DARK patches on the ocean surface could help warn coastal communities of the approach of a menacing wave.
Tsunamis are huge waves that swell out from the sites of underwater earthquakes or landslides, travelling across oceans at around 800 kilometres per hour. In deep water, they only raise the surface of the ocean by a few centimetres, but when they near a shore they billow up into giant tidal waves.
People on the shore have sometimes reported seeing mysterious strips of dark water several kilometres wide moving across the sea surface before tsunamis strike. It was unclear why, but now calculations by Oleg Godin at the University of Colorado in Boulder suggest the source is a flow of air skimming the surface.
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Godin calculates that a tsunami will create a rapid airflow just a few millimetres thick that ruffles the sea surface, making it reflect less sunlight (Geophysical Research Letters, DOI: 10.1029/2003JC002030). It probably also makes the ocean reflect radio waves less well, so satellites might be able to spot a radar shadow that could give minutes to hours of advance warning of a tsunami’s arrival.