The interface of two air masses along a sea breeze can generate a “shear zone” — a line along which small-scale eddies and whirls can invisibly dance near the surface. Once in a while, a vortex becomes entrained into the updraft of a developing cloud, vertically stretching it and forming a waterspout. But there are signs that this behemoth spout might have formed in a slightly different way.

Waterspouts this large are often the product of supercell or rotating thunderstorms, distinguishing them as true tornadoes over water. Occasionally supercell thunderstorms develop in the tropics but seldom if ever in Malaysia. At a little under 5½ degrees north of the equator, the region lacks the strong temperature (air mass) contrasts that generate significant wind shear (changes in wind speed and direction with increasing altitude). Such shear leads to the genesis of a supercell’s rotation.

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Also, this waterspout spun clockwise — opposite to the orientation of more than 98 percent of supercellular tornadoes in the northern hemisphere.

So this spout is a little unique.

What might have happened is that a small region on the storm’s eastern flank took on the characteristics of an anticyclonic supercell, in which a column of strong rotation (known as a mesocyclone) developed but spun clockwise. Notice the low-hanging “tail cloud” behind the waterspout, marking rising air ingested by the storm from left to right. Meanwhile, curtains of rain crash down from the foreground.

The separation of updraft and downdraft is a marked trait of mature supercells, but the direction of spin is highly unusual. Dark bands on the ocean’s surface show clockwise circulation.

The environment did have a fair deal of wind shear that may have been locally generated. Winds at the mid-levels were easterly, but high aloft, they were blowing gently from the west as upper-level “outflow” exited a complex of intense storms over the Malacca Strait. At the same time, if even a slight breeze from the north or northwest developed at the surface, the winds would become bottled up in the channel between the mainland and Penang Island — generating broad clockwise rotation that could encourage a rotating updraft within the storm.

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With true supercells, waterspouts that are tornadic in nature can sometimes penetrate far inland. But this morning’s funnel fell apart within a few minutes of moving onto land. That indicates wind flow over the water was extremely important for sustaining it.

In any case, it provided quite the after-lunch sight to Malaysians who scrambled to take cover. Winds probably topping 80 mph tossed debris hundreds of meters into the air, falling to the ground as lethal projectiles.

According to Storyful, there were no immediate reports of injuries.