Cumulonimbus

Cumulonimbus is the only cloud that makes thunder. Everything else in the sky drifts, thickens, or drizzles. This one generates lightning, hail, damaging wind and, given the right conditions, tornadoes. The name is ordinary Latin plumbing — cumulus, a heap, and nimbus, a rain cloud — and it undersells the thing considerably.

It is also the only genus that refuses to stay in its lane. Cloud classification sorts by altitude: low clouds below 6,500 feet, mid-level clouds up to 20,000, high clouds above that. A cumulonimbus crosses all three at once, from a base sometimes barely 1,000 feet off the ground to a top that can pass 60,000. That is why it gets a category of its own — vertical development.

How one builds

A thunderstorm needs three things: moisture, unstable air, and something to give it a shove.

Moisture is usually the easy part. Instability means the atmosphere cools quickly with height, so a parcel of air that starts rising keeps rising — it stays warmer than its surroundings, and warmer means buoyant. The shove can be a front, a mountain, a sea breeze, or simply a bare field that heated faster than the woods beside it.

Once the parcel is moving, condensation does the rest. Water vapour releases latent heat as it condenses, which warms the parcel, which makes it more buoyant, which makes it rise faster and condense more. That feedback loop is the engine, and it is why storm towers grow visibly — you can watch a cumulus congestus harden and climb over the course of twenty minutes.

The cloud earns the name cumulonimbus at the moment its top glaciates: the crisp cauliflower edge, made of water droplets, turns soft and fibrous as it freezes into ice crystals.

The anvil is a ceiling, not a roof

The tower does not rise forever. Somewhere around 40,000 to 60,000 feet it reaches the tropopause, where temperature stops falling with height and begins to rise. Above that boundary the air is stable, and a rising parcel suddenly finds itself colder than its surroundings. It has nowhere left to go but sideways.

That spreading sheet of ice crystals is the anvil, and it is the most useful thing you can read from a distance, because it points. The anvil streams the way the upper-level winds blow, which is usually where the storm is going.

Occasionally a small dome bulges above the otherwise flat anvil top. That is an overshooting top — the updraft was strong enough to punch briefly into the stratosphere before falling back. It is a reliable sign the storm is serious.

The downdraft, and the wind before the rain

Every updraft eventually builds its own opposition. Once precipitation grows heavy enough to fall, it drags air down with it, and rain evaporating on the way cools that air further. Cold air is dense. It accelerates.

When the downdraft reaches the ground it cannot go any further, so it spreads out horizontally in all directions. The leading edge of that cold outflow is the gust front — and on a surface chart it is drawn exactly like a cold front, same blue triangles, because functionally that is what it is. A small one, manufactured by the storm rather than by the general circulation.

This is why the wind picks up and the temperature drops several minutes before the rain arrives. You are standing in outflow. If a low, wedge-shaped shelf cloud rolls overhead at the same moment, that is the gust front made visible.

Why some last an hour and others last all afternoon

In still air, a thunderstorm kills itself. The updraft and the downdraft occupy the same column, so cold outflow undercuts the warm inflow feeding the storm, and within thirty to sixty minutes it starves. That is the ordinary single-cell storm: builds over a summer afternoon, gone by dinner.

Wind shear changes the outcome. When wind speed or direction changes with height, the storm tilts. The updraft leans away from the downdraft rather than sitting on top of it, the two stop fighting each other, and the inflow stays clean. Now it can run for hours.

Push the shear further and the updraft begins to rotate. A cumulonimbus with a persistently rotating updraft is a supercell — uncommon, long-lived, and responsible for a disproportionate share of large hail and violent tornadoes.

What to do about it

The National Weather Service calls a thunderstorm severe at 58 mph winds or one-inch hail. Lightning does not wait for that threshold — every cumulonimbus produces it, which is why the official guidance is blunt: when thunder roars, go indoors.

Sound covers roughly a mile every five seconds, so counting between the flash and the bang gives you the distance. But the useful version is simpler. If you can hear thunder at all, you are already within range of the next strike.