After the terrible flooding caused by Hurricane Harvey, Irma is now being classed as the second strongest Atlantic hurricane in history. Both hurricanes are reached category 5, the maximum strength. This is the first time since records began more than 150 years ago that two category 5 hurricanes have hit in the same season. And it's not over yet.
After devastating the Caribbean, Irma is expected to make landfall in Florida on 10 September. Two more hurricanes, Jose and Katia, are already forming, and there will be more after them. The seas are still exceptionally warm, which scientists explain is the reason for the high intensity of Harvey and Irma.
The hurricane season in the Atlantic lasts from the beginning of June to the end of November each year. Hurricanes (called cyclones or typhoons over other oceans) form over warm, tropical or subtropical seas. The Atlantic hurricanes typically effect the Caribbean and Gulf of Mexico but can make land much further north, as was the case with the devastating Hurricane Sandy which hit New York state in 2012.
This animation from National Hurricane Center shows the intensity of different categories of hurricane. However, the wind force is not the only factor that determines how seriously a hurricane effects a population. The size and density of population, building standards and infrastructure all play their part.
Hurricane Harvey, which caused terrible flooding in Texas and Louisiana in late August, thankfully caused relatively few deaths – estimated at 70. But the economic cost is likely to surpass even Hurricane Katrina, which hit New Orleans in 2005. The Texas governor, Republican Ted Abbott, has warned his state may need up to $180 billion in federal assistance. Houston is the fourth largest city in the nation. An estimated 40,000 homes and a million cars have been destroyed, not to mention infrastructure.
However the effects of the season's hurricanes are far worse in small, economically disadvantaged Caribbean nations such as Haiti, which will need major international aid for emergency intervention and reconstruction.