First published September 25, 2018.
The famous "tsunami from heaven" clip shows a wet microburst: a sudden, concentrated column of rain-cooled air and water dropping out of a thunderstorm and spreading out when it hits the surface. It was filmed over Millstätter See, a lake in Carinthia in southern Austria, and sped up as a time-lapse, which is why the rain seems to fall in one solid, collapsing block.
The footage was shot by Peter Maier, a filmmaker from Switzerland who was reportedly 27 at the time, from the balcony of his hotel above the lake. It began spreading online in June 2018, when Austrian weather and tech sites reported that the video had become an internet hit, and it has been re-shared many times since.
Watch the time-lapse
In the clip, a dark cloud base hangs over the water and the mountains. Then a thick grey shaft of rain drops from it, hits the lake and billows outward along the surface, almost like a wave rolling away from the impact point. Maier's own description, a tsunami from heaven, is a vivid comparison rather than a weather term: no sea wave is involved, only rain and air.
What a microburst is
A downburst is a strong current of sinking air beneath a thunderstorm. When the damaging winds it produces cover an area of less than about 4 kilometres (2.5 miles) across, meteorologists call it a microburst. The term was introduced by the Japanese-American storm researcher Ted Fujita, who studied these events in the 1970s and 1980s.
Microbursts happen when a large mass of rain, and sometimes hail, falls through drier air below the cloud. Some of the rain evaporates, which cools the surrounding air. Cold air is heavier than warm air, so the whole column accelerates toward the ground. When it reaches the surface it has nowhere to go but sideways, and it fans out in all directions. Most microbursts last only a few minutes, and the strongest winds can reach speeds comparable to a weak tornado.
Wet and dry microbursts
Meteorologists separate microbursts into two broad types. A wet microburst comes with heavy rain that reaches the ground, so you can see its outline as a dense curtain of water. That is what Maier filmed. A dry microburst forms under high-based clouds in dry climates; most of the rain evaporates before reaching the ground, so the only visible sign may be wispy streaks of virga or a ring of dust thrown up by the wind.
The wet kind is the one that produces spectacular photos like this, because the rain shaft makes an invisible process visible. Similar clips from Arizona, Australia and elsewhere are sometimes labelled "rain bombs" on social media, an informal nickname for the same phenomenon.
Why microbursts matter to pilots
Microbursts are beautiful from a distance and dangerous up close, especially for aircraft taking off or landing. A plane flying into one first meets a headwind that gives it extra lift, then a powerful downdraft, and then a tailwind that suddenly reduces lift close to the ground.
Wind shear of this kind was blamed for the crash of Delta Air Lines Flight 191 at Dallas/Fort Worth International Airport on 2 August 1985, in which more than 130 people died. Accidents like that one led to Doppler weather radar at major airports, onboard wind-shear warning systems and specific pilot training for escaping a microburst.
How the time-lapse changes what you see
A time-lapse compresses several minutes into a few seconds, so the rain shaft seems to plunge and spread faster than it did in real life. That does not make the event less real; it simply makes its shape easier to read. What the camera cannot show is wind speed, rainfall totals or exactly how long the burst lasted, so any figures attached to the clip online should be treated with caution unless they come from a weather service.
Staying safe when you see one coming
If you are outdoors and see a dark, dense rain curtain dropping out of a storm cloud nearby, treat it like any severe thunderstorm. Get off the water and into a sturdy building, stay away from trees and loose objects that the sudden gust could break or throw, and keep listening for local weather warnings. A microburst can flatten trees and damage roofs even when no tornado is present. Watching one from a hotel balcony far across a lake, as Maier did, is the safe way to enjoy it.
Sources: Video by Peter Maier on YouTube; wetter.at (June 2018); The Kid Should See This; National Weather Service: microbursts; Wikipedia: Microburst; Wikipedia: Delta Air Lines Flight 191; Wikipedia: Millstätter See.
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