A towering thundershower shot up off the coast of Gulf Shores, Ala., on July 17, 2016. Rick Geiss was sitting on the beach in front of his condominium when he snapped a photo with his iPhone.

This week, he shared it with meteorologist Jacob Dunne, who posted it to Twitter, calling it "quite possibly the best example of a mushroom cloud I have ever seen." The photo has gone viral, wowing weather enthusiasts on social media.

Brian Stelter, host of CNN's Reliable Sources, called it "incredible."

The contrast between the white billowing clouds and dark, menacing rain shaft is stunning. And the cloud-form is so symmetrical that some have questioned the photograph's authenticity.

But the photo presents a textbook example of what are known as pulse, pop-up or air mass thundershowers. They form in the absence of a large-scale thunderstorm trigger such as a cold front, and are simply fueled by the sun's energy on hot, lazy summer days.

In an article published in 2013, Jeff Halverson, the Post Capital Weather Gang's severe weather expert, described how these "pulse" storms evolve:

"A buoyant plume or bubble of rapidly rising air — the updraft — ascends during the first 15 minutes of the storm. Large amounts of water vapor are drawn inward and upward, condensing into a mass of rain.

"After 15-20 minutes, the rain falls out. The drag of the descending rain mass creates the downdraft, which is further enhanced by evaporative cooling (evaporation chills the air, increasing its density). As the downdraft plummets straight down through the updraft, it snuffs out its buoyancy. Without a self-sustaining updraft, the storm's final minutes are literally spent raining itself out."

These kinds of storms usually last 30 minutes or less because of a lack of wind shear, or increasing wind speed with altitude. This causes their updrafts to be vertical, pointing straight up into the sky.