Cloud PointField Guide

Supplementary feature — murus

Wall Cloud

The lowered base where tornadoes are born

Written by Laurence WayneEntry No. 32Rare

Wall Cloud: The lowered base where tornadoes are born
Plate XXXIIPhotograph by Stefan Klein, CC BY-SA 4.0.

A wall cloud — officially murus — is a localised, often abrupt lowering of cloud from the base of a cumulonimbus, usually beneath a supercell or severe storm. It forms in the rain-free area beneath the storm's strongest updraught, where rain-cooled, very humid inflow air condenses at a lower level than the surrounding cloud base. When a wall cloud shows clear rotation, it can spawn a funnel cloud or tornado.

Worth knowing

  • A rotating wall cloud is direct visual evidence of a mesocyclone — the spinning heart of a supercell — and most strong tornadoes descend from one, which is why storm spotters watch a wall cloud so closely.
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

A wall cloud is…

  1. aA lowering of cloud beneath a thunderstorm base
  2. bA high, wispy ice cloud
  3. cA ring of light around the sun
  4. dFog clinging to a hillside
Show answer

AnswerA lowering of cloud beneath a thunderstorm base

Murus is an abrupt, localised lowering from the base of a cumulonimbus storm cloud.

A rotating wall cloud is a warning sign of…

  1. aClearing skies ahead
  2. bA mesocyclone and a possible tornado
  3. cLight, harmless drizzle
  4. dA halo forming around the sun
Show answer

AnswerA mesocyclone and a possible tornado

Rotation in a wall cloud reveals a mesocyclone, from which tornadoes can form.

Why does a wall cloud hang lower than the surrounding storm base?

  1. aIt is heavier than the rest of the cloud
  2. bRain-cooled, very humid inflow air condenses at a lower level as it is drawn into the updraught
  3. cThe sun heats it from below
  4. dCold air sinks down into it
Show answer

AnswerRain-cooled, very humid inflow air condenses at a lower level as it is drawn into the updraught

Moist, rain-cooled air pulled into the updraught reaches its condensation level lower down, forming the distinct lowered wall.

Nearby in the guide