Since 1972, hurricanes have been classified according to the Saffir-Simpson scale, which relates wind strength to potential damage and ranks them from weakest to strongest on a scale of 1 to 5. A hurricane is considered "major" if it reaches at least Category 3 on that scale, which means that its sustained winds are above 110 mph. It's tempting to think that the "major" designation is only because Category 3 is the midpoint between 1 (the weakest) and 5 (the strongest) but there's more to it.
When detailed engineering surveys of hurricane damage were being conducted during the 1960s and early 1970s, they revealed an interesting statistic. The increase in the amount of significant damage from tropical storm strength to Category 1 hurricane and then Category 2 was gradual and steady. At Category 3 strength, significant structural damage spikes sharply upward.
To explain why, note that Category 3 strength indicates sustained winds between 111-130 miles an hour. Most building codes, meanwhile, have historically mandated the ability to withstand winds of around 120 miles an hour. Therefore, once a storm reaches Category 3 status, there is a greater likelihood that the winds, especially with gusts, will exceed the limit that most structures have been designed to tolerate and the potential for significant structural damage increases markedly. It is really this potential that causes a hurricane to be considered "major".
Quick Fact: Major hurricanes have been extremely rare in northeast Florida in the last century. Even Hurricane Dora in 1964, which approached the area as a major hurricane, was determined in post-analysis to have actually made landfall at only Category 2 strength.
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