What are w, and how do they form? An expert explains

Joseph Golden / NOAA

Waterspouts are extraordinary, impressive weather events. Observers describethem as looking like “the start of an alien invasion” and post their snaps across social media.

But what are these enigmatic offshore twisters, and what causes them to form?

What is a waterspout?

A waterspout is a spinning column of air that sucks up water (usually from the ocean) to make a twisting funnel of water and cloud connecting the sea and the sky.

They are spectacular but short lived, usually lasting no more than five minutes (but occasionally up to ten minutes). Winds inside the waterspout can be faster than 100 kilometres per hour, and they can do great damage to boats at sea.

If they drift ashore, waterspouts can create even more havoc: the Lennox Head tornado in 2010 destroyed a dozen homes in northern NSW.

Waterspouts are in some ways like the tornadoes that form over land. But where tornadoes are associated with huge supercell thunderstorms, waterspouts can form during smaller storms or even just showers or the presence of the right kind of clouds

How do waterspouts form?

Waterspouts can form when winds blowing in two different directions run into each other. Along the line where the two winds meet (called a “convergence line” or “shear line”), there is a lot of rotating air near the surface.

The collision of the two winds makes air move upwards because it has nowhere else to go. This rising air carries water vapour high into the sky where it creates rain showers, storms and cumulus clouds.

As the air rises, it can tilt some of the horizontal spinning air near the surface into the vertical direction. When this vertical spin concentrates at a particular point it starts sucking up water — and you have yourself a waterspout.

Because waterspouts form on the line where two winds meet, you sometimes see a line of waterspouts in a row where the spinning low-level air is sucked upwards at a few different points.

When and where are waterspouts most common?

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