 pool 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 organized storm mode in this module is a variation on solving that problem. A multicell solves it by building the next updraft somewhere the cold air has not reached yet. A supercell solves it by tilting, so the precipitation falls out beside the updraft rather than into it. A squall line 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 top 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 ran in Florida and the second in Ohio, and the model they 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.

Check yourself

An ordinary single cell has been going about 35 minutes. The rain has begun to slacken, the top has gone soft and fibrous, and the wind at the surface is now blowing from the storm toward you. What is happening?

1. The cold pool has spread out under the inflow and cut the updraft off, and the cell is dissipating
2. The storm is intensifying, and the outflow is a sign of a strengthening downdraft
3. The updraft has tilted downshear, which is the start of supercell organization
4. Nothing conclusive, because surface wind direction says nothing about storm stage

Sources

1. NWS Weather Spotter's Field Guide (YPA-201154, June 2011), section 2
2. Byers & Braham 1949, The Thunderstorm: Report of the Thunderstorm Project, US Government Printing Office, 287 pp (public domain)
3. NWS JetStream, the thunderstorm and stability sections
4. NSSL Severe Weather 101, thunderstorm basics
5. Takahashi 1978, J. Atmos. Sci. 35, 1536–1548: riming electrification as a charge generation mechanism
6. Saunders, Keith & Mitzeva 1991, J. Geophys. Res. 96, 11007–11017: the effect of liquid water on thunderstorm charging