Wind piling up water
Why Does the Sea Rise in a Storm?
A big storm both sucks the sea upward and shoves it toward the shore.
-
Step 1 of 5
Low pressure lets the sea rise.
Air normally presses down on the ocean. In a deep low it presses less, so the surface bulges upward — roughly a centimetre for each unit of pressure lost.
-
Step 2 of 5
Wind does most of the work.
Hours of gale-force wind blowing toward land drags surface water with it. With nowhere to go, that water simply stacks up against the coast.
-
Step 3 of 5
Shallow water makes it much worse.
Deep water lets the surge flow away underneath. A shallow shelf or a funnel-shaped bay gives it nowhere to escape, so the height multiplies.
-
Step 4 of 5
The tide decides how bad it gets.
A three-metre surge at low tide may do nothing. The same surge landing at high spring tide adds to an already high sea and overtops the defences.
-
Step 5 of 5
Which is why surge, not wind, does the damage.
Most deaths in tropical cyclones come from water, not air. The wind speed makes the headlines, but the raised sea is what floods the town.
Forecasters issue surge warnings separately from wind warnings, because the two peak at different moments.
The short version
Low pressure lifts the sea and sustained wind piles it against the coast, so a storm's water does more damage than its wind.
Try it yourself
Blow steadily across a tray of water from one end. The level at the far end climbs while you blow, and drops the moment you stop.
Topics