Shelf clouds are a stunning feature of many spring and summertime thunderstorms that often pack more bark than bite. Other than for their incredible beauty, shelf clouds are usually newsworthy because they tend to freak people out.

This past week, two shelf clouds caught the attention of both social media and international news. The incredible shelf cloud over Sydney was probably one of the most photographed weather events of 2014, as the striking formation enveloped the Sydney skyline before bringing torrential rain and intense lightning. Another one occurred just two days later in the skies over Florida as an intense line of thunderstorms moved across the state.

What are they?

A shelf cloud is a low-hanging, well-defined, wedge-shaped formation that occurs along the leading edge of a gust front in a thunderstorm. Shelf clouds most often form just ahead of intense lines of thunderstorms.

One of the best examples of this type of cloud is in the picture above, taken by Waldo Jaquith in Cape Hatteras, NC in May 2012.

What are the dangers?

Even though they look ominous and people often mistake them for tornadoes, shelf clouds themselves are harmless. What they indicate, however, is potentially more dangerous.

These clouds most often form along intense lines of thunderstorms. These storms, called squall lines or bow echoes, tend to produce damaging winds when they hit. The most well-defined and photogenic shelf clouds occur with the most intense type of severe thunderstorm called a "derecho."

How do they form?

The main circulation of air in a thunderstorm occurs within an "updraft" and a "downdraft." Updrafts feed warm, moist air into the thunderstorm to provide it with the energy it needs to survive. A downdraft consists of the dense, rain-cooled air that sinks to the surface underneath a thunderstorm.

The air from a downdraft pools up at the surface beneath the storm in what's known as a "cold pool." This cold pool can do one of two things — it can either choke off the updraft and kill the storm, or it can start moving out ahead of the storm, tilting the updraft and letting the storm begin to race along the landscape.

As the cold pool begins to race away from the thunderstorm (now called an "outflow boundary" or a "gust front"), the storm's updraft tilts along the outflow's leading edge, allowing the storm to continue to ingest warm, moist air as it moves along in the direction of its outflow.

As the updraft's warm, moist air rises up along the outflow boundary, it cools and condenses into a shelf cloud.

Examples

Sydney opera house with shelf cloud & lightning is the best one I've seen yet. #australiawx pic.twitter.com/fz8VT5q9Pv — Julie Hayes (@wxjoules) March 5, 2014

Corpus Christi, TX shelf cloud MT @StormySkies2112 I didn't take this picture! Awesome shot pic.twitter.com/po1rS2v1It #txwx — Jim Cantore (@JimCantore) November 7, 2013

What's your caption for this photo? Breathtaking shot of a shelf cloud over Janesville, WI- 2009. Credit: Matt Heeren pic.twitter.com/xXVAolWv5f — AcuRite (@AcuRite) August 1, 2013

As spring sets in over the next few months, storm systems will encounter a stronger temperature gradient as they move across the United States and have more opportunities to produce severe weather. Shelf clouds will become a common sight for many once severe weather season ramps up.

If you see one, remember that the cloud itself is completely harmless, but it's a tell-tale sign that you should get inside before the storm hits.

[Photo by Waldo Jaquith on Flickr through Creative Commons]