Every thunderstorm on earth is the same machine running at different sizes: warm moist air rising, cooling, condensing, and eventually collapsing under its own rain.
A thunderstorm needs exactly three things. Remove any one and it can't happen.
Moisture: fuel. Dew points in the 60s support storms; 70s make them heavy.
Instability: air that keeps rising once nudged, because it stays warmer than its surroundings as it climbs.
Lift: a trigger. A front, a sea breeze, terrain, or simple daytime heating off a hot surface.
Add a fourth ingredient, wind shear (wind changing speed or direction with height), and ordinary storms become organized, long-lived, and potentially severe.
PRE STORM
T+00 MIN
Surface heating. A parcel of warm moist air is about to be nudged upward.
CLOUD TOPNONE
UPDRAFT0 FT/MIN
RAINNONE
IDEALIZED DIAGRAM. NOT A LIVE OBSERVATION.
INGREDIENTS. SWITCH ONE OFF.
THE STORM MACHINE
One ordinary single-cell storm, from the first thermal off a hot field to its own collapse. Scroll to run it. Drag the timeline to scrub it by hand.
The three switches are the ingredients above. Turn any one off and watch exactly where the machine stops.
Stage one: the towering cumulus
A parcel of warm moist air rises and cools. At the condensation level its moisture becomes visible as a cloud, and condensation releases latent heat, which makes the parcel warmer than the air around it, so it rises faster. The cloud builds vertically, sometimes thousands of feet in minutes. Everything is updraft; nothing falls yet.
Stage two: mature
Eventually the cloud's water load exceeds what the updraft can suspend, and rain begins. Falling rain drags air down with it, creating a downdraft alongside the still-active updraft. This is the dangerous stage: heaviest rain, lightning, hail, and gusty outflow winds all at once.
The classic anvil top forms here, the storm hits the stratosphere, can't rise further, and spreads horizontally downwind.
THE OUTFLOW ARRIVES FIRST
The cold air rushing out of a mature storm spreads along the ground ahead of the rain. That sudden cool wind shift on a hot afternoon, often with a temperature drop and a small pressure jump, is a measured announcement that the storm is close.
Stage three: dissipating
The downdraft eventually chokes off the updraft that fed it, cutting the storm off from its own fuel supply. Rain lightens, the cloud loses definition, and the anvil drifts on alone. A garden-variety storm runs this entire cycle in under an hour.
Why afternoons
Instability is usually generated by surface heating, and the surface reaches peak temperature in mid to late afternoon. That's why summer storms cluster between roughly 2 and 7 PM, and why nocturnal storms usually require a different lift mechanism, a front, or a boundary left behind by an earlier storm.
See it on your own sky
Fat Tail shows these numbers measured by the airport sensor nearest you, and explains each one when you tap it. Free forever, on the App Store now.