Explainer-How hurricanes cause dangerous, destructive storm surges

Storm surges often are the most deadly and destructive phenomena to accompany a hurricane or tropical cyclone.

Here is an explanation of how they form and why they are so dangerous.

WHAT IS A STORM SURGE?

As a hurricane approaches a coast, the churning winds force ocean water up onto land. Atmospheric pressure from the storm also helps squeeze the water ashore. The shallower the continental shelf, the higher the threat of a dangerous surge. The waters may take a couple of days to fully subside.

According to the U.S. National Hurricane Center, surges are often the greatest threat to life and property from a hurricane.

WHAT MAKES STORM SURGES SO DANGEROUS?

Water is heavy: about 1,700 pounds (770 kg) per cubic yard. And it can move quickly in a surge, sweeping people to their deaths, tossing about boats and vehicles and pulverizing buildings and other structures. Six inches (15 cm) of fast-moving water is enough to knock over an adult, according to the hurricane center. Surges become even more dangerous when they coincide with high tide.

A powerful storm surge can cause long-term damage by sweeping away roads, eroding beaches and contaminating land with salt water, harming wildlife and agriculture.

WHAT WAS THE DEADLIEST STORM SURGE ON RECORD?

In 1970, a tropical cyclone formed in the Bay of Bengal and hit northeastern India and what was then East Pakistan. Storm surge of up to 35 feet (10.7 m) washed over barrier islands, including Bhola, the largest. The death toll was estimated between 300,000 and 500,000 people, making it the deadliest tropical cyclone on record, according to the Atlantic Oceanographic and Meteorological Laboratory.

WHAT WAS THE LARGEST STORM SURGE?

The highest storm tide noted in historical accounts was produced by the 1899 Cyclone Mahina, estimated at almost 44 feet (13.4 m) at Bathurst Bay, Australia.

In the United States, one of the largest storm surges recorded was during Hurricane Katrina that struck Gulf Coast states in 2005. The maximum high water mark observation of storm surge was 27.8 feet (8.5 m) at Pass Christian in Mississippi, according to the National Hurricane Center. The hurricane killed as many as 1,500 people.

In Florida, Hurricane Michael in 2018 brought storm surges that were 20.6 feet (6.3 m) high and destroyed Mexico Beach on Florida's Panhandle.

HOW DOES CLIMATE CHANGE AFFECT STORM SURGE?

Warmer ocean temperatures create more intense and frequent hurricanes that pack higher rainfall and greater storm surge risk due to rising sea levels, according to climate scientists.

HOW TO PREPARE FOR A STORM SURGE

The most important way to survive storm surge is to evacuate if you are in a flood zone. If you cannot evacuate, experts advise moving to the highest level of the building and staying indoors. They warn against attempting to drive through floodwaters.

(Compiled by Reuters staff; Editing by Will Dunham)