Cloud building blocks for spotters
- View
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.
In this lesson you will
- Order the convective progression from cumulus to congestus to cumulonimbus calvus to capillatus and incus, given photographs.
- Distinguish towering cumulus from cumulonimbus by whether the top has gone fibrous.
- Define the anvil, and identify one in a photograph.
- State that the cloud species map to storm maturity, not directly to severity.
Four stages, one question
The question at every stage is the same one module 2 asked: what is the updraft 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
- Towering cumulus, TCu to a pilot. Distinctly vertical, hard cauliflower edges, no 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 glaciated 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.
Hard cauliflower top means still growing. Fibrous, hazy, soft-edged top means glaciated. That one distinction separates a towering cumulus from a cumulonimbus, and you can make it from several miles away with no equipment at all.
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 organized 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.
"Any tall cloud is a thunderstorm." It is not. Module 2 lesson 2 gave the NWS definition: a thunderstorm produces lightning and thunder. A towering cumulus with no anvil and no lightning is a shower, however tall and however dark it looks from underneath.
Check yourself
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?
- Towering cumulus, still growing. Not yet a thunderstorm
- Cumulonimbus capillatus, because the tower is deep
- A severe storm, because hard tops indicate a strong updraft
- Cumulonimbus incus in its early stage
Hint
One feature separates towering cumulus from cumulonimbus, and it is about the texture of the top.
Answer
Towering cumulus, still growing. Not yet a thunderstorm
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.