Scud and the look-alikes

ScudRagged low cloud that forms in humid air near a storm and drifts about. It rises slowly and does not rotate, which is what separates it from a funnel, and it is the single most common thing mistaken for one.

Note the giveaway in the first definition: scud is cool moist air, associated with outflow. A funnel hangs from a rotating updraftThe 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., which is inflow. They are produced by opposite halves of the storm, and rising scud in front of a dark base is the single most photographed false tornado there is.

The five questions

The Field Guide gives a checklist for exactly this moment. It is worth memorising, because it works in the order the questions are written: a no anywhere near the top makes the rest moot.

1. Can I see it clearly?
2. Is the feature attached to a thunderstorm base?
3. Is the feature in the section of the storm where tornadoes and funnels typically develop, near the updraft?
4. Is there organized rotation present within the feature?
5. If it appears to be a tornado, is there debris?

Question three is the one that eliminates most bad reports before motion is even considered. If the feature is out along the leading edge of the storm rather than back under the updraft, you are looking at the outflow side, and nothing there is a tornado cue.

Shelf cloudA low, wedge-shaped arcus cloud on the leading edge of a storm, attached to the cloud base above it. It is an outflow feature, it means damaging wind rather than rotation, and it is the feature most often misreported as a wall cloud.: outflow, and never a rotation report

Module 3 lesson 6 is the full treatment of shelf versus wall, and that discrimination is not repeated here. What belongs in this lesson is the one-line consequence: a shelf cloud is outflow. It is not reportable as rotation, whatever it looks like.

The rest of the field

Rain shafts

A dense column of rain, seen edge-on against brighter sky, looks like a wide dark funnel. It has no rotation and its base is diffuse where it meets the ground.

Dust devils

Fair-weather, surface-heating driven, not attached to any cloud base. Question two of the checklist disposes of them.

Gustnadoes

A real whirl, on the gust frontThe leading edge of cool air rushing out of a storm, ahead of the rain. It is what makes the wind pick up sharply just before a storm arrives., unconnected to storm-scale rotation. Lesson 6.

Towers and structures

Grain elevators, chimneys and radio masts against a dark base at distance. Sounds ridiculous until it is dusk and raining.

Cold-air funnels

Small funnels under showers with no severe storm present. Rarely reach the ground and are usually harmless, though a local office may still want to hear.