Naarah, 8, of Pittsburgh asked how much rain a tropical storm hurricane can make and whether storm size matters. Scientists say the answer is both huge totals and sharp differences from place to place, which is why a single storm can flood one town while sparing another.
The rain can add up to trillions of gallons. One town in Texas got over 60 inches in five days from Hurricane Harvey in 2017, and Hurricane Lane dumped 58 inches on Kahuna Falls, Hawaii, the following year.
Texas and Kahuna Falls
The bigger risk is not the storm’s total rainfall. It is how much falls on each location. A hurricane can spread rain across a broad area, but the damage to any one place depends on whether the storm stalls, tracks over it, or keeps moving.
That is why size matters. The average hurricane is about 300 miles wide, but Super Typhoon Tip in 1979 grew to half the size of the lower 48 U.S. states and had a radius of about 690 miles, or 1,380 miles wide. Tropical Storm Marco in 2008 was about 23 miles across.
Hurricane Helene in Florida
Warm ocean water helps drive that spread. A hurricane needs a tropical thunderstorm complex, lots of moisture in the air, winds that do not change speed much with height, and warm sea surface waters below in order to form. Water over 80 degrees Fahrenheit can fuel hurricanes because it evaporates, adds moisture to the air, and supplies energy to the storm.
A 2025 study found that hurricane size appears to be influenced by how warm the ocean water is compared with its usual temperature for that time of year. If a hurricane moves over water that is much warmer than normal, it tends to grow in size rapidly as well as growing in strength. As Helene moved over water near Florida that was much warmer than normal for late September, it picked up lots of moisture.
Super Typhoon Tip in 1979
That leaves the question Naarah asked most directly: why some storms become wide enough to spread rain and wind over far more of the map. The answer is that unusually warm water can help a storm grow larger, but the biggest danger still comes from where the rain falls, not just how much the whole storm produces.
