Cloud PointField Guide

Supplementary feature (arcus)

Shelf cloud

The menacing wedge on a storm's leading edge

Written by Laurence WayneEntry No. 14Uncommon

Shelf cloud: The menacing wedge on a storm's leading edge
Plate XIVPhotograph by Will y theweatherguy473737, CC BY-SA 4.0.

A shelf cloud is a long, low, wedge-shaped cloud that hangs across the leading edge of an advancing thunderstorm, often looking like a vast ramp lowering from the sky. It forms as cold air spilling out of the storm wedges beneath the warm, moist air ahead and shoves it upward to condense. When you see one bearing down, the storm's gusty winds and rain are usually only moments away.

Worth knowing

  • A shelf cloud is often mistaken for a tornado-maker, but it's the opposite kind of feature: it rides the storm's outflow and doesn't rotate. Its real threat is sudden, strong straight-line gust-front winds.
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

Where and how does a shelf cloud appear?

  1. aAs a long, low wedge across the leading edge of an approaching storm
  2. bAs a coloured ring around the sun
  3. cAs thin streaks high in a clear sky
  4. dAs a cloud that glows at night
Show answer

AnswerAs a long, low wedge across the leading edge of an approaching storm

A shelf cloud is a low, long, wedge-shaped cloud stretched along the front of an advancing thunderstorm.

What forms a shelf cloud?

  1. aCold outflow air from the storm wedging under warm air ahead and lifting it
  2. bIce crystals forming high in the stratosphere
  3. cSunlight refracting through falling rain
  4. dWarm air slowly sinking over the sea
Show answer

AnswerCold outflow air from the storm wedging under warm air ahead and lifting it

Cold, dense air rushing out ahead of the storm (the gust front) slides under the warm, moist inflow and forces it upward, where it condenses into the low shelf.

How can you tell a (non-rotating) shelf cloud from a rotating wall cloud that might drop a tornado?

  1. aA shelf cloud spins rapidly while a wall cloud never moves
  2. bA shelf cloud is an outflow feature that juts forward and doesn't rotate; a wall cloud is a rotating lowering at the storm's rear
  3. cThey are really the same thing under different names
  4. dA shelf cloud only ever appears at night
Show answer

AnswerA shelf cloud is an outflow feature that juts forward and doesn't rotate; a wall cloud is a rotating lowering at the storm's rear

A shelf cloud rides the storm's outflow on its leading edge and does not rotate — its danger is straight-line wind. A wall cloud is a rotating, inflow lowering at the storm's rear flank, and is where tornadoes can form.

Nearby in the guide