Everything you have learned to identify can be made unrecognizable by the angle you are standing at, the light behind it, or the ten miles between you and it.

In this lesson you will

  • Explain how viewing angle and distance can make scud or a beaver tail look like a funnel.
  • State the rule: watch a feature for one to two minutes for rotation before deciding.
  • Describe night observation technique, identifying features by lightning illumination.
  • List how backlighting, haze and rain-wrapping reduce reliability, and how to caveat a report.

The angle problem

A long horizontal cloud viewed end-on looks like a vertical one. That single sentence accounts for a large share of false funnel reports, and NWS Northern Indiana attributes several documented ones to beaver tails and scud seen from a poor angle.

Distance compounds it. At ten miles, a feature's depth collapses and you lose the parallax that told you which of two clouds was nearer. A rain shaft in the middle distance and a lowering in the foreground can appear to be one continuous column reaching the ground.

The rule that fixes most of it

Do this every time
NWS Northern Indiana: "it is always best to stop and watch the feature for a minute or two. If the cloud is not rotating, it is not a tornado or a funnel cloud." A single glance is a photograph, and rotation is a property of a sequence. You cannot see it in one frame, which is precisely why one frame produces so many bad reports.

If you can safely move, move. A hundred meters sideways changes the geometry enough to separate two clouds that were lined up, and it costs less than a false report does.

The one exception is lesson 10. On a QLCS the feature may not last two minutes, and the instruction there is to report it with your uncertainty attached rather than to wait for a certainty that will never arrive.

Light

  • Backlit, with the sun behind the storm, everything is a silhouette. Shape survives, texture and depth do not, and a shelf cloud and a wall cloud both reduce to a dark mass.
  • Front-lit, with the sun behind you, is the photograph in the textbook. Structure, striations and the separation between precipitation and updraft are all visible. It is also common on the dry, high-visibility Plains and uncommon in a humid one.
  • Haze and low contrast, which is most of a humid summer anywhere east of the Plains, flattens everything and makes distance almost impossible to judge.
  • Rain-wrapped, from lesson 9, is the case where the feature is there and simply not available to you.

Night

Most Southeast tornadoes happen when you cannot see. The technique is to work from lightning illumination: face the storm, let your eyes adapt, and read the structure in the flashes rather than trying to see continuously. Repeated flashes over a minute give you something close to the sequence that daylight would have.

It is slower, it is less reliable, and it is worth doing. It is also the point at which positioning and safety stop being secondary considerations, which is module 8.

Reporting what you are not sure about

The Field Guide settles this: "If you are unsure of what you are seeing, make your report, but also express your uncertainty."

That is permission and an obligation at once. Do not sit on an observation because you are not certain, and do not launder uncertainty out of it either. State the conditions you were working in: distance, angle, light, whether it was rain-wrapped, how long you watched. A forecaster can weigh "possible lowering, about eight miles away, backlit, watched thirty seconds" perfectly well. What they cannot weigh is a confident-sounding report with none of that attached.

Teaching point
And the rule that outranks all of the above, from lesson 7: the absence of a visible funnel never rules out a tornado. Poor observing conditions are a reason to caveat your report, never a reason to conclude that nothing is happening.

Check yourself

You glance up and see what appears to be a funnel descending from a storm base about eight miles away, backlit by the late sun. What do you do first?

  1. Watch it for a minute or two to see whether it is rotating, and move if you safely can
  2. Report a funnel cloud immediately, since a delayed report has no value
  3. Dismiss it, because backlit features at eight miles cannot be identified reliably
  4. Wait until it either reaches the ground or dissipates, then report what happened
Hint

Rotation is a property of a sequence, not of a single glance.

Answer

Watch it for a minute or two to see whether it is rotating, and move if you safely can

Watch it. Rotation cannot be seen in one frame, and backlighting at eight miles is exactly the setup that turns scud and beaver tails into funnels. If it is rotating you will know inside a minute, and then you report it with the distance, angle and lighting attached. Dismissing it is equally wrong: poor conditions are a caveat, not a conclusion.

Sources

  1. NWS Northern Indiana, "Low hanging clouds mistaken for Funnel Clouds and Tornadoes" (August 2013)
  2. NWS Weather Spotter's Field Guide (YPA-201154, June 2011), glossary, reporting guidance
  3. SPC public domain tornado images, scanned from the NSSL storm intercept archive