A powerful Hurricane Irma is threatening millions of people in the Caribbean and Florida. Some answers to questions about Irma and hurricanes:

WHERE DO THESE STORMS COME FROM?

Irma is a classic Cape Verde storm, which begin near the islands off the west coast of Africa. Some of the worst hurricanes start as puffs of unstable air and storminess there and chug west, gaining strength over the warm open Atlantic. Another storm, Jose, has followed in Irma's footsteps. Some of those storms fizzle from wind shear or other weather conditions. Still others curve harmlessly north into the mid-North Atlantic and are called "fish storms". Storms also start in the Gulf of Mexico, like Katia which formed off Mexico and was declared a hurricane today.

WHY ARE STORMS HAPPENING NOW?

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The hurricane season starts on June 1 and ends on November 30. That's usually when the water is warm enough and other weather conditions are conducive to storm formation. Hurricanes need water that's at least 26 degrees Celsius. Peak hurricane season is from mid-August to mid-October with the peak of the peak being September 10 or 11.

WHAT'S AN AVERAGE SEASON LIKE?

An average season produces 12 named storms, according to the National Weather Service. Katia is the 11th this season. Storms get names when winds reach 63km/h. The average season produces six hurricanes and three of those become major at 178 km/h winds or higher. So far this year, there have been six hurricanes: two major ones, Harvey and Irma; two new ones today, Katia and Jose; and Franklin and Gert.

DID FORECASTERS SEE THIS BUSY YEAR COMING?

Yes. In May, the weather service predicted a 70 per cent likelihood of 11 to 17 named storms with five to 9 becoming hurricanes. They predicted two to four major hurricanes. In early August, it was changed to a 60 per cent chance of 14 to 19 named storms, five to 9 hurricanes and two to five major hurricanes.

ARE BACK-TO-BACK BIG HURRICANES UNUSUAL?

Major storms can and do form back-to-back and did so last year with Matthew and Nicole, but having more than one hit the US in a season is strange. If Irma hits Florida as a category four or five storm, it will be the first time in historical record that the US was hit by two category four or five storms in one year, said Colorado State University meteorology professor Phil Klotzbach.

WHY IS IRMA SO STRONG?

Hurricanes use warm water as fuel. Irma has been over water that is 0.7 to 1 degree C warmer than normal. And that warm water goes deeper than usual. High altitude winds, which can fight or even decapitate storms, are not strong, also helping Irma. While over the open Atlantic Ocean yesterday, Irma's 295km/h winds set a record for that region. In the entire Atlantic, Caribbean, and Gulf of Mexico, only Hurricane Allen in 1980 was stronger with 305km/h winds. Others with 295km/h winds: a 1935 Florida storm, 1988's Gilbert, and 2005's Wilma.

HOW UNUSUAL IS IRMA?

This is only the second time since satellite-tracking began about 40 years ago that one maintained 295km/h winds for more than 24 hours, said Klotzbach. The other was the massive killer typhoon Haiyan that killed more than 6000 people in the Philippines in 2013.

IS THIS GLOBAL WARMING?

Scientists take weeks or months to conduct intricate studies, using computer simulations, to see if a storm was worsened by man-made climate change. There have been a limited number of hurricanes since record-keeping began in 1851, which makes it difficult to do robust statistical analyses. However, scientists have long said future global warming would make some of the worst storms stronger and wetter and recently have linked climate change to future rapid intensification of storms. There's been scientific debate over whether global warming means more storms, but the stronger and wetter is generally accepted by scientists.

WASN'T THERE A HURRICANE DROUGHT?

In the US, yes. Until Harvey last month, no major hurricane had hit the United States since Wilma. That 2005 storm also was the last major hurricane to hit Florida. Scientists say the 12-year landfall drought was likely chance. There were still the same number of hurricanes brewing, they just missed the United States or were not major hurricanes. During that span, Superstorm Sandy was a minor hurricane in terms of wind speed but catastrophic in damage when it hit in 2012.

HOW ARE STORMS FORECAST?

Forecasters at the National Hurricane Centre rely on dozens of computer simulations and their own expert experience. They use real-time readings of wind, temperature, air pressure, humidity and more. But those real-time readings are sparse and spread out. Figuring out a storm's path and strength is tricky and usually forecasts do go out farther than five days.

-AP