On Thursday, Harvey was a tropical storm. By the next day it was a Category 2 hurricane, and it strengthened to a Category 4 before hitting southeastern Texas on Friday night. That makes it the strongest tropical cyclone to strike the mainland United States in 12 years. What’s behind this storm’s sudden increase in power?
Harvey’s winds quickened from about 35 to 109 miles per hour between Thursday and Friday. That’s called “rapid intensification,” says Suzana Camargo, an ocean and climate scientist at the Earth Institute’s Lamont-Doherty Earth Observatory. She says the hurricane’s fast power-up was influenced by two factors.
The first is warm water. Warm water dumps heat energy into a developing storm. More heat allows the low-pressure circulating system to draw in more air from its surroundings, which makes the storm spin faster and grow. “A thick layer of warm water provides more fuel for the storm,” Camargo explains. The warm waters of the Gulf of Mexico, which Harvey is feeding off of, make it something of a breeding ground for hurricanes.
— NWS Corpus Christi (@NWSCorpus) August 25, 2017
A satellite view of Hurricane Harvey as the storm approaches the Texas Coast, via Corpus Christi National Weather Service
But warm water isn’t enough to cause a rapid intensification; it also depends on wind speeds throughout the troposphere (the lowest region of the atmosphere). If you think of the storm as a big tower, explains Camargo, you can’t have the winds blowing in one direction at the bottom of the storm but a different direction at the top—that would topple the column of air and the storm would be kaput. The same thing happens if the winds have different strengths at different heights. Basically, having uniform winds from the bottom to the top of the storm allows it to grow in strength.
Rapid changes in strength like Hurricane Harvey’s are not all that rare. A recent study led by Chia-Ying Lee, a postdoctoral researcher at the International Research Institute for Climate and Society, observed that out of about 500 hurricanes that formed in the North Atlantic, 96 underwent rapid intensification—so about one in five. The phenomenon is more common in the Pacific Ocean.
Update, August 30:
As Hurricane Harvey continued moving over the warm waters of the Gulf, it continued to gain energy and speed. By the time it made landfall in Texas, its winds whipped up to 130 miles per hour as a Category 4 storm. Making matters worse, the storm is moving slowly, giving it extra time to dump devastating amounts of rainfall onto some areas of the Lone Star State. Mandatory evacuations were instated in seven Texas counties due to flood concerns, and rescue workers have helped to save thousands more trapped by the storm. The storm has claimed 30 lives so far, and has destroyed countless homes. Experts estimate the damages will total in the tens of billions of dollars.
Note for journalists: Earth Institute experts are available to answer questions about hurricane physics, rapid intensification, flooding, emergency response, and more.