What an ominous sight heading your way! Kevin Sheely captured this incredible shot of a shelf cloud near Warrenville, Illinois. On this day, numerous storms moved into and formed within Illinois.
To understand what creates this shelf-like appearance, we first need to understand that the rain that is falling from the core of the storm cools the atmosphere. This cold air is denser than the surrounding air so it sinks to the ground and then spreads outward from where the rain is falling. This cold air spreading outward is referred to as “outflow” and the leading edge of this rapidly outward moving air is called an outflow boundary or gust front. This boundary (like other weather fronts) separates air of different temperatures/densities: in this case, the cold air from the storm and the warm air surrounding it.
The warm, moist air ahead of it is less dense so it is lifted up and over the spreading cold air. This air is lifted and cooled, to the point where it condenses and forms the shelf cloud extending out ahead of the main storm along this outflow.
Notice in that diagram that the warm air out ahead is being lifted into relatively stable air above. This leads to the layered characteristics of the shelf cloud as it extends outward instead of continuing to grow upward like the parent storm.
This rapidly expanding air is often responsible for strong, potentially damaging winds at the surface. As these storms, with their shelf clouds, passed through Illinois on the 25th, downed trees were left in their path. These storm reports from the Storm Prediction Center show a cluster of blues in northern IL, indicating strong winds.
So while the shelf clouds themselves aren’t dangerous, if you see them coming, you can expect strong, cool gusty winds shortly after, followed by heavy rain and possibly even hail.
Luckily these outflow boundaries can also be detected by radar to help warn of this impending gustiness. As the cold air lifts up the warm air ahead of it, it’s also lifting up dust, insects, birds, etc. These can be detected by radar, but the power return is much less than the heavy rain falling. So while the heavy rain in these storms appear orange and red on these radar images, you can see the “fine lines” of blue ahead of these storms indicating the location of the gust front. Notice in these series of radar images that storms produce these boundaries, which then go on to produce new storms that then produce their own gust fronts. And so the cycle continued on this day…
We’ve learned that these shelf clouds are seen at the leading edge of the storm, indicating cold gusty winds to come, but what about behind the storm? Well, storms can only reach a certain height in the atmosphere, at which level they spread out horizontally. This is often seen as an anvil. The air below the anvil is typically drier and therefore sometimes mammatus clouds can form underneath. These bulbous beauties were seen on this day in other parts of Illinois by Bill Morris.
And that’s not all! Moving ahead to the next day (8/26/2014), another round of storms moved through Illinois, allowing for another opportunity to photograph shelf clouds. This picture was submitted to us by Melissa Godbee. Even though this is looking at the storm from the side compared to the head-on view shown by Kevin, can you still see the resemblance?
Have you seen these ominous, yet beautiful, clouds where you live?