Cloud building blocks for spotters
Before you can tell two features apart you have to be able to name them. This is the vocabulary the other ten lessons are written in.
Four stages, one question
The question at every stage is the same one module 2 asked: what is the The column of rising air inside a storm. Everything a thunderstorm does, from hail to tornadoes, is downstream of how strong and how organised this is. doing? The cloud is the only readout you have from the ground, and its top is the part of it most directly connected to the answer.
- Cumulus
- A fair-weather heap cloud, wider than it is tall or about square. On its own it says the air is unstable enough to make a cloud and not much else.
- Cumulus congestus
- A tall, hard-edged, cauliflower-textured convective tower with no anvil. Capable of heavy showers, and not yet a thunderstorm., TCu to a pilot. Distinctly vertical, hard cauliflower edges, no The flat, spreading top of a thunderstorm, where the updraft has run out of buoyancy and is pushed sideways by the winds aloft. An anvil left behind by a storm that has died is an orphan anvil.. Capable of moderate to heavy showers, and not yet a thunderstorm.
- Cumulonimbus calvus
- The top is starting to lose its cauliflower outline. The WMO description is "a whitish mass without sharp outlines. No fibrous or striated parts are visible". The transition is under way.
- Cumulonimbus capillatus
- A "clearly fibrous or striated structure, frequently in the shape of an anvil". The top has Turned from water droplets to ice crystals. A glaciated cloud top loses its crisp cauliflower edges and goes soft and fibrous, which is the visual sign a storm has matured. into ice crystals. Mature, and frequently with squalls and hail.
- Cumulonimbus incus
- Capillatus with the anvil properly formed and spread out at the equilibrium level. This is the mature thunderstorm.
The anvil
The anvil is the updraft hitting the equilibrium level, running out of buoyancy, and spreading sideways because it cannot go up. Module 2 lesson 1 has the thermodynamics; what matters here is that an anvil is evidence the storm has reached its ceiling and is organised enough to hold a top there.
Its shape is a rough read on the winds aloft, since the ice is being carried downwind as it spreads. Lesson 2 uses that: an anvil spreading the wrong way, upwind, means the updraft is strong enough to fight the flow.
What the species do not tell you
Nothing on this page is a severity classification. A cumulonimbus capillatus incus is a mature thunderstorm, which describes its stage of life and not how dangerous it is. Plenty of severe weather comes out of storms that never look impressive, and plenty of magnificent anvils sit over storms doing nothing at all.
You are watching a tall convective tower. Its edges are crisp and cauliflower-textured all the way to the top, and there is no anvil. What are you looking at?
One feature separates towering cumulus from cumulonimbus, and it is about the texture of the top.Crisp cauliflower edges mean the top is still liquid and still growing: towering cumulus. Cumulonimbus begins when the top glaciates and goes fibrous. A hard top does say the updraft is vigorous, but vigorous and severe are different claims, and neither is a thunderstorm until there is lightning.