How thunderstorms form

01

What three ingredients does a thunderstorm need?[1]

Moisture, instability and lift. Moisture supplies the water that condenses and releases latent heat. Instability means a rising parcel stays warmer than its surroundings and keeps climbing. Lift provides the initial push, usually from surface heating, a front, or air forced over terrain.

The recipe is common enough that around 1,800 thunderstorms are estimated to be in progress worldwide at any moment. A mature storm tower routinely reaches 10 to 12 km high, where it flattens against the stable air of the tropopause and spreads into the familiar anvil shape.

Moisture

usually low-level humid air, often drawn from a warm ocean or gulf

Instability

a steep temperature drop with height, so rising air stays buoyant

Lift

daytime heating, a cold front, a sea breeze, or terrain forcing air upward

Wind shear

not required, but changing wind with height is what organizes severe storms

02

What are the stages of a thunderstorm?[1]

Storms pass through three stages. The cumulus stage is dominated by rising air building the cloud. The mature stage has updraft and downdraft side by side, and produces the heaviest rain and nearly all the lightning. The dissipating stage is downdraft only, and the storm rains itself out.

A single-cell storm completes all three stages in roughly 30 to 60 minutes. The mature stage is the dangerous one and typically lasts 15 to 30 minutes of that.

The transition to dissipating happens because the downdraft eventually undercuts the updraft that feeds the storm, cutting off its warm moist inflow. Storms that survive far longer do so because wind shear tilts the updraft so its own rain falls clear of it.

03

What types of thunderstorm are there?[1]

Four broad kinds. Single-cell storms grow and die inside an hour. Multicell clusters are groups of cells at different life stages that outlast any one member. Squall lines are chains of storms arranged along a front. Supercells have a single rotating updraft and produce most large hail and strong tornadoes.

Single-cell

one updraft, 30 to 60 minutes of life, usually only briefly severe

Multicell cluster

new cells form on the flank as old ones die, so the cluster persists for hours

Squall line

storms in a line, often ahead of a cold front, known for damaging straight-line winds

Supercell

built around a rotating updraft called a mesocyclone. The source of most violent tornadoes and giant hail

04

How does a thundercloud become electrically charged?[1][2]

Charge separation happens when soft hail collides with tiny ice crystals in the presence of supercooled water. The collisions transfer charge, and the storm's updraft then sorts the particles by weight, carrying light positively-charged crystals upward while heavier negative graupel sinks.

The result is a cloud with a broadly positive top and a negative base, with charge differences large enough to produce potentials of hundreds of millions of volts.

This is why lightning and heavy precipitation go together. A storm needs a vigorous updraft carrying ice and graupel through the same volume, which is exactly the structure that also produces intense radar reflectivity.

Thunderclouds are not the only route to a charged cloud. Volcanic ash plumes and the pyrocumulus clouds above intense wildfires separate charge the same general way, through particles colliding in a violent updraft, and both produce genuine lightning of their own.