Cloud PointField Guide

Supplementary feature — tuba

Funnel Cloud & Tornado

The spinning column reaching for the ground

Written by Laurence WayneEntry No. 40Rare

Funnel Cloud & Tornado: The spinning column reaching for the ground
Plate XLPhotograph by Justin Hobson. Original uploader was Justin1569 (Justin Hobson) at en.wikipedia, CC BY 2.5.

Tuba is the WMO name for a cloud column or inverted cone protruding from a cloud base — the cloudy form of a more or less intense vortex. While the vortex hangs in the air it is a funnel cloud; when it reaches the ground or water it becomes a tornado (or waterspout). The visible funnel is condensation: the very low pressure inside the spinning vortex makes inflowing air expand and cool to its dew point, condensing vapour into the cone.

Worth knowing

  • The visible funnel is not the vortex itself — it is only condensation. The rotating column of wind is already there before any cloud forms and reaches wider than the funnel, which is why a tornado can be doing damage at ground level before the funnel visibly 'connects'.
Note

Still unsure it's the same cloud? Our app, Cloud Point— on iOS and Android — reads a photo and returns the likely type with altitude, useful when a formation is fading faster than you can look it up.

Test yourselfThree levels

Choose a difficulty

When does a funnel cloud officially become a tornado?

  1. aWhen it turns grey
  2. bWhen it touches the ground or a water surface
  3. cWhen it produces lightning
  4. dWhen it grows over a kilometre tall
Show answer

AnswerWhen it touches the ground or a water surface

Contact with the ground (or water) is the defining step from funnel cloud to tornado.

What makes the funnel visible?

  1. aSmoke from fires below
  2. bCondensation, as low pressure in the vortex cools inflowing air to its dew point
  3. cReflected sunlight off debris
  4. dStatic electricity glowing
Show answer

AnswerCondensation, as low pressure in the vortex cools inflowing air to its dew point

The pressure drop inside the vortex expands and cools the air until vapour condenses into the visible cone.

Which statement about the funnel is correct?

  1. aThe funnel is the wind itself
  2. bThe funnel is only condensation; the rotating wind exists before and beyond the visible cloud
  3. cA tornado cannot exist without a fully visible funnel
  4. dThe funnel is made of ice crystals
Show answer

AnswerThe funnel is only condensation; the rotating wind exists before and beyond the visible cloud

The visible funnel is condensation, not the vortex; the spinning wind is present before and wider than the cloud.

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