Funnel, tornado, debris cloud, gustnado
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The one definition every spotter is expected to know cold, and the three near-misses around it.
In this lesson you will
- State the NWS definitions of funnel cloud and tornado.
- Apply the debris-cloud rule: a debris or dust whirl confirms a tornado with no condensation funnel present.
- Explain why a gustnado is not a tornado under NWS convention, and why local practice varies.
- Define landspout and cold-air funnel.
The criterion
A condensation funnel extending from the base of a towering cumulus or Cb, associated with a rotating column of air that is not in contact with the ground (and hence different from a tornado). 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.
That single paragraph contains the whole distinction, and clause b is the half people forget. The test is not "does the funnel reach the ground". It is "is the circulation reaching the ground", and a debris cloud proves that on its own.
A debris cloud appearing beneath a thunderstorm will confirm the presence of a tornado, even in the absence of a condensation funnel.
| What you can see | What to call it |
|---|---|
| Funnel aloft, nothing beneath it | Funnel cloud |
| Funnel in contact with the ground | Tornado |
| Funnel aloft, debris or dust whirl beneath it | Tornado |
| No funnel at all, debris whirl beneath the storm | Tornado |
| Debris whirl on the gust front, no storm-scale rotation | Gustnado, see below |
Three of the five are tornadoes and only one of those has the picture everyone has in their head.
Gustnado
A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. Gustnadoes are not associated with storm-scale rotation (i.e. mesocyclones); they are more likely to be associated visually with a shelf cloud than with a wall cloud.
Read that definition carefully, because it argues with itself: it opens with "a small tornado" and then says the thing that makes a tornado a tornado, storm-scale rotation, is absent. In practice the NWS convention is that a gustnado is not a tornado and it is not warned or verified as one.
Local practice genuinely varies here. Some offices want gustnadoes reported anyway, as wind, and some grant latitude on brief spin-ups that are hard to classify in the moment. Find out what your WFO wants before the day you need to know. This lesson cannot tell you, and any course that claims to has not checked.
The practical tell is location, and it is the same tell as lesson 5: a whirl on the gust front, under or near a shelf cloud is a gustnado. A whirl under the updraft base, near a wall cloud is a tornado until proven otherwise.
Landspout and cold-air funnel
- Landspout
- A non-supercell tornado, forming from the ground up along a boundary rather than from a mesocyclone down. It is a tornado, and is reported as one. Typically narrow, tube-like, and often over open country.
- Cold-air funnel
- A small funnel under a shower or weak storm in a cold air mass, with no severe storm present. Rarely reaches the ground and rarely does damage. Report it, and expect it to be logged as a curiosity rather than a warning trigger.
Do not approach a feature to check whether there is debris under it. Module 8 covers positioning properly, but the short version is that the debris field is the part of a tornado that kills people who were trying to confirm a tornado.
Check yourself
You can see a rotating column with a condensation funnel about halfway to the ground, and a clear whirl of dust lifting on the ground directly beneath it. What is it?
- A tornado: the dust whirl beneath confirms ground contact
- A funnel cloud, because the condensation funnel does not reach the ground
- A gustnado, because the visible rotation is at ground level
- Not classifiable without radar confirmation
Hint
The definition has two clauses, a and b. Only one of them is about the funnel touching down.
Answer
A tornado: the dust whirl beneath confirms ground contact
Clause b: a condensation funnel is a tornado if "a debris cloud or dust whirl is visible beneath it". The condensation funnel marks where the air is cold enough to condense, not where the circulation stops. Waiting for the funnel to reach the ground before reporting costs lead time on a tornado that is already there.
Sources
- Branick, A Comprehensive Glossary of Weather Terms for Storm Spotters, NOAA Technical Memorandum NWS SR-145
- NWS online glossary
- NWS Weather Spotter's Field Guide (YPA-201154, June 2011)
- NSSL Severe Weather 101, tornado basics and tornado types
- SPC public domain tornado images, scanned from the NSSL storm intercept archive