Funnel, tornado, and the reporting threshold
- View
There is a bright line between a funnel cloud and a tornado, it is written down, and it is not about how the thing looks.
In this lesson you will
- State that a condensation funnel is a tornado if it is in contact with the ground or a debris or dust whirl is visible beneath it.
- Classify a rotating funnel aloft, with no ground contact and no debris, as a funnel cloud.
- State that a debris cloud confirms a tornado even with no condensation funnel.
- Identify the rope stage as a dissipating tornado.
- Distinguish a funnel from scud by the presence of rotation.
The test
Learn this sentence. It is the one module 7 will make you use verbatim, and it decides what you say on the radio.
"A condensation funnel is a tornado, not a funnel cloud, if either a) it is in contact with the ground or b) a debris cloud or dust whirl is visible beneath it."
Everything else follows. A rotating funnel hanging aloft with no ground contact and nothing being kicked up underneath it is a funnel cloud. It is reportable, and it is a different report with different words and a different urgency.
The half everyone forgets
Clause (b) runs in both directions, and the NWS glossary says the harder direction out loud: "a debris cloud appearing beneath a thunderstorm will confirm the presence of a tornado, even in the absence of a condensation funnel."
A tornado is a column of rotating air. The visible funnel is condensation, which happens when the pressure drops far enough for the moisture present to condense, and on a drier day it may never happen at all while the circulation is doing full damage at the ground. No funnel does not mean no tornado.
The rope stage
As a tornado dies, the circulation stretches, tilts and narrows, and the funnel "takes on a contorted, rope-like appearance before it completely dissipates". That is the rope stage, and it is still a tornado. It is dissipating, not dissipated.
Funnel or scud
This is lesson 5 arriving at the moment it matters. The discriminator is rotation, and only rotation. A funnel rotates continuously about a vertical axis and holds a coherent shape while doing it. Scud drifts, rises, changes shape and does not rotate.
| What you see | What it is | Report |
|---|---|---|
| Rotation, ground contact or debris beneath | Tornado | Tornado, immediately |
| Rotation, aloft only, nothing beneath | Funnel cloud | Funnel cloud |
| Debris whirl beneath a storm, no funnel | Tornado | Tornado, immediately |
| A lowering with no rotation | Scud, or a wall cloud that is not rotating | Not reportable as rotation |
One more case, because humid climates produce it constantly and lesson 9 is about it: the funnel may be there and simply invisible behind rain. A rain-wrapped tornado breaks none of the rules above. It just denies you the observation the rules are built on.
Check yourself
Beneath a storm base you can see a swirl of dust and debris being lifted and rotated at the surface. There is no condensation funnel anywhere above it. What do you report?
- A tornado. A debris cloud confirms one even with no condensation funnel
- A funnel cloud, until a condensation funnel becomes visible
- Nothing. Without a funnel there is no way to confirm a tornado
- A gustnado, since surface debris with no funnel is by definition a gustnado
Hint
The threshold has two clauses joined by "or", and this is the second one.
Answer
A tornado. A debris cloud confirms one even with no condensation funnel
A tornado. The NWS is explicit that "a debris cloud appearing beneath a thunderstorm will confirm the presence of a tornado, even in the absence of a condensation funnel". The funnel is condensation and it is optional; the rotating column of air is the tornado. Lesson 8 covers when surface debris is instead a gustnado, and the difference is whether the circulation connects to the storm above.