How a storm forms
CSENHow a thunderstorm forms
Three ingredients, three stages, and one reason an ordinary storm dies before you have finished moving the car.
A thunderstorm needs three things at once. Moist air near the ground, because the rain has to be made of something. Instability — an atmosphere in which a rising bubble of air stays warmer than its surroundings and therefore keeps climbing on its own. And something to give it the initial shove: a sun-warmed slope, a front, converging winds. Take any one away and the day ends in fair-weather cloud.
That rising motion is convection. Air expands and cools as it climbs, and once it cools to its dew point the water vapour in it condenses into cloud. Condensation releases latent heat, which warms the bubble again — and that is the engine of the whole thing. A flat cumulus grows into a towering cumulus congestus, and once the tower reaches high enough for its top to glaciate into the familiar anvil, it is a cumulonimbus: a thunderstorm cloud.
Every storm cell then passes through three stages. In the cumulus stage the updraft dominates and no rain has fallen yet. In the mature stage both currents run at once: the updraft feeds moisture upward while falling rain drags air down with it. The shear between them is what makes a storm a storm — turbulence, gusts, lightning. In the dissipating stage the updraft stops, the moisture is spent, and only weakening downdrafts remain.
And here is the crux: in the mature stage the downdraft undercuts the updraft and cuts off its supply of warm moist air. An ordinary storm, in other words, smothers itself with its own rain. That is why a single cell typically lasts twenty to thirty minutes. Storms that run for hours get around it — either cells regenerate one after another along a line, or it is a supercell, whose tilted updraft lets the rain fall beside it rather than through it, so the cycle never breaks.
All of this is visible on radar. A new cell appears as a small blob that strengthens through yellow into red within a frame or two, holds briefly, then fades. Play the timeline and you also get the direction it is travelling — which a single still frame can never tell you.
FAQ
Why do storms mostly form in the afternoon?
Because convection is driven by the heated surface. The sun works all morning, ground temperature peaks in the afternoon, and only then is the contrast with the air aloft big enough to send a bubble high enough. Storms born of a front, or of cold air moving in overnight, ignore this rhythm.
How long does a thunderstorm last?
A single cell usually twenty to thirty minutes. Longer episodes mean there are several cells regenerating in turn — or a supercell, which does not smother itself and can run for hours.
What is atmospheric instability?
A state in which temperature falls with height fast enough that rising air stays warmer than its surroundings and keeps climbing by itself. In a stable atmosphere the bubble soon matches its surroundings and stops — which is why heat alone is not enough to make a storm.
Why is the rain so heavy if the storm is so short?
Because all the water the cloud gathered while it grew falls within the few minutes of the mature stage. The brevity and the intensity are the same fact: the cell empties itself at once, and in doing so ends itself.
Where can I see whether a storm is heading my way?
On the live storm board — it combines radar cores with lightning detection and, for a chosen location, works out how long the rain will take to arrive and where it is coming from.