Seeing cloud formations growing into a vicious tornado is a threatening enough sight. But what about those ominous formations known as "shelf clouds"?

Aside from looking scary, shelf clouds can also portend some pretty heinous things, especially as they tend to arrive at the forefront of a "squall line" of thunderstorms.

As Weather.com puts it, a swarm of shelf clouds can appear to be "[r]esembling an advancing army." But what you're actually seeing with these thick, bulging clouds looming over the horizon is "the boundary between a thunderstorm (or line of thunderstorms') downdraft and updraft."

"Rain-chilled air descends in a thunderstorm, then spreads laterally when reaching the earth's surface," Weather.com says. "Warmer, more moist air is lifted at the leading edge, or gust front, of this rain-cooled air. When this warm, moist air condenses, you see the shelf cloud."

After the shelf cloud passes by, onlookers will likely feel an "abrupt" shifting of the wind as it changes both its speed and direction. Minutes later, there will just as likely be an onslaught of heavy rain or hail, Weather.com says.

The shelf clouds do also come in a variety called "roll cloud" in which the formation is not attached to its "parent thunderstorm." These roll clouds are shaped rather like large rolling pins in the sky and are a much rarer variety than the typical shelf cloud.

For more pictures of particularly scary looking shelf clouds, check out this slide show courtesy of the Mother Nature Network that you can view right here.

Do you have any wild stories (or, better yet, pictures) of shelf clouds invading your sky? Let us know in the comments below!

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