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Gustnado

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A gustnado is a brief and shallow vortex of wind near the ground. It forms within the downdraft of a thunderstorm. The term itself is a portmanteau. It combines the words "gust front" and "tornado."

Gustnado April26 2019
Gustnado April26 2019

While they look like tornadoes, they are different. They are considered non-tornadic straight-line wind features. They form specifically within the outflow of a storm. This outflow is the air moving away from the storm. Most meteorologists do not classify them as true tornadoes. They are more similar to minor whirlwinds.

To understand how a gustnado works, we must look at the storm's structure. A thunderstorm has an updraft, which is rising air. It also has a downdraft, which is falling air. A gustnado forms in the downdraft. As this air hits the ground, it creates a gust front. This is a boundary of strong winds. Within this gust front, there can be large horizontal shear. Horizontal shear means wind speeds change quickly in different directions. This shear can cause shear instability. This instability can lead to the formation of a spinning vortex.

Scientists believe the spinning comes from vertical vorticity. Vorticity is a measure of the rotation in the air. This rotation might come from tilting horizontal vorticity into a vertical position. It could also come from vertical vorticity already present in the environment. In supercell storms, these often form along the rear-flank gust front. This is a specific part of the storm's outflow. The process involves the upward tilting of vorticity within descending air. This often happens in the presence of baroclinity, which involves temperature or density differences.

Gustnadoes have several distinct characteristics that separate them from classic tornadoes. First, they differ in their vertical development. A classic tornado usually connects the ground to the cloud base. A gustnado is shallow and stays near the surface. It rarely connects to the ambient cloud base. Second, they differ in their intensity and longevity. A gustnado typically lasts only a few seconds to a few minutes. It may even appear in simultaneous swarms or multiple generations. Third, they are associated with the outflow rather than the inflow. Classic tornadoes are associated with mesocyclones in the updraft.

There are specific ways to categorize these events. Most gustnadoes have winds equivalent to an F0 or F1 tornado. These are relatively low on the intensity scale. However, a rare type can become a landspout. A landspout occurs if the rotating column connects the surface to the cloud base. Some people call them "spin-up tornadoes." This term is often used for these rare tornadic gustnadoes. It is also used for brief, true tornadoes associated with a mesovortex. Most gustnadoes, however, remain categorized as whirlwinds.

We can see the impact of these winds through specific examples. On April 26, 2019, a gustnado occurred near Swan Quarter, North Carolina.

Gustnado April26 2019
Gustnado April26 2019

The National Weather Service confirmed this event. It produced an estimated maximum wind speed of 80 mph. In metric units, this is 130 km/h. This wind caused damage to buildings in the area. Fortunately, there were no reported injuries or deaths. While injuries are uncommon, these winds still pose hazards. They can be dangerous for people who are driving.

It is important to distinguish gustnadoes from other wind events. For example, they are not the same as dust devils. Dust devils are whirlwinds that form from superheated surface layers. They usually occur on sunny, warm days with light winds. Gustnadoes are much more powerful because they are driven by severe thunderstorms. We also see them compared to the 2011 Indiana State Fair collapse. In that case, early reports speculated a gustnado was involved. However, engineers found the structure failed due to lateral load-bearing issues. This shows why careful investigation of wind loads is necessary.

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