Why Do Ash Vortices Form?

Though some people may think they are rare, ash vortices are actually more common than you may think and are similar to dust devils. “USGS Hawaiian Volcano Observatory geologists conducted post-episode-45 servicing of cameras along the south rim of Kīlauea caldera and witnessed a large dust devil, which is puahiohio in ʻŌlelo Hawaiʻi, driven by heat rising from the hot lava flows on floor of Halema‘uma‘u crater,” stated a previous entry on their website. USGS maintains a photo gallery of Puahiohio or dust devils on at Kīlauea and Mauna Loa.

Ash vortices form because of intense heat associated with lava flows or vents. As you probably know, warm air likes to rise. Intense, concentrated heating propels hot air upward, and it can rotate. This is similar to what happens with dust devils in the desert. In both cases, the presence of large temperature differences over a short distance and angular momentum are important. Like dust devils, spin is produced by very localized changes in wind or shear so they can rotate clockwise or counterclockwise. From my perspective, the vortices that I watched on the webcam were mostly circulating in a clockwise manner.

As with an ice skater who spins faster as he brings in the arms, conservation of angular momentum can enhance vortex spin. The rotating updraft is stretched upward and spin increases. It picks up volcanic ash, dust, glass or other debris to produce the visible vortex that many have called a “volnado.” In the image above, debris is seen pelting the webcam as a vortex approached. The image below shows muliple vortices swirling to the left of the lava plume.

How Do These Vortices Differ From Tornadoes?

These vortices differ from tornadoes in several ways. They are caused by temperature gradients on the ground and form “upward.” The typical tornado that most people are familiar with forms “downward” from supercell thunderstorm clouds with a rotating updraft. There are weaker “landspout” tornadoes that can form from the ground up. Localized spin near the ground can be stretched into the vertical by an updraft associated with a developing cloud. Waterspouts also form in a similar manner. In both cases, they are usually much weaker than than traditional tornaodes.