What is a storm surge, and why is it so dangerous?
When a big storm hits a coast, the wind is often not what does the most harm: it is the sea that climbs the shore.
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A storm surge is a rise of the sea above its normal tide, pushed onto the land by a storm. It is commonly associated with low-pressure weather systems such as tropical cyclones, and it is measured as the rise in water level above the normal tidal level, not counting the waves on top. The main cause is wind: the US National Hurricane Center says surge is primarily caused by a storm's strong onshore winds, which push water toward the coast over a long stretch of open sea until it piles up on the downwind shore. A smaller effect comes from the low air pressure, which lets the sea bulge upward by about a centimetre for every hectopascal of pressure drop.
Step 1: Wind pushes water ashore
Strong wind over a long builds wind setup on the downwind coast.
Step 2: Low pressure lifts the sea
About 10 mm for every the pressure drops.
Step 3: Waves run up the beach
Run-up can exceed twice the wave height.
Step 4: The tide adds its level
Surge plus makes the .
It is dangerous because water is heavy and does not stop at the beach. The US National Hurricane Center says that storm surge and large waves produced by hurricanes pose the greatest threat to life and property along the coast. Surge can reach heights well over 20 feet and can span hundreds of miles of coastline. Land that lies only a few metres above the sea is at particular risk.
The deadliest surge on record was caused by the 1970 Bhola cyclone, which killed up to 500,000 people in the area of the Bay of Bengal. After the North Sea flood of 1953, countries such as the Netherlands and Britain built storm-surge barriers that stay open for ships but close when a surge threatens.
Try it
How much the sea lifts as the pressure drops
y = 0.01x + 0
Challenge · At 10 mm per hPa, how big a pressure drop does it take to lift the sea by 0.5 m? Then try the lowest and highest slopes to see how much the estimate matters.
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Recap
The wind shoves the water ashore, the low pressure lifts it, and the tide and shape of the coast decide how high it goes.
💡 A trick to remember it · Think of the sea as water in a tray: tilt it with the wind, press on it less, and it slops over the edge.
Surprising fact · Up to 500,000 people died in the 1970 Bhola cyclone, the deadliest surge on record.
Connects to
- 🌀 What is the hurricane "cone of uncertainty", and why do so many people misread it?
- 🌊 Why do some earthquakes send a giant wave across an entire ocean?
- 🌀 How does a cluster of thunderstorms turn into a hurricane?
- 🌊 Does the water in an ocean wave actually travel?
- 🌊 Why does the sea rise when the world warms, even before the ice melts?
- 🌊 Why do some tides rise five storeys high while others barely move?
Sources (4)
No source, no claim. Every fact in this lesson (17 claims) cites at least one of these.