More than a million people have been ordered to evacuate the state of Florida, US, as Hurricane Milton makes landfall from the west coast, the Guardian reported on October 8. Milton is the third fastest-strengthening storm ever recorded in the Atlantic, according to the US National Weather Service.

While much of the southern United States was still recovering from Hurricane Helene, which made landfall in late September, the rapid strengthening of Hurricane Milton took many by surprise. In just one day, Milton strengthened into a Category 5 hurricane, the strongest on the Saffir-Simpson scale, with winds of up to 290 km/h as it moved across the Gulf of Mexico toward central Florida.

Milton experienced “rapid intensification,” a term used when a storm’s winds increase by at least 35 mph (55 km/h) in 24 hours. Milton’s rate of intensification broke that norm, with winds increasing by 90 mph (145 km/h) in about 25 hours, according to research organization Climate Central.

That makes Milton one of the strongest storms ever to threaten the United States. “This storm is approaching the mathematical limits of what the Earth’s atmosphere above this ocean can produce,” said Noah Bergren, a meteorologist in Florida.

A hurricane is made up of many variables. In Milton, they combined to create a nightmare. The storm gets a lot of energy from the high sea surface temperatures in the Gulf of Mexico, which are much higher than normal. That energy translates into higher wind speeds. The Gulf has set multiple temperature records this year, and the water is like a bathtub in the summer. Milton’s core passes through some unusually warm waters, about 2 to 3 degrees Celsius warmer than average for this time of year.

Milton also absorbs moisture from the very humid atmosphere. As a rule, a warmer atmosphere can hold 7% more water vapor for every degree Celsius increase. In addition, the air is very unstable and can rise more easily, helping the storm form and maintain its shape.

Because of La Nina, there isn’t much wind shear, and wind speeds and directions are relatively consistent at different altitudes. So Milton is able to maintain its shape very well, according to Kim Wood, an atmospheric scientist at the University of Arizona. “All of these things combine to make the storm more efficient at using the energy it has,” Wood explains.

The perfect combination of warm seas, moist air and low wind shear continues to be helped by Milton’s path through the western Gulf of Mexico, which hasn’t seen much major storm activity this season. When a storm moves over warm water, it absorbs much of that heat and uses it as fuel, cooling the water. But in the western Gulf, Wood said, there’s nothing else to cool the water.

Milton was also a very compact storm with a highly symmetrical, circular core. Helene’s core, on the other hand, took longer to coalesce, and the storm was more spread out. Wind speeds within Milton increased by 90 mph in about 25 hours, faster than nearly any other storm on record, after Hurricane Wilma in 2005 and Hurricane Felix in 2007.

Climate scientists have long feared that climate change could cause hurricanes to intensify more quickly and reach greater peak intensity. Milton appears to be confirming this.

A study published in the journal Scientific Reports last year found that tropical storms in the Atlantic are now about 29 percent more likely to experience “rapid intensification” than they were between 1971 and 1990. Another study found that natural variability alone is not enough to explain the increase in such storms, suggesting that climate change is also a factor.