How Hurricane Irma became so enormously powerful

Hurricane Irma, the most powerful Atlantic Ocean hurricane in recorded history, slammed into the U.S. Virgin Islands and Puerto Rico today. The Category 5 hurricane is expected to smash apart buildings and mow down trees with sustained winds of up to 295 km/h and cause massive flooding with heavy rain and storm surges of up to six metres.

How does a hurricane get that big?

There are some key ingredients that turn a tropical cyclone into a mega-storm, says Ian Folkins, a researcher with the department of physics and atmospheric science at Dalhousie University in Halifax who studies tropical cyclones:

• High sea surface temperatures.
• Low wind shear, or in other words very little variation in wind speed and direction within a local area, especially between the ground and higher up in the atmosphere.
• Moist air in the atmosphere.

Hurricanes can begin when the surface temperature of the water is around 27 C or above, as is common in the tropics, Folkins says.

Paradoxically, what you need in the initial stages is weak winds.- Ian Folkins, researcher

"Water vapour is the fuel, so if you have warm water, there's typically more water vapour coming off the surface."

Water evaporates from the surface and rises, leaving an area of lower pressure underneath. The surrounding air gets drawn in to take the place of the rising air, and certain wind conditions can cause that air to rotate.

"Paradoxically," says Folkins, "what you need in the initial stages is weak winds."

Stronger winds and higher wind shear can stop a storm from developing with a stacked structure of rotating winds and clouds.

Once the inward spiral begins, it forces more air to rise at the centre. As the air rises, it cools and condenses into storm clouds — especially if there's lots of moisture —  and releases rainfall and heat, which makes the air more buoyant and accelerates the upward motion, Folkins says.

Like the drain in your tub

He likens it to a drain in your bathtub: "Except in the atmosphere, the drain is pointing up."

Kevin Trenberth, a climate and global dynamics researcher at the U.S. National Center for Atmospheric Research, says the process heats up the atmosphere.

"That causes the storm to be more intense and to continue to grow or live longer than it otherwise would."

As it grows, the wind speeds increase. A hurricane is a tropical cyclone, the name given to a storm once the wind speeds reach 119 km/h.

Folkins says that once a hurricane gets going, it can last for weeks,

"They'll keep going until the source of fuel is taken away from them, and that source is warm water."

According to the U.S. National Hurricane Center, "Irma is forecast to remain within favourable atmospheric conditions and over warm waters during the next three to four days. Therefore, Irma is likely to remain a very powerful hurricane during this time."

Storm stoppers

A few things can stop a hurricane.

If the warm surface water is a thin layer, the hurricane's winds will stir it up to the point that it mixes with the cold water underneath.

"And that will kill the hurricane," Folkins says.

If a hurricane passes over a land mass, it will become disconnected from the warm water the fuels it, and eventually the storm will fizzle out.

Hurricanes also compete with other storms for energy.

Because they push air upward and cause rain, the drier air has to sink back down somewhere else, Folkins says. That tends to suppress other storms.

Hurricanes also tend to leave cooler water in their wake, says Trenberth. "As a result, none of these storms ever go on the same track."

The competition with other storms doesn't just happen locally, Trenberth says. A lot of cyclone activity in the Pacific will tend to suppress storms in the Atlantic, which is what happened in 2015.

For now, though, Irma has what it needs to keep on its destructive course toward Florida.