Why Irma is so strong and other questions about hurricanes
A powerful hurricane is threatening millions of people. Its name is Hurricane Irma. It is threatening the Caribbean and Florida. Here are answers to questions about Irma and hurricanes.
WHERE DO THESE STORMS COME FROM?
Irma is a classic Cape Verde storm. They begin near the islands off the west coast of Africa. Some of the worst hurricanes start as puffs of unstable air and storminess there. They chug west. They gain 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. They go into the mid-North Atlantic. They are called "fish storms." Storms also start in the Gulf of Mexico. Katia formed off Mexico. It was declared a hurricane on Wednesday.
WHY ARE STORMS HAPPENING NOW?
Hurricane season starts June 1. It ends Nov. 30. That's usually when the water is warm enough. Other weather conditions are also conducive to storm formation. Hurricanes need water that's at least 79 degrees. Peak hurricane season is from mid-August to mid-October. The peak of the peak is Sept. 10 or 11.
WHAT'S AN AVERAGE SEASON LIKE?
An average season produces 12 named storms. That's according to the National Weather Service. Wednesday's Katia is the 11th this season. Storms get names when winds reach 39 mph. The average season produces six hurricanes. Three of those become major. that’s when there are 111 mph winds or higher. So far this year, there have been six hurricanes. Two have been major ones. Those are Harvey and Irma. Two are new ones named Wednesday, Katia and Jose. And there is also Franklin and Gert.
DID FORECASTERS SEE THIS BUSY YEAR COMING?
Yes. In May, the weather service predicted a 70 percent likelihood of 11 to 17 named storms. Five to nine of them would become hurricanes. They predicted 2 to 4 major hurricanes. In early August, it was changed to a 60 percent chance of 14 to 19 named storms. Five to nine would be hurricanes. And 2 to 5 would be major hurricanes.
ARE BACK-TO-BACK BIG HURRICANES UNUSUAL?
Major storms can and do form back-to-back. They did so last year. That was with Matthew and Nicole. But having more than one hit the U.S. in a season is strange. If Irma hits Florida as a category 4 or 5 storm, it will be the first time in historical record that the U.S. was hit by two category 4 or 5 storms in one year. That's according to 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 1.2 to 1.8 degrees warmer than normal. And that warm water goes deeper than usual. High altitude winds can fight or even decapitate storms. But those winds are not strong. This is also helping Irma. While over the open Atlantic Ocean on Tuesday, Irma's 185 mph winds set a record for that region. In the entire Atlantic, Caribbean and Gulf of Mexico, only Hurricane Allen in 1980 was stronger. It had 190 mph winds. There have been others with 185 mph winds. A 1935 Florida storm. In 1988 there was Gilbert. And in 2005 there was Wilma.
HOW UNUSUAL IS IRMA?
This is only the second time since satellite-tracking began about 40 years ago that one maintained 185 mph winds for more than 24 hours, said Klotzbach. The other was the massive killer typhoon Haiyan. It killed more than 6,000 people in the Philippines in 2013.
IS THIS GLOBAL WARMING?
Scientists take weeks or months to conduct detailed studies. They use computer simulations. They look to see if a storm was worsened by man-made climate change. There have been a limited number of hurricanes since record-keeping began. That was in 1851. This makes it hard to do full statistical analyses. However, scientists have long said future global warming would make some of the worst storms stronger. They would also be wetter. Recently they have linked climate change to future rapid strengthening of storms. There's been scientific debate over whether global warming means more storms. But the stronger and wetter theory is generally accepted by scientists.
WASN'T THERE A HURRICANE DROUGHT?
In the U.S., yes. Until Harvey last month, no major hurricane had hit the United States since Wilma. That 2005 storm was also 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 they were not major hurricanes. During that span, Superstorm Sandy was a minor hurricane. That’s in terms of wind speed. But it was catastrophic in damage when it hit in 2012.
HOW ARE STORMS FORECAST?
Forecasters at the National Hurricane Center rely on dozens of computer simulations. They also rely on 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 they are spread out. Figuring out a storm's path and strength is tricky. And usually forecasts do not go out further than five days.