What is a shelf cloud?

 

The line of storms Friday evening gave us some spectacular views of what is known as a shelf cloud. Often times the shelf cloud can appear to be expanding from one end of the horizon to the other when viewed off in the distance. We received several pictures of the incoming shelf cloud Friday evening which can sometimes look very ominous and be mistaken for a wall cloud. So, what exactly is a shelf cloud?

A shelf cloud is often times associated with a line of thunderstorms, known as a squall line. Ahead of these storms is a very warm and humid air mass. As rain falls from the incoming storms it works to cool the surrounding air, causing that air to surge out ahead of the storms. This rain, cooled air - because it is more dense - will then push the warm, humid air ahead of it up and over. This process causes clouds to form, giving us the shelf appearance. This process can be very turbulent within the atmosphere which can cause some low hanging clouds, or even the clouds under the shelf to appear as if they are rotating. These features can often times be mistaken for funnel clouds. And while squall line thunderstorms can sometimes produce tornadoes, tornadoes most of the time form from supercell thunderstorms. A wall cloud is a lowering from a supercell storm that can sometimes produce a funnel cloud, as well as a tornado.

Strong winds are typically the biggest weather hazard with incoming squall line storms, followed by very heavy rain. Sometimes, there can be a spin-up tornado along the leading edge -especially within an area where the line itself has surged ahead or become a little kinked. Embedded within those features can be mini-supercells.

While the incoming storms from Friday weakened a bit the further east they traveled across northern Illinois, they sure did give us a great visual of what was going on in the atmosphere! Thank you to everyone who shared their photos!