Types of Thunderstorms Explained

"Thunderstorm" covers everything from a brief afternoon shower with a few rumbles to a long-lived supercell that drops softball hail and produces a violent tornado. What separates them isn't just strength but structure: how the storm organizes its rising and sinking air. Once you can recognize the main storm types on radar, you can make a good guess about how long a storm will last and what hazards it's likely to bring.

The ingredients

Every thunderstorm needs three things:

• Moisture in the lower atmosphere, to form clouds and rain. In much of the U.S., it comes north from the Gulf of Mexico.
• Instability: warm, moist air near the ground with colder air above it, so that a rising parcel of air keeps rising on its own, like a hot-air balloon.
• Lift, something to start air rising: daytime heating, a cold front, a sea breeze, a mountain slope, or the outflow from another storm.

A fourth ingredient decides how organized a storm becomes: wind shear, meaning a change in wind speed or direction with height. With little shear, a storm's rain falls straight back down through its own updraft and chokes it off. With strong shear, the updraft tilts, rain falls away from it, and the storm can sustain itself for hours. Shear is the main difference between a storm that lasts 30 minutes and one that lasts all afternoon.

Single-cell storms

The basic thunderstorm, sometimes called an ordinary or "pulse" storm, forms where moisture and instability are high but shear is weak, typical of humid summer afternoons. It goes through a life cycle of about 30 to 60 minutes: a towering cumulus cloud grows (developing stage), rain and lightning begin as the downdraft forms (mature stage), and then the downdraft takes over and the storm rains itself out (dissipating stage).

On radar, single cells pop up as isolated blobs that blossom and fade in place within a few frames. They're usually not severe, but a strong one can briefly produce small hail, heavy rain, and a sudden gust of wind as it collapses. Their lightning is just as dangerous as any other storm's.

Multicell clusters

Most thunderstorms are actually clusters of cells at different stages. As each cell matures, the cool air flowing out of its downdraft (the outflow or gust front) lifts the warm air ahead of it, triggering a new cell next door. The cluster as a whole can last for hours even though each cell lasts less than an hour.

On radar, a multicell cluster looks like a group of cores in different stages, often with new cells forming on one side (frequently the south or southwest side in the central and eastern U.S.) while old ones decay on the other. Because new cells keep forming in the same area, clusters can produce heavy rain and flash flooding (see Flash Flooding & Rainfall Totals Explained), along with moderate hail and gusty winds.

Squall lines and bow echoes

When storms organize along a boundary, typically a cold front, they can form a continuous line that may stretch for hundreds of miles. Meteorologists call it a squall line or a quasi-linear convective system (QLCS). The leading edge brings a sudden wind shift, a sharp temperature drop, and a burst of heavy rain, usually followed by a long band of lighter, steady rain behind it.

On radar, squall lines are easy to recognize: a narrow band of reds and oranges on the leading edge with a broad trail of green behind. Watch for two features:

• Bow echoes: a section of the line bulges forward into a bow or crescent shape. The bow is driven by a strong rear-inflow jet pushing down and forward, and the apex of the bow is where the most damaging straight-line winds occur.
• Kinks and notches in the line, and the ends of bows, especially the northern end, can spin up brief tornadoes. These QLCS tornadoes are often weak, form quickly, and are wrapped in rain, which makes them hard to see and to warn for.

Supercells

A supercell is a thunderstorm with a deep, persistent rotating updraft, called a mesocyclone. It needs strong shear that changes direction with height as well as speed, so that the updraft itself starts to rotate. Supercells are the least common storm type, but they produce most of the strong and violent tornadoes and nearly all of the giant hail. A supercell can last for several hours and travel more than a hundred miles.

On radar, supercells tend to be isolated storms that stand apart from the clutter, often moving noticeably to the right of the general storm motion. The classic signature is a hook echo on the storm's rear flank, where precipitation wraps around the rotating updraft. Some supercells drop so much rain and hail that the hook is buried in heavy precipitation (forecasters call these high-precipitation supercells); these are especially dangerous because any tornado is hidden in rain. Others drop almost no rain at all.

For more on where and when supercells are most common, see the Tornado Alley Weather Guide, and for their hail, Hail Explained.

Downbursts and straight-line winds

Not all damaging wind comes from tornadoes. A downburst is a powerful column of sinking air that hits the ground and spreads out in all directions. Rain evaporating into dry air beneath a storm chills the air, making it heavier, and it plunges to the ground. A small one, less than about 2.5 miles across, is called a microburst; microbursts can produce winds over 100 mph, comparable to a strong tornado, and are a serious hazard to aircraft during takeoff and landing.

Downburst damage is usually distinguished from tornado damage by its pattern: trees and debris all blown outward in the same direction or fanned out from a central point, rather than twisted or thrown in converging directions. A storm doesn't need to look impressive on radar to produce one. Some of the most damaging microbursts come from modest-looking storms over dry ground in the West.

Derechos

A derecho (Spanish for "straight") is a widespread, long-lived windstorm produced by a fast-moving line of storms, often a series of bow echoes. To count as a derecho, the damage path needs to stretch hundreds of miles. They most often happen in late spring and summer across the Midwest and Plains, sometimes continuing to the East Coast, and they can cause tornado-like damage over enormous areas and knock out power to millions. The August 2020 derecho that crossed Iowa, with gusts estimated well over 100 mph, was one of the costliest thunderstorm events in U.S. history.

What makes a thunderstorm "severe"

The National Weather Service classifies a thunderstorm as severe when it produces at least one of: hail one inch in diameter or larger, wind gusts of 58 mph or more, or a tornado. Heavy rain and frequent lightning don't make a storm officially severe, though either can be deadly. See NWS Watches vs. Warnings Explained for the alerts that go with severe storms, and Severe Weather Outlooks Explained for how forecasters flag the risk days in advance.