CINCINNATI, OH (FOX19) - In addition to rain, lightning, hail, and gusty winds, Monday's afternoon storms triggered a rare sight for some in the Tri-State.
The FOX19 NOW newsroom was flooded with pictures of long, thick, horizontal clouds.
Many of these clouds were surrounded by otherwise quiet skies.
This type of cloud is called a roll cloud. They form when rain cooled air surges out ahead of thunderstorms in the form of outflow boundaries or gust fronts.
These outflow boundaries act as mini cold fronts, cooling things down a little at the surface and also forcing warm, moist air up.
As this warm, moist air rises it cools and condenses forming a cloud. As the outflow boundary moves, so does the cloud.
When the weather around the thunderstorms is quiet, outflow boundaries can travel many miles away from their parent thunderstorm, allowing these clouds to form in otherwise dry conditions.
Roll clouds don't form every time outflow boundaries do, though.
Sometimes the air around outflow boundaries is too dry and no clouds form.
Other times the air is so humid that new thunderstorms form along the outflow boundaries.
When the humidity is just right and surface and upper level winds are light, roll clouds can form and last as long as the outflow boundary does.
Though ominous looking, roll clouds are completely harmless.
They are closely related to shelf clouds, but shelf clouds are still attached to their parent thunderstorm.
A shelf cloud means stormy weather will soon begin.
Roll clouds sometimes mean rain is on the way, but other times the thunderstorms fizzle before they ever reach where the roll clouds were.
If you ever catch a unique sight in the sky, send us your pictures through Fox 19 First Alert Weather app.
You can also check out other viewer photos in the app.