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What's A Shelf Cloud?

A shelf cloud resembles it's name of course, and is defined as a low cloud and horizontal (not vertically prominent like many storm clouds). A shelf cloud is usually curved or semicircular and sticks out like a shelf from a parent cloud (usually a thunderstorm cloud).

Shelf clouds form due to outflow. What the heck is outflow you ask? Allow me to explain....

What goes up must come down. That's why storms have both updrafts of air rushing upward and downdrafts of air rushing back down to earth. Typically the downdraft also carries rain to the ground. When rain initially forms it steals heat energy from the air. So, the result is a rain-cooled shaft of air rushing down to the ground. This is called outflow.

To help explain what happens next, imagine if you dumped a can of paint on the ground. The paint will splatter outwards as it hits the ground. The same thing happens with air from the downdraft of a thunderstorm.

As the rain-cooled air smacks the surface of earth, it will splatter out ahead of the storm. This is the reason why it becomes gusty before a strong storm approaches. I wanted to explain the reason for the gusty winds because a shelf cloud is associated with those gusty winds and is created by the outflow of the storm.

How does the shelf cloud form?

As the rush of rain-cooled downdraft air spreads outward, it will cause the humid air ahead of the storm to lift & condense into this cool cloud formation.

So basically, the shelf cloud is a result of thunderstorm downdrafts spreading outward into warm and humid air. For this reason, a tornado will not develop out of a shelf cloud because tornadoes thrive on inflow. However, that rush of air from the storm could produce damaging winds, especially when the shelf cloud is associated with a squall line of thunderstorms.

  Picture Courtesy of NOAA Photo Library